AAnn IInnttrroodduuccttiioonn ttoo tthhee CC sshheellll _W_i_l_l_i_a_m _J_o_y _(_r_e_v_i_s_e_d _f_o_r _4_._3_B_S_D _b_y _M_a_r_k _S_e_i_d_e_n_) Computer Science Division Department of Electrical Engineering and Computer Science University of California, Berkeley Berkeley, California 94720 _A_B_S_T_R_A_C_T _C_s_h is a new command language interpreter for UNIX(R) systems. It incorporates good features of other shells and a _h_i_s_t_o_r_y mechanism similar to the _r_e_d_o of INTERLISP. While incorporating many features of other shells which make writing shell programs (shell scripts) easier, most of the fea- tures unique to _c_s_h are designed more for the interactive UNIX user. UNIX users who have read a general introduc- tion to the system will find a valuable basic explanation of the shell here. Simple terminal interaction with _c_s_h is possible after reading just the first section of this document. The sec- ond section describes the shell's capabilities which you can explore after you have begun to become acquainted with the shell. Later sections introduce features which are useful, but not nec- essary for all users of the shell. Additional information includes an appendix listing special characters of the shell and a glossary of terms and commands introduced in this manual. IInnttrroodduuccttiioonn A _s_h_e_l_l is a command language interpreter. _C_s_h is the name of one particular command interpreter on UNIX. The primary purpose of _c_s_h is to translate command lines typed at a terminal into system actions, such as invocation of other programs. _C_s_h is a user program just like any you might write. Hopefully, _c_s_h will be a very useful program for you in interacting with the UNIX system. In addition to this document, you will want to refer to a copy of the UNIX User Reference Manual. The _c_s_h USD:4-2 An Introduction to the C shell documentation in section 1 of the manual provides a full description of all features of the shell and is the defini- tive reference for questions about the shell. Many words in this document are shown in _i_t_a_l_i_c_s_. These are important words; names of commands, and words which have special meaning in discussing the shell and UNIX. Many of the words are defined in a glossary at the end of this document. If you don't know what is meant by a word, you should look for it in the glossary. AAcckknnoowwlleeddggeemmeennttss Numerous people have provided good input about previous versions of _c_s_h and aided in its debugging and in the debug- ging of its documentation. I would especially like to thank Michael Ubell who made the crucial observation that history commands could be done well over the word structure of input text, and implemented a prototype history mechanism in an older version of the shell. Eric Allman has also provided a large number of useful comments on the shell, helping to unify those concepts which are present and to identify and eliminate useless and marginally useful features. Mike O'Brien suggested the pathname hashing mechanism which speeds command execution. Jim Kulp added the job control and directory stack primitives and added their documentation to this introduction. An Introduction to the C shell USD:4-3 11.. TTeerrmmiinnaall uussaaggee ooff tthhee sshheellll 11..11.. TThhee bbaassiicc nnoottiioonn ooff ccoommmmaannddss A _s_h_e_l_l in UNIX acts mostly as a medium through which other _p_r_o_g_r_a_m_s are invoked. While it has a set of _b_u_i_l_t_i_n functions which it performs directly, most commands cause execution of programs that are, in fact, external to the shell. The shell is thus distinguished from the command interpreters of other systems both by the fact that it is just a user program, and by the fact that it is used almost exclusively as a mechanism for invoking other programs. _C_o_m_m_a_n_d_s in the UNIX system consist of a list of strings or _w_o_r_d_s interpreted as a _c_o_m_m_a_n_d _n_a_m_e followed by _a_r_g_u_m_e_n_t_s_. Thus the command mail bill consists of two words. The first word _m_a_i_l names the com- mand to be executed, in this case the mail program which sends messages to other users. The shell uses the name of the command in attempting to execute it for you. It will look in a number of _d_i_r_e_c_t_o_r_i_e_s for a file with the name _m_a_i_l which is expected to contain the mail program. The rest of the words of the command are given as _a_r_g_u_- _m_e_n_t_s to the command itself when it is executed. In this case we specified also the argument _b_i_l_l which is inter- preted by the _m_a_i_l program to be the name of a user to whom mail is to be sent. In normal terminal usage we might use the _m_a_i_l command as follows. % mail bill I have a question about the csh documentation. My document seems to be missing page 5. Does a page five exist? Bill EOT % Here we typed a message to send to _b_i_l_l and ended this message with a ^D which sent an end-of-file to the mail pro- gram. (Here and throughout this document, the notation ``^_x'' is to be read ``control-_x'' and represents the strik- ing of the _x key while the control key is held down.) The mail program then echoed the characters `EOT' and transmit- ted our message. The characters `% ' were printed before and after the mail command by the shell to indicate that input was needed. After typing the `% ' prompt the shell was reading com- mand input from our terminal. We typed a complete command USD:4-4 An Introduction to the C shell `mail bill'. The shell then executed the _m_a_i_l program with argument _b_i_l_l and went dormant waiting for it to complete. The mail program then read input from our terminal until we signalled an end-of-file via typing a ^D after which the shell noticed that mail had completed and signaled us that it was ready to read from the terminal again by printing another `% ' prompt. This is the essential pattern of all interaction with UNIX through the shell. A complete command is typed at the terminal, the shell executes the command and when this exe- cution completes, it prompts for a new command. If you run the editor for an hour, the shell will patiently wait for you to finish editing and obediently prompt you again when- ever you finish editing. An example of a useful command you can execute now is the _t_s_e_t command, which sets the default _e_r_a_s_e and _k_i_l_l characters on your terminal - the erase character erases the last character you typed and the kill character erases the entire line you have entered so far. By default, the erase character is the delete key (equivalent to `^?') and the kill character is `^U'. Some people prefer to make the erase character the backspace key (equivalent to `^H'). You can make this be true by typing tset -e which tells the program _t_s_e_t to set the erase character to tset's default setting for this character (a backspace). 11..22.. FFllaagg aarrgguummeennttss A useful notion in UNIX is that of a _f_l_a_g argument. While many arguments to commands specify file names or user names, some arguments rather specify an optional capability of the command which you wish to invoke. By convention, such arguments begin with the character `-' (hyphen). Thus the command ls will produce a list of the files in the current _w_o_r_k_i_n_g _d_i_r_e_c_t_o_r_y. The option _-_s is the size option, and ls -s causes _l_s to also give, for each file the size of the file in blocks of 512 characters. The manual section for each command in the UNIX reference manual gives the available options for each command. The _l_s command has a large number of useful and interesting options. Most other commands have either no options or only one or two options. It is hard to remember options of commands which are not used very An Introduction to the C shell USD:4-5 frequently, so most UNIX utilities perform only one or two functions rather than having a large number of hard to remember options. 11..33.. OOuuttppuutt ttoo ffiilleess Commands that normally read input or write output on the terminal can also be executed with this input and/or output done to a file. Thus suppose we wish to save the current date in a file called `now'. The command date will print the current date on our terminal. This is because our terminal is the default _s_t_a_n_d_a_r_d _o_u_t_p_u_t for the date command and the date command prints the date on its standard output. The shell lets us _r_e_d_i_r_e_c_t the _s_t_a_n_d_a_r_d _o_u_t_p_u_t of a command through a notation using the _m_e_t_a_c_h_a_r_a_c_- _t_e_r `>' and the name of the file where output is to be placed. Thus the command date > now runs the _d_a_t_e command such that its standard output is the file `now' rather than the terminal. Thus this command places the current date and time into the file `now'. It is important to know that the _d_a_t_e command was unaware that its output was going to a file rather than to the terminal. The shell performed this _r_e_d_i_r_e_c_t_i_o_n before the command began executing. One other thing to note here is that the file `now' need not have existed before the _d_a_t_e command was executed; the shell would have created the file if it did not exist. And if the file did exist? If it had existed previously these previous contents would have been discarded! A shell option _n_o_c_l_o_b_b_e_r exists to prevent this from happening acci- dentally; it is discussed in section 2.2. The system normally keeps files which you create with `>' and all other files. Thus the default is for files to be permanent. If you wish to create a file which will be removed automatically, you can begin its name with a `#' character, this `scratch' character denotes the fact that the file will be a scratch file.* The system will remove ----------- *Note that if your erase character is a `#', you will have to precede the `#' with a `\'. The fact that the `#' character is the old (pre-CRT) stan- dard erase character means that it seldom appears in a file name, and allows this convention to be used for scratch files. If you are using a CRT, USD:4-6 An Introduction to the C shell such files after a couple of days, or sooner if file space becomes very tight. Thus, in running the _d_a_t_e command above, we don't really want to save the output forever, so we would more likely do date > #now 11..44.. MMeettaacchhaarraacctteerrss iinn tthhee sshheellll The shell has a large number of special characters (like `>') which indicate special functions. We say that these notations have _s_y_n_t_a_c_t_i_c and _s_e_m_a_n_t_i_c meaning to the shell. In general, most characters which are neither let- ters nor digits have special meaning to the shell. We shall shortly learn a means of _q_u_o_t_a_t_i_o_n which allows us to use _m_e_t_a_c_h_a_r_a_c_t_e_r_s without the shell treating them in any spe- cial way. Metacharacters normally have effect only when the shell is reading our input. We need not worry about placing shell metacharacters in a letter we are sending via _m_a_i_l_, or when we are typing in text or data to some other program. Note that the shell is only reading input when it has prompted with `% ' (although we can type our input even before it prompts). 11..55.. IInnppuutt ffrroomm ffiilleess;; ppiippeelliinneess We learned above how to _r_e_d_i_r_e_c_t the _s_t_a_n_d_a_r_d _o_u_t_p_u_t of a command to a file. It is also possible to redirect the _s_t_a_n_d_a_r_d _i_n_p_u_t of a command from a file. This is not often necessary since most commands will read from a file whose name is given as an argument. We can give the command sort < data to run the _s_o_r_t command with standard input, where the com- mand normally reads its input, from the file `data'. We would more likely say sort data letting the _s_o_r_t command open the file `data' for input itself since this is less to type. We should note that if we just typed sort then the sort program would sort lines from its _s_t_a_n_d_a_r_d ----------- your erase character should be a ^H, as we demon- strated in section 1.1 how this could be set up. An Introduction to the C shell USD:4-7 _i_n_p_u_t_. Since we did not _r_e_d_i_r_e_c_t the standard input, it would sort lines as we typed them on the terminal until we typed a ^D to indicate an end-of-file. A most useful capability is the ability to combine the standard output of one command with the standard input of another, i.e. to run the commands in a sequence known as a _p_i_p_e_l_i_n_e_. For instance the command ls -s normally produces a list of the files in our directory with the size of each in blocks of 512 characters. If we are interested in learning which of our files is largest we may wish to have this sorted by size rather than by name, which is the default way in which _l_s sorts. We could look at the many options of _l_s to see if there was an option to do this but would eventually discover that there is not. Instead we can use a couple of simple options of the _s_o_r_t command, com- bining it with _l_s to get what we want. The _-_n option of sort specifies a numeric sort rather than an alphabetic sort. Thus ls -s | sort -n specifies that the output of the _l_s command run with the option _-_s is to be _p_i_p_e_d to the command _s_o_r_t run with the numeric sort option. This would give us a sorted list of our files by size, but with the smallest first. We could then use the _-_r reverse sort option and the _h_e_a_d command in combination with the previous command doing ls -s | sort -n -r | head -5 Here we have taken a list of our files sorted alphabeti- cally, each with the size in blocks. We have run this to the standard input of the _s_o_r_t command asking it to sort numerically in reverse order (largest first). This output has then been run into the command _h_e_a_d which gives us the first few lines. In this case we have asked _h_e_a_d for the first 5 lines. Thus this command gives us the names and sizes of our 5 largest files. The notation introduced above is called the _p_i_p_e mecha- nism. Commands separated by `|' characters are connected together by the shell and the standard output of each is run into the standard input of the next. The leftmost command in a pipeline will normally take its standard input from the terminal and the rightmost will place its standard output on the terminal. Other examples of pipelines will be given later when we discuss the history mechanism; one important use of pipes which is illustrated there is in the routing of information to the line printer. USD:4-8 An Introduction to the C shell 11..66.. FFiilleennaammeess Many commands to be executed will need the names of files as arguments. UNIX _p_a_t_h_n_a_m_e_s consist of a number of _c_o_m_p_o_n_e_n_t_s separated by `/'. Each component except the last names a directory in which the next component resides, in effect specifying the _p_a_t_h of directories to follow to reach the file. Thus the pathname /etc/motd specifies a file in the directory `etc' which is a subdirec- tory of the _r_o_o_t directory `/'. Within this directory the file named is `motd' which stands for `message of the day'. A _p_a_t_h_n_a_m_e that begins with a slash is said to be an _a_b_s_o_- _l_u_t_e pathname since it is specified from the absolute top of the entire directory hierarchy of the system (the _r_o_o_t). _P_a_t_h_n_a_m_e_s which do not begin with `/' are interpreted as starting in the current _w_o_r_k_i_n_g _d_i_r_e_c_t_o_r_y, which is, by default, your _h_o_m_e directory and can be changed dynamically by the _c_d change directory command. Such pathnames are said to be _r_e_l_a_t_i_v_e to the working directory since they are found by starting in the working directory and descending to lower levels of directories for each _c_o_m_p_o_n_e_n_t of the pathname. If the pathname contains no slashes at all then the file is contained in the working directory itself and the pathname is merely the name of the file in this directory. Absolute pathnames have no relation to the working directory. Most filenames consist of a number of alphanumeric characters and `.'s (periods). In fact, all printing char- acters except `/' (slash) may appear in filenames. It is inconvenient to have most non-alphabetic characters in file- names because many of these have special meaning to the shell. The character `.' (period) is not a shell-metachar- acter and is often used to separate the _e_x_t_e_n_s_i_o_n of a file name from the base of the name. Thus prog.c prog.o prog.errs prog.output are four related files. They share a _b_a_s_e portion of a name (a base portion being that part of the name that is left when a trailing `.' and following characters which are not `.' are stripped off). The file `prog.c' might be the source for a C program, the file `prog.o' the corresponding object file, the file `prog.errs' the errors resulting from a compilation of the program and the file `prog.output' the output of a run of the program. If we wished to refer to all four of these files in a command, we could use the notation prog.* An Introduction to the C shell USD:4-9 This expression is expanded by the shell, before the command to which it is an argument is executed, into a list of names which begin with `prog.'. The character `*' here matches any sequence (including the empty sequence) of characters in a file name. The names which match are alphabetically sorted and placed in the _a_r_g_u_m_e_n_t _l_i_s_t of the command. Thus the command echo prog.* will echo the names prog.c prog.errs prog.o prog.output Note that the names are in sorted order here, and a differ- ent order than we listed them above. The _e_c_h_o command receives four words as arguments, even though we only typed one word as an argument directly. The four words were gen- erated by _f_i_l_e_n_a_m_e _e_x_p_a_n_s_i_o_n of the one input word. Other notations for _f_i_l_e_n_a_m_e _e_x_p_a_n_s_i_o_n are also avail- able. The character `?' matches any single character in a filename. Thus echo ? ?? ??? will echo a line of filenames; first those with one charac- ter names, then those with two character names, and finally those with three character names. The names of each length will be independently sorted. Another mechanism consists of a sequence of characters between `[' and `]'. This metasequence matches any single character from the enclosed set. Thus prog.[co] will match prog.c prog.o in the example above. We can also place two characters around a `-' in this notation to denote a range. Thus chap.[1-5] might match files chap.1 chap.2 chap.3 chap.4 chap.5 if they existed. This is shorthand for chap.[12345] USD:4-10 An Introduction to the C shell and otherwise equivalent. An important point to note is that if a list of argu- ment words to a command (an _a_r_g_u_m_e_n_t _l_i_s_t_) contains filename expansion syntax, and if this filename expansion syntax fails to match any existing file names, then the shell con- siders this to be an error and prints a diagnostic No match. and does not execute the command. Another very important point is that files with the character `.' at the beginning are treated specially. Nei- ther `*' or `?' or the `[' `]' mechanism will match it. This prevents accidental matching of the filenames `.' and `..' in the working directory which have special meaning to the system, as well as other files such as _._c_s_h_r_c which are not normally visible. We will discuss the special role of the file _._c_s_h_r_c later. Another filename expansion mechanism gives access to the pathname of the _h_o_m_e directory of other users. This notation consists of the character `~' (tilde) followed by another user's login name. For instance the word `~bill' would map to the pathname `/usr/bill' if the home directory for `bill' was `/usr/bill'. Since, on large systems, users may have login directories scattered over many different disk volumes with different prefix directory names, this notation provides a convenient way of accessing the files of other users. A special case of this notation consists of a `~' alone, e.g. `~/mbox'. This notation is expanded by the shell into the file `mbox' in your _h_o_m_e directory, i.e. into `/usr/bill/mbox' for me on Ernie Co-vax, the UCB Computer Science Department VAX machine, where this document was pre- pared. This can be very useful if you have used _c_d to change to another directory and have found a file you wish to copy using _c_p_. If I give the command cp thatfile ~ the shell will expand this command to cp thatfile /usr/bill since my home directory is /usr/bill. There also exists a mechanism using the characters `{' and `}' for abbreviating a set of words which have common parts but cannot be abbreviated by the above mechanisms because they are not files, are the names of files which do not yet exist, are not thus conveniently described. This An Introduction to the C shell USD:4-11 mechanism will be described much later, in section 4.2, as it is used less frequently. 11..77.. QQuuoottaattiioonn We have already seen a number of metacharacters used by the shell. These metacharacters pose a problem in that we cannot use them directly as parts of words. Thus the com- mand echo * will not echo the character `*'. It will either echo an sorted list of filenames in the current _w_o_r_k_i_n_g _d_i_r_e_c_t_o_r_y_, or print the message `No match' if there are no files in the working directory. The recommended mechanism for placing characters which are neither numbers, digits, `/', `.' or `-' in an argument word to a command is to enclose it with single quotation characters `'', i.e. echo '*' There is one special character `!' which is used by the _h_i_s_- _t_o_r_y mechanism of the shell and which cannot be _e_s_c_a_p_e_d by placing it within `'' characters. It and the character `'' itself can be preceded by a single `\' to prevent their spe- cial meaning. Thus echo \'\! prints '! These two mechanisms suffice to place any printing character into a word which is an argument to a shell command. They can be combined, as in echo \''*' which prints '* since the first `\' escaped the first `'' and the `*' was enclosed between `'' characters. 11..88.. TTeerrmmiinnaattiinngg ccoommmmaannddss When you are executing a command and the shell is wait- ing for it to complete there are several ways to force it to stop. For instance if you type the command USD:4-12 An Introduction to the C shell cat /etc/passwd the system will print a copy of a list of all users of the system on your terminal. This is likely to continue for several minutes unless you stop it. You can send an INTER- RUPT _s_i_g_n_a_l to the _c_a_t command by typing ^C on your termi- nal.* Since _c_a_t does not take any precautions to avoid or otherwise handle this signal the INTERRUPT will cause it to terminate. The shell notices that _c_a_t has terminated and prompts you again with `% '. If you hit INTERRUPT again, the shell will just repeat its prompt since it handles INTERRUPT signals and chooses to continue to execute com- mands rather than terminating like _c_a_t did, which would have the effect of logging you out. Another way in which many programs terminate is when they get an end-of-file from their standard input. Thus the _m_a_i_l program in the first example above was terminated when we typed a ^D which generates an end-of-file from the stan- dard input. The shell also terminates when it gets an end- of-file printing `logout'; UNIX then logs you off the sys- tem. Since this means that typing too many ^D's can acci- dentally log us off, the shell has a mechanism for prevent- ing this. This _i_g_n_o_r_e_e_o_f option will be discussed in sec- tion 2.2. If a command has its standard input redirected from a file, then it will normally terminate when it reaches the end of this file. Thus if we execute mail bill < prepared.text the mail command will terminate without our typing a ^D. This is because it read to the end-of-file of our file `pre- pared.text' in which we placed a message for `bill' with an editor program. We could also have done cat prepared.text | mail bill since the _c_a_t command would then have written the text through the pipe to the standard input of the mail command. When the _c_a_t command completed it would have terminated, closing down the pipeline and the _m_a_i_l command would have received an end-of-file from it and terminated. Using a pipe here is more complicated than redirecting input so we would more likely use the first form. These commands could also have been stopped by sending an INTERRUPT. Another possibility for stopping a command is to sus- pend its execution temporarily, with the possibility of ----------- *On some older Unix systems the DEL or RUBOUT key has the same effect. "stty all" will tell you the INTR key value. An Introduction to the C shell USD:4-13 continuing execution later. This is done by sending a STOP signal via typing a ^Z. This signal causes all commands running on the terminal (usually one but more if a pipeline is executing) to become suspended. The shell notices that the command(s) have been suspended, types `Stopped' and then prompts for a new command. The previously executing command has been suspended, but otherwise unaffected by the STOP signal. Any other commands can be executed while the origi- nal command remains suspended. The suspended command can be continued using the _f_g command with no arguments. The shell will then retype the command to remind you which command is being continued, and cause the command to resume execution. Unless any input files in use by the suspended command have been changed in the meantime, the suspension has no effect whatsoever on the execution of the command. This feature can be very useful during editing, when you need to look at another file before continuing. An example of command sus- pension follows. % mail harold Someone just copied a big file into my directory and its name is ^Z Stopped % ls funnyfile prog.c prog.o % jobs [1] + Stopped mail harold % fg mail harold funnyfile. Do you know who did it? EOT % In this example someone was sending a message to Harold and forgot the name of the file he wanted to mention. The mail command was suspended by typing ^Z. When the shell noticed that the mail program was suspended, it typed `Stopped' and prompted for a new command. Then the _l_s command was typed to find out the name of the file. The _j_o_b_s command was run to find out which command was suspended. At this time the _f_g command was typed to continue execution of the mail program. Input to the mail program was then continued and ended with a ^D which indicated the end of the message at which time the mail program typed EOT. The _j_o_b_s command will show which commands are suspended. The ^Z should only be typed at the beginning of a line since everything typed on the current line is discarded when a signal is sent from the keyboard. This also happens on INTERRUPT, and QUIT signals. More information on suspending jobs and controlling them is given in section 2.6. USD:4-14 An Introduction to the C shell If you write or run programs which are not fully debugged then it may be necessary to stop them somewhat ungracefully. This can be done by sending them a QUIT sig- nal, sent by typing a ^\. This will usually provoke the shell to produce a message like: Quit (Core dumped) indicating that a file `core' has been created containing information about the running program's state when it termi- nated due to the QUIT signal. You can examine this file yourself, or forward information to the maintainer of the program telling him/her where the _c_o_r_e _f_i_l_e is. If you run background commands (as explained in section 2.6) then these commands will ignore INTERRUPT and QUIT sig- nals at the terminal. To stop them you must use the _k_i_l_l command. See section 2.6 for an example. If you want to examine the output of a command without having it move off the screen as the output of the cat /etc/passwd command will, you can use the command more /etc/passwd The _m_o_r_e program pauses after each complete screenful and types `--More--' at which point you can hit a space to get another screenful, a return to get another line, a `?' to get some help on other commands, or a `q' to end the _m_o_r_e program. You can also use more as a filter, i.e. cat /etc/passwd | more works just like the more simple more command above. For stopping output of commands not involving _m_o_r_e you can use the ^S key to stop the typeout. The typeout will resume when you hit ^Q or any other key, but ^Q is normally used because it only restarts the output and does not become input to the program which is running. This works well on low-speed terminals, but at 9600 baud it is hard to type ^S and ^Q fast enough to paginate the output nicely, and a pro- gram like _m_o_r_e is usually used. An additional possibility is to use the ^O flush output character; when this character is typed, all output from the current command is thrown away (quickly) until the next input read occurs or until the next shell prompt. This can be used to allow a command to complete without having to suffer through the output on a slow terminal; ^O is a tog- gle, so flushing can be turned off by typing ^O again while An Introduction to the C shell USD:4-15 output is being flushed. 11..99.. WWhhaatt nnooww?? We have so far seen a number of mechanisms of the shell and learned a lot about the way in which it operates. The remaining sections will go yet further into the internals of the shell, but you will surely want to try using the shell before you go any further. To try it you can log in to UNIX and type the following command to the system: chsh myname /bin/csh Here `myname' should be replaced by the name you typed to the system prompt of `login:' to get onto the system. Thus I would use `chsh bill /bin/csh'. YYoouu oonnllyy hhaavvee ttoo ddoo tthhiiss oonnccee;; iitt ttaakkeess eeffffeecctt aatt nneexxtt llooggiinn.. You are now ready to try using _c_s_h_. Before you do the `chsh' command, the shell you are using when you log into the system is `/bin/sh'. In fact, much of the above discussion is applicable to `/bin/sh'. The next section will introduce many features particular to _c_s_h so you should change your shell to _c_s_h before you begin reading it. USD:4-16 An Introduction to the C shell 22.. DDeettaaiillss oonn tthhee sshheellll ffoorr tteerrmmiinnaall uusseerrss 22..11.. SShheellll ssttaarrttuupp aanndd tteerrmmiinnaattiioonn When you login, the shell is started by the system in your _h_o_m_e directory and begins by reading commands from a file _._c_s_h_r_c in this directory. All shells which you may start during your terminal session will read from this file. We will later see what kinds of commands are usefully placed there. For now we need not have this file and the shell does not complain about its absence. A _l_o_g_i_n _s_h_e_l_l, executed after you login to the system, will, after it reads commands from _._c_s_h_r_c_, read commands from a file _._l_o_g_i_n also in your home directory. This file contains commands which you wish to do each time you login to the UNIX system. My _._l_o_g_i_n file looks something like: set ignoreeof set mail=(/usr/spool/mail/bill) echo "${prompt}users" ; users alias ts \ 'set noglob ; eval `tset -s -m dialup:c100rv4pna -m plugboard:?hp2621nl *`'; ts; stty intr ^C kill ^U crt set time=15 history=10 msgs -f if (-e $mail) then echo "${prompt}mail" mail endif This file contains several commands to be executed by UNIX each time I login. The first is a _s_e_t command which is interpreted directly by the shell. It sets the shell vari- able _i_g_n_o_r_e_e_o_f which causes the shell to not log me off if I hit ^D. Rather, I use the _l_o_g_o_u_t command to log off of the system. By setting the _m_a_i_l variable, I ask the shell to watch for incoming mail to me. Every 5 minutes the shell looks for this file and tells me if more mail has arrived there. An alternative to this is to put the command biff y in place of this _s_e_t_; this will cause me to be notified immediately when mail arrives, and to be shown the first few lines of the new message. Next I set the shell variable `time' to `15' causing the shell to automatically print out statistics lines for commands which execute for at least 15 seconds of CPU time. The variable `history' is set to 10 indicating that I want the shell to remember the last 10 commands I type in its _h_i_s_t_o_r_y _l_i_s_t, (described later). An Introduction to the C shell USD:4-17 I create an _a_l_i_a_s ``ts'' which executes a _t_s_e_t(1) com- mand setting up the modes of the terminal. The parameters to _t_s_e_t indicate the kinds of terminal which I usually use when not on a hardwired port. I then execute ``ts'' and also use the _s_t_t_y command to change the interrupt character to ^C and the line kill character to ^U. I then run the `msgs' program, which provides me with any system messages which I have not seen before; the `-f' option here prevents it from telling me anything if there are no new messages. Finally, if my mailbox file exists, then I run the `mail' program to process my mail. When the `mail' and `msgs' programs finish, the shell will finish processing my _._l_o_g_i_n file and begin reading com- mands from the terminal, prompting for each with `% '. When I log off (by giving the _l_o_g_o_u_t command) the shell will print `logout' and execute commands from the file `.logout' if it exists in my home directory. After that the shell will terminate and UNIX will log me off the system. If the system is not going down, I will receive a new login mes- sage. In any case, after the `logout' message the shell is committed to terminating and will take no further input from my terminal. 22..22.. SShheellll vvaarriiaabblleess The shell maintains a set of _v_a_r_i_a_b_l_e_s_. We saw above the variables _h_i_s_t_o_r_y and _t_i_m_e which had values `10' and `15'. In fact, each shell variable has as value an array of zero or more _s_t_r_i_n_g_s_. Shell variables may be assigned val- ues by the set command. It has several forms, the most use- ful of which was given above and is set name=value Shell variables may be used to store values which are to be used in commands later through a substitution mecha- nism. The shell variables most commonly referenced are, however, those which the shell itself refers to. By chang- ing the values of these variables one can directly affect the behavior of the shell. One of the most important variables is the variable _p_a_t_h_. This variable contains a sequence of directory names where the shell searches for commands. The _s_e_t command with no arguments shows the value of all variables currently defined (we usually say _s_e_t_) in the shell. The default value for path will be shown by _s_e_t to be USD:4-18 An Introduction to the C shell % set argv () cwd /usr/bill home /usr/bill path (. /usr/ucb /bin /usr/bin) prompt % shell /bin/csh status 0 term c100rv4pna user bill % This output indicates that the variable path points to the current directory `.' and then `/usr/ucb', `/bin' and `/usr/bin'. Commands which you may write might be in `.' (usually one of your directories). Commands developed at Berkeley, live in `/usr/ucb' while commands developed at Bell Laboratories live in `/bin' and `/usr/bin'. A number of locally developed programs on the system live in the directory `/usr/local'. If we wish that all shells which we invoke to have access to these new programs we can place the command set path=(. /usr/ucb /bin /usr/bin /usr/local) in our file _._c_s_h_r_c in our home directory. Try doing this and then logging out and back in and do set again to see that the value assigned to _p_a_t_h has changed. One thing you should be aware of is that the shell examines each directory which you insert into your path and determines which commands are contained there. Except for the current directory `.', which the shell treats specially, this means that if commands are added to a directory in your search path after you have started the shell, they will not necessarily be found by the shell. If you wish to use a command which has been added in this way, you should give the command rehash to the shell, which will cause it to recompute its internal table of command locations, so that it will find the newly added command. Since the shell has to look in the current directory `.' on each command, placing it at the end of the path specification usually works equivalently and reduces ----------- |- Another directory that might interest you is /usr/new, which contains many useful user-con- tributed programs provided with Berkeley Unix. An Introduction to the C shell USD:4-19 overhead. Other useful built in variables are the variable _h_o_m_e which shows your home directory, _c_w_d which contains your current working directory, the variable _i_g_n_o_r_e_e_o_f which can be set in your _._l_o_g_i_n file to tell the shell not to exit when it receives an end-of-file from a terminal (as described above). The variable `ignoreeof' is one of sev- eral variables which the shell does not care about the value of, only whether they are _s_e_t or _u_n_s_e_t_. Thus to set this variable you simply do set ignoreeof and to unset it do unset ignoreeof These give the variable `ignoreeof' no value, but none is desired or required. Finally, some other built-in shell variables of use are the variables _n_o_c_l_o_b_b_e_r and _m_a_i_l_. The metasyntax > filename which redirects the standard output of a command will over- write and destroy the previous contents of the named file. In this way you may accidentally overwrite a file which is valuable. If you would prefer that the shell not overwrite files in this way you can set noclobber in your _._l_o_g_i_n file. Then trying to do date > now would cause a diagnostic if `now' existed already. You could type date >! now if you really wanted to overwrite the contents of `now'. The `>!' is a special metasyntax indicating that clobbering the file is ok.|- ----------- |-The space between the `!' and the word `now' is critical here, as `!now' would be an invocation of the _h_i_s_t_o_r_y mechanism, and have a totally differ- ent effect. USD:4-20 An Introduction to the C shell 22..33.. TThhee sshheellll''ss hhiissttoorryy lliisstt The shell can maintain a _h_i_s_t_o_r_y _l_i_s_t into which it places the words of previous commands. It is possible to use a notation to reuse commands or words from commands in forming new commands. This mechanism can be used to repeat previous commands or to correct minor typing mistakes in commands. The following figure gives a sample session involving typical usage of the history mechanism of the shell. In this example we have a very simple C program which has a bug (or two) in it in the file `bug.c', which we `cat' out on our terminal. We then try to run the C compiler on it, referring to the file again as `!$', meaning the last argu- ment to the previous command. Here the `!' is the history mechanism invocation metacharacter, and the `$' stands for the last argument, by analogy to `$' in the editor which stands for the end of the line. The shell echoed the com- mand, as it would have been typed without use of the history mechanism, and then executed it. The compilation yielded error diagnostics so we now run the editor on the file we were trying to compile, fix the bug, and run the C compiler again, this time referring to this command simply as `!c', which repeats the last command which started with the letter `c'. If there were other commands starting with `c' done recently we could have said `!cc' or even `!cc:p' which would have printed the last command starting with `cc' with- out executing it. After this recompilation, we ran the resulting `a.out' file, and then noting that there still was a bug, ran the editor again. After fixing the program we ran the C com- piler again, but tacked onto the command an extra `-o bug' telling the compiler to place the resultant binary in the file `bug' rather than `a.out'. In general, the history mechanisms may be used anywhere in the formation of new com- mands and other characters may be placed before and after the substituted commands. We then ran the `size' command to see how large the binary program images we have created were, and then an `ls -l' command with the same argument list, denoting the argu- ment list `*'. Finally we ran the program `bug' to see that its output is indeed correct. To make a numbered listing of the program we ran the `num' command on the file `bug.c'. In order to compress out blank lines in the output of `num' we ran the output through the filter `ssp', but misspelled it as spp. To correct this we used a shell substitute, placing the old text and new text between `^' characters. This is similar to the substi- tute command in the editor. Finally, we repeated the same command with `!!', but sent its output to the line printer. An Introduction to the C shell USD:4-21 % cat bug.c main() { printf("hello); } % cc !$ cc bug.c "bug.c", line 4: newline in string or char constant "bug.c", line 5: syntax error % ed !$ ed bug.c 29 4s/);/"&/p printf("hello"); w 30 q % !c cc bug.c % a.out hello% !e ed bug.c 30 4s/lo/lo\\n/p printf("hello\n"); w 32 q % !c -o bug cc bug.c -o bug % size a.out bug a.out: 2784+364+1028 = 4176b = 0x1050b bug: 2784+364+1028 = 4176b = 0x1050b % ls -l !* ls -l a.out bug -rwxr-xr-x 1 bill 3932 Dec 19 09:41 a.out -rwxr-xr-x 1 bill 3932 Dec 19 09:42 bug % bug hello % num bug.c | spp spp: Command not found. % ^spp^ssp num bug.c | ssp 1 main() 3 { 4 printf("hello\n"); 5 } % !! | lpr num bug.c | ssp | lpr % USD:4-22 An Introduction to the C shell There are other mechanisms available for repeating com- mands. The _h_i_s_t_o_r_y command prints out a number of previous commands with numbers by which they can be referenced. There is a way to refer to a previous command by searching for a string which appeared in it, and there are other, less useful, ways to select arguments to include in a new com- mand. A complete description of all these mechanisms is given in the C shell manual pages in the UNIX Programmer's Manual. 22..44.. AAlliiaasseess The shell has an _a_l_i_a_s mechanism which can be used to make transformations on input commands. This mechanism can be used to simplify the commands you type, to supply default arguments to commands, or to perform transformations on com- mands and their arguments. The alias facility is similar to a macro facility. Some of the features obtained by aliasing can be obtained also using shell command files, but these take place in another instance of the shell and cannot directly affect the current shells environment or involve commands such as _c_d which must be done in the current shell. As an example, suppose that there is a new version of the mail program on the system called `newmail' you wish to use, rather than the standard mail program which is called `mail'. If you place the shell command alias mail newmail in your _._c_s_h_r_c file, the shell will transform an input line of the form mail bill into a call on `newmail'. More generally, suppose we wish the command `ls' to always show sizes of files, that is to always do `-s'. We can do alias ls ls -s or even alias dir ls -s creating a new command syntax `dir' which does an `ls -s'. If we say dir ~bill then the shell will translate this to ls -s /mnt/bill An Introduction to the C shell USD:4-23 Thus the _a_l_i_a_s mechanism can be used to provide short names for commands, to provide default arguments, and to define new short commands in terms of other commands. It is also possible to define aliases which contain multiple com- mands or pipelines, showing where the arguments to the orig- inal command are to be substituted using the facilities of the history mechanism. Thus the definition alias cd 'cd \!* ; ls ' would do an _l_s command after each change directory _c_d com- mand. We enclosed the entire alias definition in `'' char- acters to prevent most substitutions from occurring and the character `;' from being recognized as a metacharacter. The `!' here is escaped with a `\' to prevent it from being interpreted when the alias command is typed in. The `\!*' here substitutes the entire argument list to the pre-alias- ing _c_d command, without giving an error if there were no arguments. The `;' separating commands is used here to indicate that one command is to be done and then the next. Similarly the definition alias whois 'grep \!^ /etc/passwd' defines a command which looks up its first argument in the password file. WWaarrnniinngg:: The shell currently reads the _._c_s_h_r_c file each time it starts up. If you place a large number of commands there, shells will tend to start slowly. A mechanism for saving the shell environment after reading the _._c_s_h_r_c file and quickly restoring it is under development, but for now you should try to limit the number of aliases you have to a reasonable number... 10 or 15 is reasonable, 50 or 60 will cause a noticeable delay in starting up shells, and make the system seem sluggish when you execute commands from within the editor and other programs. 22..55.. MMoorree rreeddiirreeccttiioonn;; >>>> aanndd >>&& There are a few more notations useful to the terminal user which have not been introduced yet. In addition to the standard output, commands also have a _d_i_a_g_n_o_s_t_i_c _o_u_t_p_u_t which is normally directed to the termi- nal even when the standard output is redirected to a file or a pipe. It is occasionally desirable to direct the diagnos- tic output along with the standard output. For instance if you want to redirect the output of a long running command into a file and wish to have a record of any error diagnos- tic it produces you can do command >& file USD:4-24 An Introduction to the C shell The `>&' here tells the shell to route both the diagnostic output and the standard output into `file'. Similarly you can give the command command |& lpr to route both standard and diagnostic output through the pipe to the line printer daemon _l_p_r_._|_= Finally, it is possible to use the form command >> file to place output at the end of an existing file.|- 22..66.. JJoobbss;; BBaacckkggrroouunndd,, FFoorreeggrroouunndd,, oorr SSuussppeennddeedd When one or more commands are typed together as a pipeline or as a sequence of commands separated by semi- colons, a single _j_o_b is created by the shell consisting of these commands together as a unit. Single commands without pipes or semicolons create the simplest jobs. Usually, every line typed to the shell creates a job. Some lines that create jobs (one per line) are sort < data ls -s | sort -n | head -5 mail harold If the metacharacter `&' is typed at the end of the commands, then the job is started as a _b_a_c_k_g_r_o_u_n_d job. This means that the shell does not wait for it to complete but immediately prompts and is ready for another command. The job runs _i_n _t_h_e _b_a_c_k_g_r_o_u_n_d at the same time that normal jobs, called _f_o_r_e_g_r_o_u_n_d jobs, continue to be read and exe- cuted by the shell one at a time. Thus du > usage & would run the _d_u program, which reports on the disk usage of your working directory (as well as any directories below it), put the output into the file `usage' and return ----------- |= A command of the form command >&! file exists, and is used when _n_o_c_l_o_b_b_e_r is set and _f_i_l_e already exists. |- If _n_o_c_l_o_b_b_e_r is set, then an error will result if _f_i_l_e does not exist, otherwise the shell will create _f_i_l_e if it doesn't exist. A form command >>! file makes it not be an error for file to not exist when _n_o_c_l_o_b_b_e_r is set. An Introduction to the C shell USD:4-25 immediately with a prompt for the next command without out waiting for _d_u to finish. The _d_u program would continue executing in the background until it finished, even though you can type and execute more commands in the mean time. When a background job terminates, a message is typed by the shell just before the next prompt telling you that the job has completed. In the following example the _d_u job finishes sometime during the execution of the _m_a_i_l command and its completion is reported just before the prompt after the _m_a_i_l job is finished. % du > usage & [1] 503 % mail bill How do you know when a background job is finished? EOT [1] - Done du > usage % If the job did not terminate normally the `Done' message might say something else like `Killed'. If you want the terminations of background jobs to be reported at the time they occur (possibly interrupting the output of other fore- ground jobs), you can set the _n_o_t_i_f_y variable. In the pre- vious example this would mean that the `Done' message might have come right in the middle of the message to Bill. Back- ground jobs are unaffected by any signals from the keyboard like the STOP, INTERRUPT, or QUIT signals mentioned earlier. Jobs are recorded in a table inside the shell until they terminate. In this table, the shell remembers the com- mand names, arguments and the _p_r_o_c_e_s_s _n_u_m_b_e_r_s of all com- mands in the job as well as the working directory where the job was started. Each job in the table is either running _i_n _t_h_e _f_o_r_e_g_r_o_u_n_d with the shell waiting for it to terminate, running _i_n _t_h_e _b_a_c_k_g_r_o_u_n_d_, or _s_u_s_p_e_n_d_e_d_. Only one job can be running in the foreground at one time, but several jobs can be suspended or running in the background at once. As each job is started, it is assigned a small identifying num- ber called the _j_o_b _n_u_m_b_e_r which can be used later to refer to the job in the commands described below. Job numbers remain the same until the job terminates and then are re- used. When a job is started in the backgound using `&', its number, as well as the process numbers of all its (top level) commands, is typed by the shell before prompting you for another command. For example, % ls -s | sort -n > usage & [2] 2034 2035 % runs the `ls' program with the `-s' options, pipes this USD:4-26 An Introduction to the C shell output into the `sort' program with the `-n' option which puts its output into the file `usage'. Since the `&' was at the end of the line, these two programs were started together as a background job. After starting the job, the shell prints the job number in brackets (2 in this case) followed by the process number of each program started in the job. Then the shell immediates prompts for a new com- mand, leaving the job running simultaneously. As mentioned in section 1.8, foreground jobs become _s_u_s_p_e_n_d_e_d by typing ^Z which sends a STOP signal to the cur- rently running foreground job. A background job can become suspended by using the _s_t_o_p command described below. When jobs are suspended they merely stop any further progress until started again, either in the foreground or the back- gound. The shell notices when a job becomes stopped and reports this fact, much like it reports the termination of background jobs. For foreground jobs this looks like % du > usage ^Z Stopped % `Stopped' message is typed by the shell when it notices that the _d_u program stopped. For background jobs, using the _s_t_o_p command, it is % sort usage & [1] 2345 % stop %1 [1] + Stopped (signal)sort usage % Suspending foreground jobs can be very useful when you need to temporarily change what you are doing (execute other com- mands) and then return to the suspended job. Also, fore- ground jobs can be suspended and then continued as back- ground jobs using the _b_g command, allowing you to continue other work and stop waiting for the foreground job to fin- ish. Thus % du > usage ^Z Stopped % bg [1] du > usage & % starts `du' in the foreground, stops it before it finishes, then continues it in the background allowing more foreground commands to be executed. This is especially helpful when a foreground job ends up taking longer than you expected and you wish you had started it in the backgound in the An Introduction to the C shell USD:4-27 beginning. All _j_o_b _c_o_n_t_r_o_l commands can take an argument that identifies a particular job. All job name arguments begin with the character `%', since some of the job control com- mands also accept process numbers (printed by the _p_s com- mand.) The default job (when no argument is given) is called the _c_u_r_r_e_n_t job and is identified by a `+' in the output of the _j_o_b_s command, which shows you which jobs you have. When only one job is stopped or running in the back- ground (the usual case) it is always the current job thus no argument is needed. If a job is stopped while running in the foreground it becomes the _c_u_r_r_e_n_t job and the existing current job becomes the _p_r_e_v_i_o_u_s job - identified by a `-' in the output of _j_o_b_s_. When the current job terminates, the previous job becomes the current job. When given, the argu- ment is either `%-' (indicating the previous job); `%#', where # is the job number; `%pref' where pref is some unique prefix of the command name and arguments of one of the jobs; or `%?' followed by some string found in only one of the jobs. The _j_o_b_s command types the table of jobs, giving the job number, commands and status (`Stopped' or `Running') of each backgound or suspended job. With the `-l' option the process numbers are also typed. % du > usage & [1] 3398 % ls -s | sort -n > myfile & [2] 3405 % mail bill ^Z Stopped % jobs [1] - Running du > usage [2] Running ls -s | sort -n > myfile [3] + Stopped mail bill % fg %ls ls -s | sort -n > myfile % more myfile The _f_g command runs a suspended or background job in the foreground. It is used to restart a previously sus- pended job or change a background job to run in the fore- ground (allowing signals or input from the terminal). In the above example we used _f_g to change the `ls' job from the background to the foreground since we wanted to wait for it to finish before looking at its output file. The _b_g command runs a suspended job in the background. It is usually used after stopping the currently running foreground job with the STOP signal. The combination of the STOP signal and the _b_g command changes a foreground job into a background job. The USD:4-28 An Introduction to the C shell _s_t_o_p command suspends a background job. The _k_i_l_l command terminates a background or suspended job immediately. In addition to jobs, it may be given pro- cess numbers as arguments, as printed by _p_s_. Thus, in the example above, the running _d_u command could have been termi- nated by the command % kill %1 [1] Terminated du > usage % The _n_o_t_i_f_y command (not the variable mentioned earlier) indicates that the termination of a specific job should be reported at the time it finishes instead of waiting for the next prompt. If a job running in the background tries to read input from the terminal it is automatically stopped. When such a job is then run in the foreground, input can be given to the job. If desired, the job can be run in the background again until it requests input again. This is illustrated in the following sequence where the `s' command in the text editor might take a long time. % ed bigfile 120000 1,$s/thisword/thatword/ ^Z Stopped % bg [1] ed bigfile & % . . . some foreground commands [1] Stopped (tty input)ed bigfile % fg ed bigfile w 120000 q % So after the `s' command was issued, the `ed' job was stopped with ^Z and then put in the background using _b_g_. Some time later when the `s' command was finished, _e_d tried to read another command and was stopped because jobs in the backgound cannot read from the terminal. The _f_g command returned the `ed' job to the foreground where it could once again accept commands from the terminal. The command An Introduction to the C shell USD:4-29 stty tostop causes all background jobs run on your terminal to stop when they are about to write output to the terminal. This pre- vents messages from background jobs from interrupting fore- ground job output and allows you to run a job in the back- ground without losing terminal output. It also can be used for interactive programs that sometimes have long periods without interaction. Thus each time it outputs a prompt for more input it will stop before the prompt. It can then be run in the foreground using _f_g_, more input can be given and, if necessary stopped and returned to the background. This _s_t_t_y command might be a good thing to put in your _._l_o_g_i_n file if you do not like output from background jobs inter- rupting your work. It also can reduce the need for redi- recting the output of background jobs if the output is not very big: % stty tostop % wc hugefile & [1] 10387 % ed text . . . some time later q [1] Stopped (tty output)wc hugefile % fg wc wc hugefile 13371 30123 302577 % stty -tostop Thus after some time the `wc' command, which counts the lines, words and characters in a file, had one line of out- put. When it tried to write this to the terminal it stopped. By restarting it in the foreground we allowed it to write on the terminal exactly when we were ready to look at its output. Programs which attempt to change the mode of the terminal will also block, whether or not _t_o_s_t_o_p is set, when they are not in the foreground, as it would be very unpleasant to have a background job change the state of the terminal. Since the _j_o_b_s command only prints jobs started in the currently executing shell, it knows nothing about background jobs started in other login sessions or within shell files. The _p_s can be used in this case to find out about background jobs not started in the current shell. 22..77.. WWoorrkkiinngg DDiirreeccttoorriieess As mentioned in section 1.6, the shell is always in a particular _w_o_r_k_i_n_g _d_i_r_e_c_t_o_r_y_. The `change directory' com- mand _c_h_d_i_r (its short form _c_d may also be used) changes the working directory of the shell, that is, changes the direc- tory you are located in. USD:4-30 An Introduction to the C shell It is useful to make a directory for each project you wish to work on and to place all files related to that pro- ject in that directory. The `make directory' command, _m_k_d_i_r_, creates a new directory. The _p_w_d (`print working directory') command reports the absolute pathname of the working directory of the shell, that is, the directory you are located in. Thus in the example below: % pwd /usr/bill % mkdir newpaper % chdir newpaper % pwd /usr/bill/newpaper % the user has created and moved to the directory _n_e_w_p_a_p_e_r_. where, for example, he might place a group of related files. No matter where you have moved to in a directory hier- archy, you can return to your `home' login directory by doing just cd with no arguments. The name `..' always means the directory above the current one in the hierarchy, thus cd .. changes the shell's working directory to the one directly above the current one. The name `..' can be used in any pathname, thus, cd ../programs means change to the directory `programs' contained in the directory above the current one. If you have several direc- tories for different projects under, say, your home direc- tory, this shorthand notation permits you to switch easily between them. The shell always remembers the pathname of its current working directory in the variable _c_w_d_. The shell can also be requested to remember the previous directory when you change to a new working directory. If the `push directory' command _p_u_s_h_d is used in place of the _c_d command, the shell saves the name of the current working directory on a _d_i_r_e_c_- _t_o_r_y _s_t_a_c_k before changing to the new one. You can see this list at any time by typing the `directories' command _d_i_r_s_. % pushd newpaper/references ~/newpaper/references ~ % pushd /usr/lib/tmac An Introduction to the C shell USD:4-31 /usr/lib/tmac ~/newpaper/references ~ % dirs /usr/lib/tmac ~/newpaper/references ~ % popd ~/newpaper/references ~ % popd ~ % The list is printed in a horizontal line, reading left to right, with a tilde (~) as shorthand for your home direc- tory--in this case `/usr/bill'. The directory stack is printed whenever there is more than one entry on it and it changes. It is also printed by a _d_i_r_s command. _D_i_r_s is usually faster and more informative than _p_w_d since it shows the current working directory as well as any other directo- ries remembered in the stack. The _p_u_s_h_d command with no argument alternates the cur- rent directory with the first directory in the list. The `pop directory' _p_o_p_d command without an argument returns you to the directory you were in prior to the current one, dis- carding the previous current directory from the stack (for- getting it). Typing _p_o_p_d several times in a series takes you backward through the directories you had been in (changed to) by _p_u_s_h_d command. There are other options to _p_u_s_h_d and _p_o_p_d to manipulate the contents of the directory stack and to change to directories not at the top of the stack; see the _c_s_h manual page for details. Since the shell remembers the working directory in which each job was started, it warns you when you might be confused by restarting a job in the foreground which has a different working directory than the current working direc- tory of the shell. Thus if you start a background job, then change the shell's working directory and then cause the background job to run in the foreground, the shell warns you that the working directory of the currently running fore- ground job is different from that of the shell. USD:4-32 An Introduction to the C shell % dirs -l /mnt/bill % cd myproject % dirs ~/myproject % ed prog.c 1143 ^Z Stopped % cd .. % ls myproject textfile % fg ed prog.c (wd: ~/myproject) This way the shell warns you when there is an implied change of working directory, even though no cd command was issued. In the above example the `ed' job was still in `/mnt/bill/project' even though the shell had changed to `/mnt/bill'. A similar warning is given when such a fore- ground job terminates or is suspended (using the STOP sig- nal) since the return to the shell again implies a change of working directory. % fg ed prog.c (wd: ~/myproject) . . . after some editing q (wd now: ~) % These messages are sometimes confusing if you use programs that change their own working directories, since the shell only remembers which directory a job is started in, and assumes it stays there. The `-l' option of _j_o_b_s will type the working directory of suspended or background jobs when it is different from the current working directory of the shell. 22..88.. UUsseeffuull bbuuiilltt--iinn ccoommmmaannddss We now give a few of the useful built-in commands of the shell describing how they are used. The _a_l_i_a_s command described above is used to assign new aliases and to show the existing aliases. With no arguments it prints the current aliases. It may also be given only one argument such as alias ls to show the current alias for, e.g., `ls'. An Introduction to the C shell USD:4-33 The _e_c_h_o command prints its arguments. It is often used in _s_h_e_l_l _s_c_r_i_p_t_s or as an interactive command to see what filename expansions will produce. The _h_i_s_t_o_r_y command will show the contents of the his- tory list. The numbers given with the history events can be used to reference previous events which are difficult to reference using the contextual mechanisms introduced above. There is also a shell variable called _p_r_o_m_p_t_. By placing a `!' character in its value the shell will there substitute the number of the current command in the history list. You can use this number to refer to this command in a history substitution. Thus you could set prompt='\! % ' Note that the `!' character had to be _e_s_c_a_p_e_d here even within `'' characters. The _l_i_m_i_t command is used to restrict use of resources. With no arguments it prints the current limitations: cputime unlimited filesize unlimited datasize 5616 kbytes stacksize 512 kbytes coredumpsizeunlimited Limits can be set, e.g.: limit coredumpsize 128k Most reasonable units abbreviations will work; see the _c_s_h manual page for more details. The _l_o_g_o_u_t command can be used to terminate a login shell which has _i_g_n_o_r_e_e_o_f set. The _r_e_h_a_s_h command causes the shell to recompute a table of where commands are located. This is necessary if you add a command to a directory in the current shell's search path and wish the shell to find it, since otherwise the hashing algorithm may tell the shell that the command wasn't in that directory when the hash table was computed. The _r_e_p_e_a_t command can be used to repeat a command sev- eral times. Thus to make 5 copies of the file _o_n_e in the file _f_i_v_e you could do repeat 5 cat one >> five The _s_e_t_e_n_v command can be used to set variables in the environment. Thus USD:4-34 An Introduction to the C shell setenv TERM adm3a will set the value of the environment variable TERM to `adm3a'. A user program _p_r_i_n_t_e_n_v exists which will print out the environment. It might then show: % printenv HOME=/usr/bill SHELL=/bin/csh PATH=:/usr/ucb:/bin:/usr/bin:/usr/local TERM=adm3a USER=bill % The _s_o_u_r_c_e command can be used to force the current shell to read commands from a file. Thus source .cshrc can be used after editing in a change to the _._c_s_h_r_c file which you wish to take effect right away. The _t_i_m_e command can be used to cause a command to be timed no matter how much CPU time it takes. Thus % time cp /etc/rc /usr/bill/rc 0.0u 0.1s 0:01 8% 2+1k 3+2io 1pf+0w % time wc /etc/rc /usr/bill/rc 52 178 1347 /etc/rc 52 178 1347 /usr/bill/rc 104 356 2694 total 0.1u 0.1s 0:00 13% 3+3k 5+3io 7pf+0w % indicates that the _c_p command used a negligible amount of user time (u) and about 1/10th of a system time (s); the elapsed time was 1 second (0:01), there was an average mem- ory usage of 2k bytes of program space and 1k bytes of data space over the cpu time involved (2+1k); the program did three disk reads and two disk writes (3+2io), and took one page fault and was not swapped (1pf+0w). The word count command _w_c on the other hand used 0.1 seconds of user time and 0.1 seconds of system time in less than a second of elapsed time. The percentage `13%' indicates that over the period when it was active the command `wc' used an average of 13 percent of the available CPU cycles of the machine. The _u_n_a_l_i_a_s and _u_n_s_e_t commands can be used to remove aliases and variable definitions from the shell, and _u_n_s_e_t_e_n_v removes variables from the environment. An Introduction to the C shell USD:4-35 22..99.. WWhhaatt eellssee?? This concludes the basic discussion of the shell for terminal users. There are more features of the shell to be discussed here, and all features of the shell are discussed in its manual pages. One useful feature which is discussed later is the _f_o_r_e_a_c_h built-in command which can be used to run the same command sequence with a number of different arguments. If you intend to use UNIX a lot you should look through the rest of this document and the csh manual pages (sec- tion1) to become familiar with the other facilities which are available to you. USD:4-36 An Introduction to the C shell 33.. SShheellll ccoonnttrrooll ssttrruuccttuurreess aanndd ccoommmmaanndd ssccrriippttss 33..11.. IInnttrroodduuccttiioonn It is possible to place commands in files and to cause shells to be invoked to read and execute commands from these files, which are called _s_h_e_l_l _s_c_r_i_p_t_s_. We here detail those features of the shell useful to the writers of such scripts. 33..22.. MMaakkee It is important to first note what shell scripts are _n_o_t useful for. There is a program called _m_a_k_e which is very useful for maintaining a group of related files or per- forming sets of operations on related files. For instance a large program consisting of one or more files can have its dependencies described in a _m_a_k_e_f_i_l_e which contains defini- tions of the commands used to create these different files when changes occur. Definitions of the means for printing listings, cleaning up the directory in which the files reside, and installing the resultant programs are easily, and most appropriately placed in this _m_a_k_e_f_i_l_e_. This format is superior and preferable to maintaining a group of shell procedures to maintain these files. Similarly when working on a document a _m_a_k_e_f_i_l_e may be created which defines how different versions of the document are to be created and which options of _n_r_o_f_f or _t_r_o_f_f are appropriate. 33..33.. IInnvvooccaattiioonn aanndd tthhee aarrggvv vvaarriiaabbllee A _c_s_h command script may be interpreted by saying % csh script ... where _s_c_r_i_p_t is the name of the file containing a group of _c_s_h commands and `...' is replaced by a sequence of argu- ments. The shell places these arguments in the variable _a_r_g_v and then begins to read commands from the script. These parameters are then available through the same mecha- nisms which are used to reference any other shell variables. If you make the file `script' executable by doing chmod 755 script and place a shell comment at the beginning of the shell script (i.e. begin the file with a `#' character) then a `/bin/csh' will automatically be invoked to execute `script' when you type script An Introduction to the C shell USD:4-37 If the file does not begin with a `#' then the standard shell `/bin/sh' will be used to execute it. This allows you to convert your older shell scripts to use _c_s_h at your con- venience. 33..44.. VVaarriiaabbllee ssuubbssttiittuuttiioonn After each input line is broken into words and history substitutions are done on it, the input line is parsed into distinct commands. Before each command is executed a mecha- nism know as _v_a_r_i_a_b_l_e _s_u_b_s_t_i_t_u_t_i_o_n is done on these words. Keyed by the character `$' this substitution replaces the names of variables by their values. Thus echo $argv when placed in a command script would cause the current value of the variable _a_r_g_v to be echoed to the output of the shell script. It is an error for _a_r_g_v to be unset at this point. A number of notations are provided for accessing compo- nents and attributes of variables. The notation $?name expands to `1' if name is _s_e_t or to `0' if name is not _s_e_t_. It is the fundamental mechanism used for checking whether particular variables have been assigned values. All other forms of reference to undefined variables cause errors. The notation $#name expands to the number of elements in the variable _n_a_m_e_. Thus % set argv=(a b c) % echo $?argv 1 % echo $#argv 3 % unset argv % echo $?argv 0 % echo $argv Undefined variable: argv. % It is also possible to access the components of a vari- able which has several values. Thus USD:4-38 An Introduction to the C shell $argv[1] gives the first component of _a_r_g_v or in the example above `a'. Similarly $argv[$#argv] would give `c', and $argv[1-2] would give `a b'. Other notations useful in shell scripts are $_n where _n is an integer as a shorthand for $argv[_n] the _n_t_h parameter and $* which is a shorthand for $argv The form $$ expands to the process number of the current shell. Since this process number is unique in the system it can be used in generation of unique temporary file names. The form $< is quite special and is replaced by the next line of input read from the shell's standard input (not the script it is reading). This is useful for writing shell scripts that are interactive, reading commands from the terminal, or even writing a shell script that acts as a filter, reading lines from its input file. Thus the sequence echo 'yes or no?\c' set a=($<) would write out the prompt `yes or no?' without a newline and then read the answer into the variable `a'. In this case `$#a' would be `0' if either a blank line or end-of- file (^D) was typed. An Introduction to the C shell USD:4-39 One minor difference between `$_n' and `$argv[_n]' should be noted here. The form `$argv[_n]' will yield an error if _n is not in the range `1-$#argv' while `$n' will never yield an out of range subscript error. This is for compatibility with the way older shells handled parameters. Another important point is that it is never an error to give a subrange of the form `n-'; if there are less than _n components of the given variable then no words are substi- tuted. A range of the form `m-n' likewise returns an empty vector without giving an error when _m exceeds the number of elements of the given variable, provided the subscript _n is in range. 33..55.. EExxpprreessssiioonnss In order for interesting shell scripts to be con- structed it must be possible to evaluate expressions in the shell based on the values of variables. In fact, all the arithmetic operations of the language C are available in the shell with the same precedence that they have in C. In par- ticular, the operations `==' and `!=' compare strings and the operators `&&' and `||' implement the boolean and/or operations. The special operators `=~' and `!~' are similar to `==' and `!=' except that the string on the right side can have pattern matching characters (like *, ? or []) and the test is whether the string on the left matches the pat- tern on the right. The shell also allows file enquiries of the form -? filename where `?' is replace by a number of single characters. For instance the expression primitive -e filename tell whether the file `filename' exists. Other primitives test for read, write and execute access to the file, whether it is a directory, or has non-zero length. It is possible to test whether a command terminates normally, by a primitive of the form `{ command }' which returns true, i.e. `1' if the command succeeds exiting nor- mally with exit status 0, or `0' if the command terminates abnormally or with exit status non-zero. If more detailed information about the execution status of a command is required, it can be executed and the variable `$status' examined in the next command. Since `$status' is set by every command, it is very transient. It can be saved if it is inconvenient to use it only in the single immediately following command. USD:4-40 An Introduction to the C shell For a full list of expression components available see the manual section for the shell. 33..66.. SSaammppllee sshheellll ssccrriipptt A sample shell script which makes use of the expression mechanism of the shell and some of its control structure follows: % cat copyc # # Copyc copies those C programs in the specified list # to the directory ~/backup if they differ from the files # already in ~/backup # set noglob foreach i ($argv) if ($i !~ *.c) continue # not a .c file so do nothing if (! -r ~/backup/$i:t) then echo $i:t not in backup... not cp\'ed continue endif cmp -s $i ~/backup/$i:t # to set $status if ($status != 0) then echo new backup of $i cp $i ~/backup/$i:t endif end This script makes use of the _f_o_r_e_a_c_h command, which causes the shell to execute the commands between the _f_o_r_e_a_c_h and the matching _e_n_d for each of the values given between `(' and `)' with the named variable, in this case `i' set to successive values in the list. Within this loop we may use the command _b_r_e_a_k to stop executing the loop and _c_o_n_t_i_n_u_e to prematurely terminate one iteration and begin the next. After the _f_o_r_e_a_c_h loop the iteration variable (_i in this case) has the value at the last iteration. We set the variable _n_o_g_l_o_b here to prevent filename expansion of the members of _a_r_g_v_. This is a good idea, in general, if the arguments to a shell script are filenames which have already been expanded or if the arguments may contain filename expansion metacharacters. It is also pos- sible to quote each use of a `$' variable expansion, but this is harder and less reliable. The other control construct used here is a statement of the form An Introduction to the C shell USD:4-41 iiff ( expression ) tthheenn command ... eennddiiff The placement of the keywords here is nnoott flexible due to the current implementation of the shell.|- The shell does have another form of the if statement of the form iiff ( expression ) ccoommmmaanndd which can be written iiff ( expression ) \ command Here we have escaped the newline for the sake of appearance. The command must not involve `|', `&' or `;' and must not be another control command. The second form requires the final `\' to iimmmmeeddiiaatteellyy precede the end-of-line. The more general _i_f statements above also admit a sequence of _e_l_s_e_-_i_f pairs followed by a single _e_l_s_e and an _e_n_d_i_f_, e.g.: iiff ( expression ) tthheenn commands eellssee iiff (expression ) tthheenn commands ... eellssee commands eennddiiff Another important mechanism used in shell scripts is the `:' modifier. We can use the modifier `:r' here to extract a root of a filename or `:e' to extract the ----------- |-The following two formats are not currently acceptable to the shell: iiff ( expression ) # WWoonn''tt wwoorrkk!! tthheenn command ... eennddiiff and iiff ( expression ) tthheenn command eennddiiff # WWoonn''tt wwoorrkk USD:4-42 An Introduction to the C shell _e_x_t_e_n_s_i_o_n_. Thus if the variable _i has the value `/mnt/foo.bar' then % echo $i $i:r $i:e /mnt/foo.bar /mnt/foo bar % shows how the `:r' modifier strips off the trailing `.bar' and the the `:e' modifier leaves only the `bar'. Other mod- ifiers will take off the last component of a pathname leav- ing the head `:h' or all but the last component of a path- name leaving the tail `:t'. These modifiers are fully described in the _c_s_h manual pages in the User's Reference Manual. It is also possible to use the _c_o_m_m_a_n_d _s_u_b_s_t_i_t_u_t_i_o_n mechanism described in the next major section to perform modifications on strings to then reenter the shell's envi- ronment. Since each usage of this mechanism involves the creation of a new process, it is much more expensive to use than the `:' modification mechanism.|= Finally, we note that the character `#' lexically introduces a shell comment in shell scripts (but not from the terminal). All subsequent characters on the input line after a `#' are discarded by the shell. This character can be quoted using `'' or `\' to place it in an argument word. 33..77.. OOtthheerr ccoonnttrrooll ssttrruuccttuurreess The shell also has control structures _w_h_i_l_e and _s_w_i_t_c_h similar to those of C. These take the forms wwhhiillee ( expression ) commands eenndd and ----------- |= It is also important to note that the current implementation of the shell limits the number of `:' modifiers on a `$' substitution to 1. Thus % echo $i $i:h:t /a/b/c /a/b:t % does not do what one would expect. An Introduction to the C shell USD:4-43 sswwiittcchh ( word ) ccaassee str1: commands bbrreeaakkssww ... ccaassee strn: commands bbrreeaakkssww ddeeffaauulltt:: commands bbrreeaakkssww eennddssww For details see the manual section for _c_s_h_. C programmers should note that we use _b_r_e_a_k_s_w to exit from a _s_w_i_t_c_h while _b_r_e_a_k exits a _w_h_i_l_e or _f_o_r_e_a_c_h loop. A common mistake to make in _c_s_h scripts is to use _b_r_e_a_k rather than _b_r_e_a_k_s_w in switches. Finally, _c_s_h allows a _g_o_t_o statement, with labels look- ing like they do in C, i.e.: loop: commands ggoottoo loop 33..88.. SSuuppppllyyiinngg iinnppuutt ttoo ccoommmmaannddss Commands run from shell scripts receive by default the standard input of the shell which is running the script. This is different from previous shells running under UNIX. It allows shell scripts to fully participate in pipelines, but mandates extra notation for commands which are to take inline data. Thus we need a metanotation for supplying inline data to commands in shell scripts. As an example, consider this script which runs the editor to delete leading blanks from the lines in each argument file: USD:4-44 An Introduction to the C shell % cat deblank # deblank -- remove leading blanks foreach i ($argv) ed - $i << 'EOF' 1,$s/^[ ]*// w q 'EOF' end % The notation `<< 'EOF'' means that the standard input for the _e_d command is to come from the text in the shell script file up to the next line consisting of exactly `'EOF''. The fact that the `EOF' is enclosed in `'' characters, i.e. quoted, causes the shell to not perform variable substitu- tion on the intervening lines. In general, if any part of the word following the `<<' which the shell uses to termi- nate the text to be given to the command is quoted then these substitutions will not be performed. In this case since we used the form `1,$' in our editor script we needed to insure that this `$' was not variable substituted. We could also have insured this by preceding the `$' here with a `\', i.e.: 1,\$s/^[ ]*// but quoting the `EOF' terminator is a more reliable way of achieving the same thing. 33..99.. CCaattcchhiinngg iinntteerrrruuppttss If our shell script creates temporary files, we may wish to catch interruptions of the shell script so that we can clean up these files. We can then do onintr label where _l_a_b_e_l is a label in our program. If an interrupt is received the shell will do a `goto label' and we can remove the temporary files and then do an _e_x_i_t command (which is built in to the shell) to exit from the shell script. If we wish to exit with a non-zero status we can do exit(1) e.g. to exit with status `1'. 33..1100.. WWhhaatt eellssee?? There are other features of the shell useful to writers of shell procedures. The _v_e_r_b_o_s_e and _e_c_h_o options and the related _-_v and _-_x command line options can be used to help trace the actions of the shell. The _-_n option causes the An Introduction to the C shell USD:4-45 shell only to read commands and not to execute them and may sometimes be of use. One other thing to note is that _c_s_h will not execute shell scripts which do not begin with the character `#', that is shell scripts that do not begin with a comment. Similarly, the `/bin/sh' on your system may well defer to `csh' to interpret shell scripts which begin with `#'. This allows shell scripts for both shells to live in harmony. There is also another quotation mechanism using `"' which allows only some of the expansion mechanisms we have so far discussed to occur on the quoted string and serves to make this string into a single word as `'' does. USD:4-46 An Introduction to the C shell 44.. OOtthheerr,, lleessss ccoommmmoonnllyy uusseedd,, sshheellll ffeeaattuurreess 44..11.. LLooooppss aatt tthhee tteerrmmiinnaall;; vvaarriiaabblleess aass vveeccttoorrss It is occasionally useful to use the _f_o_r_e_a_c_h control structure at the terminal to aid in performing a number of similar commands. For instance, there were at one point three shells in use on the Cory UNIX system at Cory Hall, `/bin/sh', `/bin/nsh', and `/bin/csh'. To count the number of persons using each shell one could have issued the com- mands % grep -c csh$ /etc/passwd 27 % grep -c nsh$ /etc/passwd 128 % grep -c -v sh$ /etc/passwd 430 % Since these commands are very similar we can use _f_o_r_e_a_c_h to do this more easily. % foreach i ('sh$' 'csh$' '-v sh$') ? grep -c $i /etc/passwd ? end 27 128 430 % Note here that the shell prompts for input with `? ' when reading the body of the loop. Very useful with loops are variables which contain lists of filenames or other words. You can, for example, do % set a=(`ls`) % echo $a csh.n csh.rm % ls csh.n csh.rm % echo $#a 2 % The _s_e_t command here gave the variable _a a list of all the filenames in the current directory as value. We can then iterate over these names to perform any chosen function. The output of a command within ``' characters is con- verted by the shell to a list of words. You can also place the ``' quoted string within `"' characters to take each An Introduction to the C shell USD:4-47 (non-empty) line as a component of the variable; preventing the lines from being split into words at blanks and tabs. A modifier `:x' exists which can be used later to expand each component of the variable into another variable splitting it into separate words at embedded blanks and tabs. 44..22.. BBrraacceess {{ ...... }} iinn aarrgguummeenntt eexxppaannssiioonn Another form of filename expansion, alluded to before involves the characters `{' and `}'. These characters spec- ify that the contained strings, separated by `,' are to be consecutively substituted into the containing characters and the results expanded left to right. Thus A{str1,str2,...strn}B expands to Astr1B Astr2B ... AstrnB This expansion occurs before the other filename expansions, and may be applied recursively (i.e. nested). The results of each expanded string are sorted separately, left to right order being preserved. The resulting filenames are not required to exist if no other expansion mechanisms are used. This means that this mechanism can be used to generate argu- ments which are not filenames, but which have common parts. A typical use of this would be mkdir ~/{hdrs,retrofit,csh} to make subdirectories `hdrs', `retrofit' and `csh' in your home directory. This mechanism is most useful when the com- mon prefix is longer than in this example, i.e. chown root /usr/{ucb/{ex,edit},lib/{ex?.?*,how_ex}} 44..33.. CCoommmmaanndd ssuubbssttiittuuttiioonn A command enclosed in ``' characters is replaced, just before filenames are expanded, by the output from that com- mand. Thus it is possible to do set pwd=`pwd` to save the current directory in the variable _p_w_d or to do ex `grep -l TRACE *.c` to run the editor _e_x supplying as arguments those files whose names end in `.c' which have the string `TRACE' in USD:4-48 An Introduction to the C shell them.* 44..44.. OOtthheerr ddeettaaiillss nnoott ccoovveerreedd hheerree In particular circumstances it may be necessary to know the exact nature and order of different substitutions per- formed by the shell. The exact meaning of certain combina- tions of quotations is also occasionally important. These are detailed fully in its manual section. The shell has a number of command line option flags mostly of use in writing UNIX programs, and debugging shell scripts. See the csh(1) manual section for a list of these options. ----------- *Command expansion also occurs in input redirected with `<<' and within `"' quotations. Refer to the shell manual section for full details. An Introduction to the C shell USD:4-49 AAppppeennddiixx -- SSppeecciiaall cchhaarraacctteerrss The following table lists the special characters of _c_s_h and the UNIX system, giving for each the section(s) in which it is discussed. A number of these characters also have spe- cial meaning in expressions. See the _c_s_h manual section for a complete list. Syntactic metacharacters ; 2.4 separates commands to be executed sequentially | 1.5 separates commands in a pipeline ( ) 2.2,3.6 brackets expressions and variable values & 2.5 follows commands to be executed without waiting for completion Filename metacharacters / 1.6 separates components of a file's pathname ? 1.6 expansion character matching any single character * 1.6 expansion character matching any sequence of characters [ ] 1.6 expansion sequence matching any single character from a set ~ 1.6 used at the beginning of a filename to indicate home directories { } 4.2 used to specify groups of arguments with common parts Quotation metacharacters \ 1.7 prevents meta-meaning of following single character ' 1.7 prevents meta-meaning of a group of characters " 4.3 like ', but allows variable and command expansion Input/output metacharacters < 1.5 indicates redirected input > 1.3 indicates redirected output Expansion/substitution metacharacters $ 3.4 indicates variable substitution ! 2.3 indicates history substitution : 3.6 precedes substitution modifiers ^ 2.3 used in special forms of history substitution ` 4.3 indicates command substitution Other metacharacters # 1.3,3.6 begins scratch file names; indicates shell comments - 1.2 prefixes option (flag) arguments to commands % 2.6 prefixes job name specifications USD:4-50 An Introduction to the C shell GGlloossssaarryy This glossary lists the most important terms introduced in the introduction to the shell and gives references to sections of the shell document for further information about them. References of the form `pr (1)' indicate that the command _p_r is in the UNIX User Reference manual in section 1. You can look at an online copy of its manual page by doing man 1 pr References of the form (2.5) indicate that more information can be found in section 2.5 of this manual. .. Your current directory has the name `.' as well as the name printed by the command _p_w_d_; see also _d_i_r_s_. The current directory `.' is usually the first _c_o_m_p_o_n_e_n_t of the search path contained in the variable _p_a_t_h, thus commands which are in `.' are found first (2.2). The character `.' is also used in separating _c_o_m_p_o_n_e_n_t_s of filenames (1.6). The character `.' at the beginning of a _c_o_m_- _p_o_n_e_n_t of a _p_a_t_h_n_a_m_e is treated specially and not matched by the _f_i_l_e_n_a_m_e _e_x_p_a_n_s_i_o_n metacharacters `?', `*', and `[' `]' pairs (1.6). .... Each directory has a file `..' in it which is a reference to its parent directory. After changing into the directory with _c_h_d_i_r, i.e. chdir paper you can return to the parent directory by doing chdir .. The current directory is printed by _p_w_d (2.7). a.out Compilers which create executable images cre- ate them, by default, in the file _a_._o_u_t_. for historical reasons (2.3). absolute pathname A _p_a_t_h_n_a_m_e which begins with a `/' is _a_b_s_o_- _l_u_t_e since it specifies the _p_a_t_h of directo- ries from the beginning of the entire direc- tory system - called the _r_o_o_t directory. _P_a_t_h_n_a_m_es which are not _a_b_s_o_l_u_t_e are called _r_e_l_a_t_i_v_e (see definition of _r_e_l_a_t_i_v_e An Introduction to the C shell USD:4-51 _p_a_t_h_n_a_m_e) (1.6). alias An _a_l_i_a_s specifies a shorter or different name for a UNIX command, or a transformation on a command to be performed in the shell. The shell has a command _a_l_i_a_s which estab- lishes _a_l_i_a_s_e_s and can print their current values. The command _u_n_a_l_i_a_s is used to remove _a_l_i_a_s_e_s (2.4). argument Commands in UNIX receive a list of _a_r_g_u_m_e_n_t words. Thus the command echo a b c consists of the _c_o_m_m_a_n_d _n_a_m_e `echo' and three _a_r_g_u_m_e_n_t words `a', `b' and `c'. The set of _a_r_g_u_m_e_n_t_s after the _c_o_m_m_a_n_d _n_a_m_e is said to be the _a_r_g_u_m_e_n_t _l_i_s_t of the command (1.1). argv The list of arguments to a command written in the shell language (a shell script or shell procedure) is stored in a variable called _a_r_g_v within the shell. This name is taken from the conventional name in the C program- ming language (3.4). background Commands started without waiting for them to complete are called _b_a_c_k_g_r_o_u_n_d commands (2.6). base A filename is sometimes thought of as con- sisting of a _b_a_s_e part, before any `.' char- acter, and an _e_x_t_e_n_s_i_o_n - the part after the `.'. See _f_i_l_e_n_a_m_e and _e_x_t_e_n_s_i_o_n (1.6) and basename (1). bg The _b_g command causes a _s_u_s_p_e_n_d_e_d job to con- tinue execution in the _b_a_c_k_g_r_o_u_n_d (2.6). bin A directory containing binaries of programs and shell scripts to be executed is typically called a _b_i_n directory. The standard system _b_i_n directories are `/bin' containing the most heavily used commands and `/usr/bin' which contains most other user programs. Programs developed at UC Berkeley live in `/usr/ucb', while locally written programs live in `/usr/local'. Games are kept in the directory `/usr/games'. You can place bina- ries in any directory. If you wish to exe- cute them often, the name of the directories should be a _c_o_m_p_o_n_e_n_t of the variable _p_a_t_h. USD:4-52 An Introduction to the C shell break _B_r_e_a_k is a builtin command used to exit from loops within the control structure of the shell (3.7). breaksw The _b_r_e_a_k_s_w builtin command is used to exit from a _s_w_i_t_c_h control structure, like a _b_r_e_a_k exits from loops (3.7). builtin A command executed directly by the shell is called a _b_u_i_l_t_i_n command. Most commands in UNIX are not built into the shell, but rather exist as files in _b_i_n directories. These commands are accessible because the directo- ries in which they reside are named in the _p_a_t_h variable. case A _c_a_s_e command is used as a label in a _s_w_i_t_c_h statement in the shell's control structure, similar to that of the language C. Details are given in the shell documentation `csh (1)' (3.7). cat The _c_a_t program catenates a list of specified files on the _s_t_a_n_d_a_r_d _o_u_t_p_u_t. It is usually used to look at the contents of a single file on the terminal, to `cat a file' (1.8, 2.3). cd The _c_d command is used to change the _w_o_r_k_i_n_g _d_i_r_e_c_t_o_r_y. With no arguments, _c_d changes your _w_o_r_k_i_n_g _d_i_r_e_c_t_o_r_y to be your _h_o_m_e direc- tory (2.4, 2.7). chdir The _c_h_d_i_r command is a synonym for _c_d. _C_d is usually used because it is easier to type. chsh The _c_h_s_h command is used to change the shell which you use on UNIX. By default, you use an different version of the shell which resides in `/bin/sh'. You can change your shell to `/bin/csh' by doing chsh your-login-name /bin/csh Thus I would do chsh bill /bin/csh It is only necessary to do this once. The next time you log in to UNIX after doing this command, you will be using _c_s_h rather than the shell in `/bin/sh' (1.9). cmp _C_m_p is a program which compares files. It is usually used on binary files, or to see if An Introduction to the C shell USD:4-53 two files are identical (3.6). For comparing text files the program _d_i_f_f, described in `diff (1)' is used. command A function performed by the system, either by the shell (a builtin _c_o_m_m_a_n_d) or by a program residing in a file in a directory within the UNIX system, is called a _c_o_m_m_a_n_d (1.1). command name When a command is issued, it consists of a _c_o_m_m_a_n_d _n_a_m_e, which is the first word of the command, followed by arguments. The conven- tion on UNIX is that the first word of a com- mand names the function to be performed (1.1). command substitution The replacement of a command enclosed in ``' characters by the text output by that command is called _c_o_m_m_a_n_d _s_u_b_s_t_i_t_u_t_i_o_n (4.3). component A part of a _p_a_t_h_n_a_m_e between `/' characters is called a _c_o_m_p_o_n_e_n_t of that _p_a_t_h_n_a_m_e. A variable which has multiple strings as value is said to have several _c_o_m_p_o_n_e_n_ts; each string is a _c_o_m_p_o_n_e_n_t of the variable. continue A builtin command which causes execution of the enclosing _f_o_r_e_a_c_h or _w_h_i_l_e loop to cycle prematurely. Similar to the _c_o_n_t_i_n_u_e command in the programming language C (3.6). control- Certain special characters, called _c_o_n_t_r_o_l characters, are produced by holding down the CONTROL key on your terminal and simultane- ously pressing another character, much like the SHIFT key is used to produce upper case characters. Thus _c_o_n_t_r_o_l_-c is produced by holding down the CONTROL key while pressing the `c' key. Usually UNIX prints an caret (^) followed by the corresponding letter when you type a _c_o_n_t_r_o_l character (e.g. `^C' for _c_o_n_t_r_o_l_-c (1.8). core dump When a program terminates abnormally, the system places an image of its current state in a file named `core'. This _c_o_r_e _d_u_m_p can be examined with the system debugger `adb (1)' or `sdb (1)' in order to determine what went wrong with the program (1.8). If the shell produces a message of the form USD:4-54 An Introduction to the C shell Illegal instruction (core dumped) (where `Illegal instruction' is only one of several possible messages), you should report this to the author of the program or a system administrator, saving the `core' file. cp The _c_p (copy) program is used to copy the contents of one file into another file. It is one of the most commonly used UNIX com- mands (1.6). csh The name of the shell program that this docu- ment describes. .cshrc The file _._c_s_h_r_c in your _h_o_m_e directory is read by each shell as it begins execution. It is usually used to change the setting of the variable _p_a_t_h and to set _a_l_i_a_s parameters which are to take effect globally (2.1). cwd The _c_w_d variable in the shell holds the _a_b_s_o_- _l_u_t_e _p_a_t_h_n_a_m_e of the current _w_o_r_k_i_n_g _d_i_r_e_c_- _t_o_r_y. It is changed by the shell whenever your current _w_o_r_k_i_n_g _d_i_r_e_c_t_o_r_y changes and should not be changed otherwise (2.2). date The _d_a_t_e command prints the current date and time (1.3). debugging _D_e_b_u_g_g_i_n_g is the process of correcting mis- takes in programs and shell scripts. The shell has several options and variables which may be used to aid in shell _d_e_b_u_g_g_i_n_g (4.4). default: The label _d_e_f_a_u_l_t_: is used within shell _s_w_i_t_c_h statements, as it is in the C language to label the code to be executed if none of the _c_a_s_e labels matches the value switched on (3.7). DELETE The DELETE or RUBOUT key on the terminal nor- mally causes an interrupt to be sent to the current job. Many users change the interrupt character to be ^C. detached A command that continues running in the _b_a_c_k_- _g_r_o_u_n_d after you logout is said to be _d_e_t_a_c_h_e_d. diagnostic An error message produced by a program is often referred to as a _d_i_a_g_n_o_s_t_i_c. Most error messages are not written to the _s_t_a_n_- _d_a_r_d _o_u_t_p_u_t, since that is often directed An Introduction to the C shell USD:4-55 away from the terminal (1.3, 1.5). Error messsages are instead written to the _d_i_a_g_n_o_s_- _t_i_c _o_u_t_p_u_t which may be directed away from the terminal, but usually is not. Thus _d_i_a_g_- _n_o_s_t_i_c_s will usually appear on the terminal (2.5). directory A structure which contains files. At any time you are in one particular _d_i_r_e_c_t_o_r_y whose names can be printed by the command _p_w_d. The _c_h_d_i_r command will change you to another _d_i_r_e_c_t_o_r_y, and make the files in that _d_i_r_e_c_t_o_r_y visible. The _d_i_r_e_c_t_o_r_y in which you are when you first login is your _h_o_m_e direc- tory (1.1, 2.7). directory stack The shell saves the names of previous _w_o_r_k_i_n_g _d_i_r_e_c_t_o_r_i_e_s in the _d_i_r_e_c_t_o_r_y _s_t_a_c_k when you change your current _w_o_r_k_i_n_g _d_i_r_e_c_t_o_r_y via the _p_u_s_h_d command. The _d_i_r_e_c_t_o_r_y _s_t_a_c_k can be printed by using the _d_i_r_s command, which includes your current _w_o_r_k_i_n_g _d_i_r_e_c_t_o_r_y as the first directory name on the left (2.7). dirs The _d_i_r_s command prints the shell's _d_i_r_e_c_t_o_r_y _s_t_a_c_k (2.7). du The _d_u command is a program (described in `du (1)') which prints the number of disk blocks is all directories below and including your current _w_o_r_k_i_n_g _d_i_r_e_c_t_o_r_y (2.6). echo The _e_c_h_o command prints its arguments (1.6, 3.6). else The _e_l_s_e command is part of the `if-then- else-endif' control command construct (3.6). endif If an _i_f statement is ended with the word _t_h_e_n, all lines following the _i_f up to a line starting with the word _e_n_d_i_f or _e_l_s_e are exe- cuted if the condition between parentheses after the _i_f is true (3.6). EOF An _e_n_d-_o_f-_f_i_l_e is generated by the terminal by a control-d, and whenever a command reads to the end of a file which it has been given as input. Commands receiving input from a _p_i_p_e receive an _e_n_d-_o_f-_f_i_l_e when the command sending them input completes. Most commands terminate when they receive an _e_n_d-_o_f-_f_i_l_e. The shell has an option to ignore _e_n_d-_o_f-_f_i_l_e from a terminal input which may help you keep USD:4-56 An Introduction to the C shell from logging out accidentally by typing too many control-d's (1.1, 1.8, 3.8). escape A character `\' used to prevent the special meaning of a metacharacter is said to _e_s_c_a_p_e the character from its special meaning. Thus echo \* will echo the character `*' while just echo * will echo the names of the file in the cur- rent directory. In this example, \ _e_s_c_a_p_es `*' (1.7). There is also a non-printing character called _e_s_c_a_p_e, usually labelled ESC or ALTMODE on terminal keyboards. Some older UNIX systems use this character to indicate that output is to be _s_u_s_p_e_n_d_e_d. Most systems use control-s to stop the output and control- q to start it. /etc/passwd This file contains information about the accounts currently on the system. It con- sists of a line for each account with fields separated by `:' characters (1.8). You can look at this file by saying cat /etc/passwd The commands _f_i_n_g_e_r and _g_r_e_p are often used to search for information in this file. See `finger (1)', `passwd(5)', and `grep (1)' for more details. exit The _e_x_i_t command is used to force termination of a shell script, and is built into the shell (3.9). exit status A command which discovers a problem may reflect this back to the command (such as a shell) which invoked (executed) it. It does this by returning a non-zero number as its _e_x_i_t _s_t_a_t_u_s, a status of zero being consid- ered `normal termination'. The _e_x_i_t command can be used to force a shell command script to give a non-zero _e_x_i_t _s_t_a_t_u_s (3.6). expansion The replacement of strings in the shell input which contain metacharacters by other strings is referred to as the process of _e_x_p_a_n_s_i_o_n. Thus the replacement of the word `*' by a sorted list of files in the current directory An Introduction to the C shell USD:4-57 is a `filename expansion'. Similarly the replacement of the characters `!!' by the text of the last command is a `history expan- sion'. _E_x_p_a_n_s_i_o_n_s are also referred to as _s_u_b_s_t_i_t_u_t_i_o_n_s (1.6, 3.4, 4.2). expressions _E_x_p_r_e_s_s_i_o_n_s are used in the shell to control the conditional structures used in the writ- ing of shell scripts and in calculating val- ues for these scripts. The operators avail- able in shell _e_x_p_r_e_s_s_i_o_n_s are those of the language C (3.5). extension Filenames often consist of a _b_a_s_e name and an _e_x_t_e_n_s_i_o_n separated by the character `.'. By convention, groups of related files often share the same _r_o_o_t name. Thus if `prog.c' were a C program, then the object file for this program would be stored in `prog.o'. Similarly a paper written with the `-me' nroff macro package might be stored in `paper.me' while a formatted version of this paper might be kept in `paper.out' and a list of spelling errors in `paper.errs' (1.6). fg The _j_o_b _c_o_n_t_r_o_l command _f_g is used to run a _b_a_c_k_g_r_o_u_n_d or _s_u_s_p_e_n_d_e_d job in the _f_o_r_e_g_r_o_u_n_d (1.8, 2.6). filename Each file in UNIX has a name consisting of up to 14 characters and not including the char- acter `/' which is used in _p_a_t_h_n_a_m_e building. Most _f_i_l_e_n_a_m_e_s do not begin with the charac- ter `.', and contain only letters and digits with perhaps a `.' separating the _b_a_s_e por- tion of the _f_i_l_e_n_a_m_e from an _e_x_t_e_n_s_i_o_n (1.6). filename expansion _F_i_l_e_n_a_m_e _e_x_p_a_n_s_i_o_n uses the metacharacters `*', `?' and `[' and `]' to provide a conve- nient mechanism for naming files. Using _f_i_l_e_n_a_m_e _e_x_p_a_n_s_i_o_n it is easy to name all the files in the current directory, or all files which have a common _r_o_o_t name. Other _f_i_l_e_n_a_m_e _e_x_p_a_n_s_i_o_n mechanisms use the metacharacter `~' and allow files in other users' directo- ries to be named easily (1.6, 4.2). flag Many UNIX commands accept arguments which are not the names of files or other users but are used to modify the action of the commands. These are referred to as _f_l_a_g options, and by convention consist of one or more letters preceded by the character `-' (1.2). Thus USD:4-58 An Introduction to the C shell the _l_s (list files) command has an option `-s' to list the sizes of files. This is specified ls -s foreach The _f_o_r_e_a_c_h command is used in shell scripts and at the terminal to specify repetition of a sequence of commands while the value of a certain shell variable ranges through a spec- ified list (3.6, 4.1). foreground When commands are executing in the normal way such that the shell is waiting for them to finish before prompting for another command they are said to be _f_o_r_e_g_r_o_u_n_d _j_o_b_s or _r_u_n_- _n_i_n_g _i_n _t_h_e _f_o_r_e_g_r_o_u_n_d. This is as opposed to _b_a_c_k_g_r_o_u_n_d. _F_o_r_e_g_r_o_u_n_d jobs can be stopped by signals from the terminal caused by typing different control characters at the keyboard (1.8, 2.6). goto The shell has a command _g_o_t_o used in shell scripts to transfer control to a given label (3.7). grep The _g_r_e_p command searches through a list of argument files for a specified string. Thus grep bill /etc/passwd will print each line in the file _/_e_t_c_/_p_a_s_s_w_d which contains the string `bill'. Actually, _g_r_e_p scans for _r_e_g_u_l_a_r _e_x_p_r_e_s_s_i_o_n_s in the sense of the editors `ed (1)' and `ex (1)'. _G_r_e_p stands for `globally find _r_e_g_u_l_a_r _e_x_p_r_e_s_s_i_o_n and print' (2.4). head The _h_e_a_d command prints the first few lines of one or more files. If you have a bunch of files containing text which you are wondering about it is sometimes useful to run _h_e_a_d with these files as arguments. This will usually show enough of what is in these files to let you decide which you are interested in (1.5). _H_e_a_d is also used to describe the part of a _p_a_t_h_n_a_m_e before and including the last `/' character. The _t_a_i_l of a _p_a_t_h_n_a_m_e is the part after the last `/'. The `:h' and `:t' modifiers allow the _h_e_a_d or _t_a_i_l of a _p_a_t_h_- _n_a_m_e stored in a shell variable to be used (3.6). An Introduction to the C shell USD:4-59 history The _h_i_s_t_o_r_y mechanism of the shell allows previous commands to be repeated, possibly after modification to correct typing mistakes or to change the meaning of the command. The shell has a _h_i_s_t_o_r_y _l_i_s_t where these commands are kept, and a _h_i_s_t_o_r_y variable which con- trols how large this list is (2.3). home directory Each user has a _h_o_m_e _d_i_r_e_c_t_o_r_y, which is given in your entry in the password file, _/_e_t_c_/_p_a_s_s_w_d. This is the directory which you are placed in when you first login. The _c_d or _c_h_d_i_r command with no arguments takes you back to this directory, whose name is recorded in the shell variable _h_o_m_e. You can also access the _h_o_m_e _d_i_r_e_c_t_o_r_i_e_s of other users in forming filenames using a _f_i_l_e_n_a_m_e _e_x_p_a_n_s_i_o_n notation and the character `~' (1.6). if A conditional command within the shell, the _i_f command is used in shell command scripts to make decisions about what course of action to take next (3.6). ignoreeof Normally, your shell will exit, printing `logout' if you type a control-d at a prompt of `% '. This is the way you usually log off the system. You can _s_e_t the _i_g_n_o_r_e_e_o_f vari- able if you wish in your _._l_o_g_i_n file and then use the command _l_o_g_o_u_t to logout. This is useful if you sometimes accidentally type too many control-d characters, logging yourself off (2.2). input Many commands on UNIX take information from the terminal or from files which they then act on. This information is called _i_n_p_u_t. Commands normally read for _i_n_p_u_t from their _s_t_a_n_d_a_r_d _i_n_p_u_t which is, by default, the ter- minal. This _s_t_a_n_d_a_r_d _i_n_p_u_t can be redirected from a file using a shell metanotation with the character `<'. Many commands will also read from a file specified as argument. Com- mands placed in _p_i_p_e_l_i_n_e_s will read from the output of the previous command in the _p_i_p_e_l_i_n_e. The leftmost command in a _p_i_p_e_l_i_n_e reads from the terminal if you neither redi- rect its _i_n_p_u_t nor give it a filename to use as _s_t_a_n_d_a_r_d _i_n_p_u_t. Special mechanisms exist for supplying input to commands in shell scripts (1.5, 3.8). USD:4-60 An Introduction to the C shell interrupt An _i_n_t_e_r_r_u_p_t is a signal to a program that is generated by typing ^C. (On older versions of UNIX the RUBOUT or DELETE key were used for this purpose.) It causes most programs to stop execution. Certain programs, such as the shell and the editors, handle an _i_n_t_e_r_- _r_u_p_t in special ways, usually by stopping what they are doing and prompting for another command. While the shell is executing another command and waiting for it to finish, the shell does not listen to _i_n_t_e_r_r_u_p_t_s_. The shell often wakes up when you hit _i_n_t_e_r_r_u_p_t because many commands die when they receive an _i_n_t_e_r_r_u_p_t (1.8, 3.9). job One or more commands typed on the same input line separated by `|' or `;' characters are run together and are called a _j_o_b. Simple commands run by themselves without any `|' or `;' characters are the simplest _j_o_b_s_. _J_o_b_s are classified as _f_o_r_e_g_r_o_u_n_d, _b_a_c_k_g_r_o_u_n_d, or _s_u_s_p_e_n_d_e_d (2.6). job control The builtin functions that control the execu- tion of jobs are called _j_o_b _c_o_n_t_r_o_l commands. These are _b_g_, _f_g_, _s_t_o_p_, _k_i_l_l (2.6). job number When each job is started it is assigned a small number called a _j_o_b _n_u_m_b_e_r which is printed next to the job in the output of the _j_o_b_s command. This number, preceded by a `%' character, can be used as an argument to _j_o_b _c_o_n_t_r_o_l commands to indicate a specific job (2.6). jobs The _j_o_b_s command prints a table showing jobs that are either running in the _b_a_c_k_g_r_o_u_n_d or are _s_u_s_p_e_n_d_e_d (2.6). kill A command which sends a signal to a job caus- ing it to terminate (2.6). .login The file _._l_o_g_i_n in your _h_o_m_e directory is read by the shell each time you login to UNIX and the commands there are executed. There are a number of commands which are usefully placed here, especially _s_e_t commands to the shell itself (2.1). login shell The shell that is started on your terminal when you login is called your _l_o_g_i_n _s_h_e_l_l. It is different from other shells which you may run (e.g. on shell scripts) in that it reads the _._l_o_g_i_n file before reading commands An Introduction to the C shell USD:4-61 from the terminal and it reads the _._l_o_g_o_u_t file after you logout (2.1). logout The _l_o_g_o_u_t command causes a login shell to exit. Normally, a login shell will exit when you hit control-d generating an _e_n_d-_o_f-_f_i_l_e_, but if you have set _i_g_n_o_r_e_e_o_f in you _._l_o_g_i_n file then this will not work and you must use _l_o_g_o_u_t to log off the UNIX system (2.8). .logout When you log off of UNIX the shell will exe- cute commands from the file _._l_o_g_o_u_t in your _h_o_m_e directory after it prints `logout'. lpr The command _l_p_r is the line printer daemon. The standard input of _l_p_r spooled and printed on the UNIX line printer. You can also give _l_p_r a list of filenames as arguments to be printed. It is most common to use _l_p_r as the last component of a _p_i_p_e_l_i_n_e (2.3). ls The _l_s (list files) command is one of the most commonly used UNIX commands. With no argument filenames it prints the names of the files in the current directory. It has a number of useful _f_l_a_g arguments, and can also be given the names of directories as argu- ments, in which case it lists the names of the files in these directories (1.2). mail The _m_a_i_l program is used to send and receive messages from other UNIX users (1.1, 2.1), whether they are logged on or not. make The _m_a_k_e command is used to maintain one or more related files and to organize functions to be performed on these files. In many ways _m_a_k_e is easier to use, and more helpful than shell command scripts (3.2). makefile The file containing commands for _m_a_k_e is called _m_a_k_e_f_i_l_e or _M_a_k_e_f_i_l_e (3.2). manual The _m_a_n_u_a_l often referred to is the `UNIX manual'. It contains 8 numbered sections with a description of each UNIX program (sec- tion 1), system call (section 2), subroutine (section 3), device (section 4), special data structure (section 5), game (section 6), mis- cellaneous item (section 7) and system admin- istration program (section 8). There are also supplementary documents (tutorials and reference guides) for individual programs which require explanation in more detail. An USD:4-62 An Introduction to the C shell online version of the _m_a_n_u_a_l is accessible through the _m_a_n command. Its documentation can be obtained online via man man If you can't decide what manual page to look in, try the _a_p_r_o_p_o_s(1) command. The supple- mentary documents are in subdirectories of /usr/doc. metacharacter Many characters which are neither letters nor digits have special meaning either to the shell or to UNIX. These characters are called _m_e_t_a_c_h_a_r_a_c_t_e_r_s. If it is necessary to place these characters in arguments to com- mands without them having their special mean- ing then they must be _q_u_o_t_e_d. An example of a _m_e_t_a_c_h_a_r_a_c_t_e_r is the character `>' which is used to indicate placement of output into a file. For the purposes of the _h_i_s_t_o_r_y mecha- nism, most unquoted _m_e_t_a_c_h_a_r_a_c_t_e_r_s form sepa- rate words (1.4). The appendix to this user's manual lists the _m_e_t_a_c_h_a_r_a_c_t_e_r_s in groups by their function. mkdir The _m_k_d_i_r command is used to create a new directory. modifier Substitutions with the _h_i_s_t_o_r_y mechanism, keyed by the character `!' or of variables using the metacharacter `$', are often sub- jected to modifications, indicated by placing the character `:' after the substitution and following this with the _m_o_d_i_f_i_e_r itself. The _c_o_m_m_a_n_d _s_u_b_s_t_i_t_u_t_i_o_n mechanism can also be used to perform modification in a similar way, but this notation is less clear (3.6). more The program _m_o_r_e writes a file on your termi- nal allowing you to control how much text is displayed at a time. _M_o_r_e can move through the file screenful by screenful, line by line, search forward for a string, or start again at the beginning of the file. It is generally the easiest way of viewing a file (1.8). noclobber The shell has a variable _n_o_c_l_o_b_b_e_r which may be set in the file _._l_o_g_i_n to prevent acciden- tal destruction of files by the `>' output redirection metasyntax of the shell (2.2, 2.5). An Introduction to the C shell USD:4-63 noglob The shell variable _n_o_g_l_o_b is set to suppress the _f_i_l_e_n_a_m_e _e_x_p_a_n_s_i_o_n of arguments contain- ing the metacharacters `~', `*', `?', `[' and `]' (3.6). notify The _n_o_t_i_f_y command tells the shell to report on the termination of a specific _b_a_c_k_g_r_o_u_n_d _j_o_b at the exact time it occurs as opposed to waiting until just before the next prompt to report the termination. The _n_o_t_i_f_y variable, if set, causes the shell to always report the termination of _b_a_c_k_g_r_o_u_n_d jobs exactly when they occur (2.6). onintr The _o_n_i_n_t_r command is built into the shell and is used to control the action of a shell command script when an _i_n_t_e_r_r_u_p_t signal is received (3.9). output Many commands in UNIX result in some lines of text which are called their _o_u_t_p_u_t_. This _o_u_t_p_u_t is usually placed on what is known as the _s_t_a_n_d_a_r_d _o_u_t_p_u_t which is normally con- nected to the user's terminal. The shell has a syntax using the metacharacter `>' for redirecting the _s_t_a_n_d_a_r_d _o_u_t_p_u_t of a command to a file (1.3). Using the _p_i_p_e mechanism and the metacharacter `|' it is also possible for the _s_t_a_n_d_a_r_d _o_u_t_p_u_t of one command to become the _s_t_a_n_d_a_r_d _i_n_p_u_t of another command (1.5). Certain commands such as the line printer daemon _p do not place their results on the _s_t_a_n_d_a_r_d _o_u_t_p_u_t but rather in more useful places such as on the line printer (2.3). Similarly the _w_r_i_t_e command places its output on another user's terminal rather than its _s_t_a_n_d_a_r_d _o_u_t_p_u_t (2.3). Commands also have a _d_i_a_g_n_o_s_t_i_c _o_u_t_p_u_t where they write their error messages. Normally these go to the terminal even if the _s_t_a_n_d_a_r_d _o_u_t_- _p_u_t has been sent to a file or another com- mand, but it is possible to direct error diagnostics along with _s_t_a_n_d_a_r_d _o_u_t_p_u_t using a special metanotation (2.5). path The shell has a variable _p_a_t_h which gives the names of the directories in which it searches for the commands which it is given. It always checks first to see if the command it is given is built into the shell. If it is, then it need not search for the command as it can do it internally. If the command is not builtin, then the shell searches for a file with the name given in each of the USD:4-64 An Introduction to the C shell directories in the _p_a_t_h variable, left to right. Since the normal definition of the _p_a_t_h variable is path (. /usr/ucb /bin /usr/bin) the shell normally looks in the current directory, and then in the standard system directories `/usr/ucb', `/bin' and `/usr/bin' for the named command (2.2). If the command cannot be found the shell will print an error diagnostic. Scripts of shell commands will be executed using another shell to interpret them if they have `execute' permission set. This is normally true because a command of the form chmod 755 script was executed to turn this execute permission on (3.3). If you add new commands to a directory in the _p_a_t_h, you should issue the command _r_e_h_a_s_h (2.2). pathname A list of names, separated by `/' characters, forms a _p_a_t_h_n_a_m_e_. Each _c_o_m_p_o_n_e_n_t_, between successive `/' characters, names a directory in which the next _c_o_m_p_o_n_e_n_t file resides. _P_a_t_h_n_a_m_e_s which begin with the character `/' are interpreted relative to the _r_o_o_t direc- tory in the filesystem. Other _p_a_t_h_n_a_m_e_s are interpreted relative to the current directory as reported by _p_w_d_. The last component of a _p_a_t_h_n_a_m_e may name a directory, but usually names a file. pipeline A group of commands which are connected together, the _s_t_a_n_d_a_r_d _o_u_t_p_u_t of each con- nected to the _s_t_a_n_d_a_r_d _i_n_p_u_t of the next, is called a _p_i_p_e_l_i_n_e_. The _p_i_p_e mechanism used to connect these commands is indicated by the shell metacharacter `|' (1.5, 2.3). popd The _p_o_p_d command changes the shell's _w_o_r_k_i_n_g _d_i_r_e_c_t_o_r_y to the directory you most recently left using the _p_u_s_h_d command. It returns to the directory without having to type its name, forgetting the name of the current _w_o_r_k_i_n_g _d_i_r_e_c_t_o_r_y before doing so (2.7). port The part of a computer system to which each terminal is connected is called a _p_o_r_t. Usu- ally the system has a fixed number of _p_o_r_t_s, some of which are connected to telephone An Introduction to the C shell USD:4-65 lines for dial-up access, and some of which are permanently wired directly to specific terminals. pr The _p_r command is used to prepare listings of the contents of files with headers giving the name of the file and the date and time at which the file was last modified (2.3). printenv The _p_r_i_n_t_e_n_v command is used to print the current setting of variables in the environ- ment (2.8). process An instance of a running program is called a _p_r_o_c_e_s_s (2.6). UNIX assigns each _p_r_o_c_e_s_s a unique number when it is started - called the _p_r_o_c_e_s_s _n_u_m_b_e_r. _P_r_o_c_e_s_s _n_u_m_b_e_r_s can be used to stop individual _p_r_o_c_e_s_s_e_s using the _k_i_l_l or _s_t_o_p commands when the _p_r_o_c_e_s_s_e_s are part of a detached _b_a_c_k_g_r_o_u_n_d job. program Usually synonymous with _c_o_m_m_a_n_d; a binary file or shell command script which performs a useful function is often called a _p_r_o_g_r_a_m. prompt Many programs will print a _p_r_o_m_p_t on the ter- minal when they expect input. Thus the edi- tor `ex (1)' will print a `:' when it expects input. The shell _p_r_o_m_p_t_s for input with `% ' and occasionally with `? ' when reading com- mands from the terminal (1.1). The shell has a variable _p_r_o_m_p_t which may be set to a dif- ferent value to change the shell's main _p_r_o_m_p_t. This is mostly used when debugging the shell (2.8). pushd The _p_u_s_h_d command, which means `push direc- tory', changes the shell's _w_o_r_k_i_n_g _d_i_r_e_c_t_o_r_y and also remembers the current _w_o_r_k_i_n_g _d_i_r_e_c_- _t_o_r_y before the change is made, allowing you to return to the same directory via the _p_o_p_d command later without retyping its name (2.7). ps The _p_s command is used to show the processes you are currently running. Each process is shown with its unique process number, an indication of the terminal name it is attached to, an indication of the state of the process (whether it is running, stopped, awaiting some event (sleeping), and whether it is swapped out), and the amount of CPU time it has used so far. The command is identified by printing some of the words used USD:4-66 An Introduction to the C shell when it was invoked (2.6). Shells, such as the _c_s_h you use to run the _p_s command, are not normally shown in the output. pwd The _p_w_d command prints the full _p_a_t_h_n_a_m_e of the current _w_o_r_k_i_n_g _d_i_r_e_c_t_o_r_y. The _d_i_r_s builtin command is usually a better and faster choice. quit The _q_u_i_t signal, generated by a control-\, is used to terminate programs which are behaving unreasonably. It normally produces a core image file (1.8). quotation The process by which metacharacters are pre- vented their special meaning, usually by using the character `' in pairs, or by using the character `\', is referred to as _q_u_o_t_a_- _t_i_o_n (1.7). redirection The routing of input or output from or to a file is known as _r_e_d_i_r_e_c_t_i_o_n of input or out- put (1.3). rehash The _r_e_h_a_s_h command tells the shell to rebuild its internal table of which commands are found in which directories in your _p_a_t_h. This is necessary when a new program is installed in one of these directories (2.8). relative pathname A _p_a_t_h_n_a_m_e which does not begin with a `/' is called a _r_e_l_a_t_i_v_e _p_a_t_h_n_a_m_e since it is inter- preted _r_e_l_a_t_i_v_e to the current _w_o_r_k_i_n_g _d_i_r_e_c_- _t_o_r_y. The first _c_o_m_p_o_n_e_n_t of such a _p_a_t_h_n_a_m_e refers to some file or directory in the _w_o_r_k_- _i_n_g _d_i_r_e_c_t_o_r_y, and subsequent _c_o_m_p_o_n_e_n_t_s between `/' characters refer to directories below the _w_o_r_k_i_n_g _d_i_r_e_c_t_o_r_y. _P_a_t_h_n_a_m_e_s that are not _r_e_l_a_t_i_v_e are called _a_b_s_o_l_u_t_e _p_a_t_h_- _n_a_m_e_s (1.6). repeat The _r_e_p_e_a_t command iterates another command a specified number of times. root The directory that is at the top of the entire directory structure is called the _r_o_o_t directory since it is the `root' of the entire tree structure of directories. The name used in _p_a_t_h_n_a_m_e_s to indicate the _r_o_o_t is `/'. _P_a_t_h_n_a_m_e_s starting with `/' are said to be _a_b_s_o_l_u_t_e since they start at the _r_o_o_t directory. _R_o_o_t is also used as the part of a _p_a_t_h_n_a_m_e that is left after removing the An Introduction to the C shell USD:4-67 _e_x_t_e_n_s_i_o_n. See _f_i_l_e_n_a_m_e for a further expla- nation (1.6). RUBOUT The RUBOUT or DELETE key is often used to erase the previously typed character; some users prefer the BACKSPACE for this purpose. On older versions of UNIX this key served as the INTR character. scratch file Files whose names begin with a `#' are referred to as _s_c_r_a_t_c_h _f_i_l_e_s, since they are automatically removed by the system after a couple of days of non-use, or more frequently if disk space becomes tight (1.3). script Sequences of shell commands placed in a file are called shell command _s_c_r_i_p_t_s. It is often possible to perform simple tasks using these _s_c_r_i_p_t_s without writing a program in a language such as C, by using the shell to selectively run other programs (3.3, 3.10). set The builtin _s_e_t command is used to assign new values to shell variables and to show the values of the current variables. Many shell variables have special meaning to the shell itself. Thus by using the _s_e_t command the behavior of the shell can be affected (2.1). setenv Variables in the environment `environ (5)' can be changed by using the _s_e_t_e_n_v builtin command (2.8). The _p_r_i_n_t_e_n_v command can be used to print the value of the variables in the environment. shell A _s_h_e_l_l is a command language interpreter. It is possible to write and run your own _s_h_e_l_l, as _s_h_e_l_l_s are no different than any other programs as far as the system is con- cerned. This manual deals with the details of one particular _s_h_e_l_l, called _c_s_h_. shell script See _s_c_r_i_p_t (3.3, 3.10). signal A _s_i_g_n_a_l in UNIX is a short message that is sent to a running program which causes some- thing to happen to that process. _S_i_g_n_a_l_s are sent either by typing special _c_o_n_t_r_o_l charac- ters on the keyboard or by using the _k_i_l_l or _s_t_o_p commands (1.8, 2.6). sort The _s_o_r_t program sorts a sequence of lines in ways that can be controlled by argument _f_l_a_g_s (1.5). USD:4-68 An Introduction to the C shell source The _s_o_u_r_c_e command causes the shell to read commands from a specified file. It is most useful for reading files such as _._c_s_h_r_c after changing them (2.8). special character See _m_e_t_a_c_h_a_r_a_c_t_e_r_s and the appendix to this manual. standard We refer often to the _s_t_a_n_d_a_r_d _i_n_p_u_t and _s_t_a_n_d_a_r_d _o_u_t_p_u_t of commands. See _i_n_p_u_t and _o_u_t_p_u_t (1.3, 3.8). status A command normally returns a _s_t_a_t_u_s when it finishes. By convention a _s_t_a_t_u_s of zero indicates that the command succeeded. Com- mands may return non-zero _s_t_a_t_u_s to indicate that some abnormal event has occurred. The shell variable _s_t_a_t_u_s is set to the _s_t_a_t_u_s returned by the last command. It is most useful in shell commmand scripts (3.6). stop The _s_t_o_p command causes a _b_a_c_k_g_r_o_u_n_d job to become _s_u_s_p_e_n_d_e_d (2.6). string A sequential group of characters taken together is called a _s_t_r_i_n_g. _S_t_r_i_n_g_s can contain any printable characters (2.2). stty The _s_t_t_y program changes certain parameters inside UNIX which determine how your terminal is handled. See `stty (1)' for a complete description (2.6). substitution The shell implements a number of _s_u_b_s_t_i_t_u_- _t_i_o_n_s where sequences indicated by metachar- acters are replaced by other sequences. Notable examples of this are history _s_u_b_s_t_i_- _t_u_t_i_o_n keyed by the metacharacter `!' and variable _s_u_b_s_t_i_t_u_t_i_o_n indicated by `$'. We also refer to _s_u_b_s_t_i_t_u_t_i_o_n_s as _e_x_p_a_n_s_i_o_n_s (3.4). suspended A job becomes _s_u_s_p_e_n_d_e_d after a STOP signal is sent to it, either by typing a _c_o_n_t_r_o_l-z at the terminal (for _f_o_r_e_g_r_o_u_n_d jobs) or by using the _s_t_o_p command (for _b_a_c_k_g_r_o_u_n_d jobs). When _s_u_s_p_e_n_d_e_d, a job temporarily stops run- ning until it is restarted by either the _f_g or _b_g command (2.6). switch The _s_w_i_t_c_h command of the shell allows the shell to select one of a number of sequences of commands based on an argument string. It An Introduction to the C shell USD:4-69 is similar to the _s_w_i_t_c_h statement in the language C (3.7). termination When a command which is being executed fin- ishes we say it undergoes _t_e_r_m_i_n_a_t_i_o_n or _t_e_r_- _m_i_n_a_t_e_s_. Commands normally terminate when they read an _e_n_d-_o_f-_f_i_l_e from their _s_t_a_n_d_a_r_d _i_n_p_u_t. It is also possible to terminate com- mands by sending them an _i_n_t_e_r_r_u_p_t or _q_u_i_t signal (1.8). The _k_i_l_l program terminates specified jobs (2.6). then The _t_h_e_n command is part of the shell's `if- then-else-endif' control construct used in command scripts (3.6). time The _t_i_m_e command can be used to measure the amount of CPU and real time consumed by a specified command as well as the amount of disk i/o, memory utilized, and number of page faults and swaps taken by the command (2.1, 2.8). tset The _t_s_e_t program is used to set standard erase and kill characters and to tell the system what kind of terminal you are using. It is often invoked in a _._l_o_g_i_n file (2.1). tty The word _t_t_y is a historical abbreviation for `teletype' which is frequently used in UNIX to indicate the _p_o_r_t to which a given termi- nal is connected. The _t_t_y command will print the name of the _t_t_y or _p_o_r_t to which your terminal is presently connected. unalias The _u_n_a_l_i_a_s command removes aliases (2.8). UNIX UNIX is an operating system on which _c_s_h runs. UNIX provides facilities which allow _c_s_h to invoke other programs such as editors and text formatters which you may wish to use. unset The _u_n_s_e_t command removes the definitions of shell variables (2.2, 2.8). variable expansion See _v_a_r_i_a_b_l_e_s and _e_x_p_a_n_s_i_o_n (2.2, 3.4). variables _V_a_r_i_a_b_l_e_s in _c_s_h hold one or more strings as value. The most common use of _v_a_r_i_a_b_l_e_s is in controlling the behavior of the shell. See _p_a_t_h, _n_o_c_l_o_b_b_e_r, and _i_g_n_o_r_e_e_o_f for exam- ples. _V_a_r_i_a_b_l_e_s such as _a_r_g_v are also used USD:4-70 An Introduction to the C shell in writing shell programs (shell command scripts) (2.2). verbose The _v_e_r_b_o_s_e shell variable can be set to cause commands to be echoed after they are history expanded. This is often useful in debugging shell scripts. The _v_e_r_b_o_s_e vari- able is set by the shell's _-_v command line option (3.10). wc The _w_c program calculates the number of char- acters, words, and lines in the files whose names are given as arguments (2.6). while The _w_h_i_l_e builtin control construct is used in shell command scripts (3.7). word A sequence of characters which forms an argu- ment to a command is called a _w_o_r_d. Many characters which are neither letters, digits, `-', `.' nor `/' form _w_o_r_d_s all by themselves even if they are not surrounded by blanks. Any sequence of characters may be made into a _w_o_r_d by surrounding it with `'' characters except for the characters `'' and `!' which require special treatment (1.1). This pro- cess of placing special characters in _w_o_r_d_s without their special meaning is called _q_u_o_t_- _i_n_g. working directory At any given time you are in one particular directory, called your _w_o_r_k_i_n_g _d_i_r_e_c_t_o_r_y. This directory's name is printed by the _p_w_d command and the files listed by _l_s are the ones in this directory. You can change _w_o_r_k_- _i_n_g _d_i_r_e_c_t_o_r_i_e_s using _c_h_d_i_r. write The _w_r_i_t_e command is an obsolete way of com- municating with other users who are logged in to UNIX (you have to take turns typing). If you are both using display terminals, use _t_a_l_k(1), which is much more pleasant.