An introduction to the C shell William Joy 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|- 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 features 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 second section describes the shells capabilities which you can explore after you have begun to become acquainted with the shell. Later sections introduce features which are useful, but not necessary for all users of the shell. Back matter includes an appendix listing spe- cial characters of the shell and a glossary of terms and commands introduced in this manual. May 27, 1987 _________________________ |- UNIX is a trademark of Bell Laboratories. An introduction to the C shell William Joy Computer Science Division Department of Electrical Engineering and Computer Science University of California, Berkeley Berkeley, California 94720 _I_n_t_r_o_d_u_c_t_i_o_n 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 programmer's manual. The _c_s_h documenta- tion in the manual provides a full description of all features of the shell and is a final 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. _A_c_k_n_o_w_l_e_d_g_e_m_e_n_t_s 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. - 2 - _1. _T_e_r_m_i_n_a_l _u_s_a_g_e _o_f _t_h_e _s_h_e_l_l _1._1. _T_h_e _b_a_s_i_c _n_o_t_i_o_n _o_f _c_o_m_m_a_n_d_s 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 - 3 - `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 `#' and the kill character is `@'. Most people who use CRT displays prefer to use the backspace (|^H) char- acter as their erase character since it is then easier to see what you have typed so far. You can make this be true by typing tset -e which tells the program _t_s_e_t to set the erase character, and its default setting for this character is a backspace. _1._2. _F_l_a_g _a_r_g_u_m_e_n_t_s 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 - 4 - remember options of commands which are not used very fre- quently, so most UNIX utilities perform only one or two functions rather than having a large number of hard to remember options. _1._3. _O_u_t_p_u_t _t_o _f_i_l_e_s 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 accidentally; 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) standard erase char- acter means that it seldom appears in a file name, and allows this convention to be used for scratch files. - 5 - 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 _1._4. _M_e_t_a_c_h_a_r_a_c_t_e_r_s _i_n _t_h_e _s_h_e_l_l 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 letters 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 special 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 `% '. _1._5. _I_n_p_u_t _f_r_o_m _f_i_l_e_s; _p_i_p_e_l_i_n_e_s 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 _________________________ If you are using a CRT, your erase character should be a |^H, as we demonstrated in section 1.1 how this could be set up. - 6 - then the sort program would sort lines from its _s_t_a_n_d_a_r_d _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 mechanism. Commands separated by `|' characters are con- nected 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 - 7 - routing of information to the line printer. _1._6. _F_i_l_e_n_a_m_e_s 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 filenames because many of these have special meaning to the shell. The character `.' (period) is not a shell- metacharacter 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 - 8 - prog.* This word 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 dif- ferent 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 as 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 - 9 - chap.[12345] 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 users' 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 reliable 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 prepared. 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 - 10 - 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 mechanism will be described much later, in section 4.2, as it is used less frequently. _1._7. _Q_u_o_t_a_t_i_o_n 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 - _1._8. _T_e_r_m_i_n_a_t_i_n_g _c_o_m_m_a_n_d_s 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 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 the DEL or RUBOUT key on your terminal.* Since _c_a_t does not take any precau- tions 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 exe- cute commands 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 accidentally log us off, the shell has a mechanism for preventing this. This _i_g_n_o_r_e_e_o_f option will be discussed in section 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 `prepared.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 _________________________ *Many users use _s_t_t_y(1) to change the interrupt charac- ter to |^C. - 12 - 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 suspend its execution temporarily, with the possibility of 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 suspension 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 - 13 - 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. 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 program `a.out's state when it ter- minated 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, 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 - 14 - 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 output is being flushed. _1._9. _W_h_a_t _n_o_w? 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'. _Y_o_u _o_n_l_y _h_a_v_e _t_o _d_o _t_h_i_s _o_n_c_e; _i_t _t_a_k_e_s _e_f_f_e_c_t _a_t _n_e_x_t _l_o_g_i_n. 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. - 15 - _2. _D_e_t_a_i_l_s _o_n _t_h_e _s_h_e_l_l _f_o_r _t_e_r_m_i_n_a_l _u_s_e_r_s _2._1. _S_h_e_l_l _s_t_a_r_t_u_p _a_n_d _t_e_r_m_i_n_a_t_i_o_n 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). - 16 - 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. _2._2. _S_h_e_l_l _v_a_r_i_a_b_l_e_s 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 values by the set command. It has several forms, the most useful 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 mechan- ism. The shell variables most commonly referenced are, how- ever, those which the shell itself refers to. By changing 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 - 17 - % 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 overhead. Other useful built in variables are the variable _h_o_m_e - 18 - 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 several 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 overwrite 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.|- _2._3. _T_h_e _s_h_e_l_l'_s _h_i_s_t_o_r_y _l_i_s_t 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 _________________________ |-The space between the `!' and the word `now' is criti- cal here, as `!now' would be an invocation of the _h_i_s- _t_o_r_y mechanism, and have a totally different effect. - 19 - 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' without 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. 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 - 20 - % 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 % - 21 - 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 Programmers Manual. _2._4. _A_l_i_a_s_e_s 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 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 - 22 - also possible to define aliases which contain multiple com- mands or pipelines, showing where the arguments to the ori- ginal 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- aliasing _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. _W_a_r_n_i_n_g: 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. _2._5. _M_o_r_e _r_e_d_i_r_e_c_t_i_o_n; >> _a_n_d >& 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 The `>&' here tells the shell to route both the diagnostic output and the standard output into `file'. Similarly you can give the command - 23 - 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.|- _2._6. _J_o_b_s; _B_a_c_k_g_r_o_u_n_d, _F_o_r_e_g_r_o_u_n_d, _o_r _S_u_s_p_e_n_d_e_d When one or more commands are typed together as a pipe- line or as a sequence of commands separated by semicolons, a single _j_o_b is created by the shell consisting of these com- mands 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 _________________________ #A command form command >&! file exists, and is used when _n_o_c_l_o_b_b_e_r is set and _f_i_l_e al- ready 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. - 24 - your working directory (as well as any directories below it), put the output into the file `usage' and return immedi- ately 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 number 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 % - 25 - runs the `ls' program with the `-s' options, pipes this out- put 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 command, 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 currently 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 pro- gress until started again, either in the foreground or the backgound. 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