-
Copyright (c) 1980, 1993
The Regents of the University of California. All rights reserved.

Redistribution and use in source and binary forms, with or without
modification, are permitted provided that the following conditions
are met:
1. Redistributions of source code must retain the above copyright
notice, this list of conditions and the following disclaimer.
2. Redistributions in binary form must reproduce the above copyright
notice, this list of conditions and the following disclaimer in the
documentation and/or other materials provided with the distribution.
3. Neither the name of the University nor the names of its contributors
may be used to endorse or promote products derived from this software
without specific prior written permission.

THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
SUCH DAMAGE.

Glossary

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 pr is in the \s-2UNIX\s0 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 pwd; see also dirs. The current directory `.' is usually the first component of the search path contained in the variable path , thus commands which are in `.' are found first (2.2). The character `.' is also used in separating components of filenames (1.6). The character `.' at the beginning of a component of a pathname is treated specially and not matched by the "filename expansion" 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 chdir , i.e. chdir paper you can return to the parent directory by doing chdir .. The current directory is printed by pwd (2.7). Compilers which create executable images create them, by default, in the file a.out. for historical reasons (2.3).

A pathname which begins with a `/' is absolute since it specifies the path of directories from the beginning of the entire directory system - called the root directory. Pathname s which are not absolute are called relative (see definition of "relative pathname" ) (1.6).

An alias specifies a shorter or different name for a \s-2UNIX\s0 command, or a transformation on a command to be performed in the shell. The shell has a command alias which establishes aliases and can print their current values. The command unalias is used to remove aliases (2.4). Commands in \s-2UNIX\s0 receive a list of argument words. Thus the command echo a b c consists of the "command name" `echo' and three argument words `a', `b' and `c'. The set of arguments after the "command name" is said to be the "argument list" of the command (1.1). The list of arguments to a command written in the shell language (a shell script or shell procedure) is stored in a variable called argv within the shell. This name is taken from the conventional name in the C programming language (3.4). Commands started without waiting for them to complete are called background commands (2.6). A filename is sometimes thought of as consisting of a base part, before any `.' character, and an extension - the part after the `.'. See filename and extension (1.6) and basename (1). The bg command causes a suspended job to continue execution in the background (2.6). A directory containing binaries of programs and shell scripts to be executed is typically called a bin directory. The standard system bin 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 binaries in any directory. If you wish to execute them often, the name of the directories should be a component of the variable path . Break is a builtin command used to exit from loops within the control structure of the shell (3.7). The breaksw builtin command is used to exit from a switch control structure, like a break exits from loops (3.7). A command executed directly by the shell is called a builtin command. Most commands in \s-2UNIX\s0 are not built into the shell, but rather exist as files in bin directories. These commands are accessible because the directories in which they reside are named in the path variable. A case command is used as a label in a switch 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). The cat program catenates a list of specified files on the "standard output" . It is usually used to look at the contents of a single file on the terminal, to `cat a file' (1.8, 2.3). The cd command is used to change the "working directory" . With no arguments, cd changes your "working directory" to be your home directory (2.4, 2.7). The chdir command is a synonym for cd . Cd is usually used because it is easier to type. The chsh command is used to change the shell which you use on \s-2UNIX\s0. By default, you use a 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 \s-2UNIX\s0 after doing this command, you will be using csh rather than the shell in `/bin/sh' (1.9). Cmp is a program which compares files. It is usually used on binary files, or to see if two files are identical (3.6). For comparing text files the program diff , described in `diff (1)' is used. A function performed by the system, either by the shell (a builtin command ) or by a program residing in a file in a directory within the \s-2UNIX\s0 system, is called a command (1.1).

When a command is issued, it consists of a "command name" , which is the first word of the command, followed by arguments. The convention on \s-2UNIX\s0 is that the first word of a command names the function to be performed (1.1).

The replacement of a command enclosed in `\`' characters by the text output by that command is called "command substitution" (4.3).

A part of a pathname between `/' characters is called a component of that pathname . A variable which has multiple strings as value is said to have several component s; each string is a component of the variable. A builtin command which causes execution of the enclosing foreach or while loop to cycle prematurely. Similar to the continue command in the programming language C (3.6). Certain special characters, called control characters, are produced by holding down the \s-2CONTROL\s0 key on your terminal and simultaneously pressing another character, much like the \s-2SHIFT\s0 key is used to produce upper case characters. Thus control- c is produced by holding down the \s-2CONTROL\s0 key while pressing the `c' key. Usually \s-2UNIX\s0 prints a caret (^) followed by the corresponding letter when you type a control character (e.g. `^C' for control- c (1.8). When a program terminates abnormally, the system places an image of its current state in a file named `core'. This "core dump" 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 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. The cp (copy) program is used to copy the contents of one file into another file. It is one of the most commonly used \s-2UNIX\s0 commands (1.6). The name of the shell program that this document describes. The file .cshrc in your home directory is read by each shell as it begins execution. It is usually used to change the setting of the variable path and to set alias parameters which are to take effect globally (2.1). The cwd variable in the shell holds the "absolute pathname" of the current "working directory" . It is changed by the shell whenever your current "working directory" changes and should not be changed otherwise (2.2). The date command prints the current date and time (1.3). Debugging is the process of correcting mistakes in programs and shell scripts. The shell has several options and variables which may be used to aid in shell debugging (4.4). The label default: is used within shell switch statements, as it is in the C language to label the code to be executed if none of the case labels matches the value switched on (3.7). The \s-2DELETE\s0 or \s-2RUBOUT\s0 key on the terminal normally causes an interrupt to be sent to the current job. Many users change the interrupt character to be ^C. A command that continues running in the background after you logout is said to be detached . An error message produced by a program is often referred to as a diagnostic . Most error messages are not written to the "standard output" , since that is often directed away from the terminal (1.3, 1.5). Error messsages are instead written to the "diagnostic output" which may be directed away from the terminal, but usually is not. Thus diagnostics will usually appear on the terminal (2.5). A structure which contains files. At any time you are in one particular directory whose names can be printed by the command pwd . The chdir command will change you to another directory , and make the files in that directory visible. The directory in which you are when you first login is your home directory (1.1, 2.7). The shell saves the names of previous "working directories" in the "directory stack" when you change your current "working directory" via the pushd command. The "directory stack" can be printed by using the dirs command, which includes your current "working directory" as the first directory name on the left (2.7). The dirs command prints the shell's "directory stack" (2.7). The du command is a program (described in `du (1)') which prints the number of disk blocks is all directories below and including your current "working directory" (2.6). The echo command prints its arguments (1.6, 3.6). The else command is part of the `if-then-else-endif' control command construct (3.6). If an if statement is ended with the word then , all lines following the if up to a line starting with the word endif or else are executed if the condition between parentheses after the if is true (3.6). An "end\f1-of\f1-file" 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 pipe receive an "end\f1-of\f1-file" when the command sending them input completes. Most commands terminate when they receive an "end\f1-of\f1-file" . The shell has an option to ignore "end\f1-of\f1-file" from a terminal input which may help you keep from logging out accidentally by typing too many control-d's (1.1, 1.8, 3.8). A character `\e' used to prevent the special meaning of a metacharacter is said to escape the character from its special meaning. Thus echo \e* will echo the character `*' while just echo * will echo the names of the file in the current directory. In this example, \e escape s `*' (1.7). There is also a non-printing character called escape , usually labelled \s-2ESC\s0 or \s-2ALTMODE\s0 on terminal keyboards. Some older \s-2UNIX\s0 systems use this character to indicate that output is to be suspended . Most systems use control-s to stop the output and control-q to start it. This file contains information about the accounts currently on the system. It consists 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 finger and grep are often used to search for information in this file. See `finger (1)', `passwd(5)', and `grep (1)' for more details. The exit command is used to force termination of a shell script, and is built into the shell (3.9). 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 "exit status" , a status of zero being considered `normal termination'. The exit command can be used to force a shell command script to give a non-zero "exit status" (3.6). The replacement of strings in the shell input which contain metacharacters by other strings is referred to as the process of expansion . Thus the replacement of the word `*' by a sorted list of files in the current directory is a `filename expansion'. Similarly the replacement of the characters `!!' by the text of the last command is a `history expansion'. Expansions are also referred to as substitutions (1.6, 3.4, 4.2). Expressions are used in the shell to control the conditional structures used in the writing of shell scripts and in calculating values for these scripts. The operators available in shell expressions are those of the language C (3.5). Filenames often consist of a base name and an extension separated by the character `.'. By convention, groups of related files often share the same root 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). The "job control" command fg is used to run a background or suspended job in the foreground (1.8, 2.6). Each file in \s-2UNIX\s0 has a name consisting of up to 14 characters and not including the character `/' which is used in pathname building. Most filenames do not begin with the character `.', and contain only letters and digits with perhaps a `.' separating the base portion of the filename from an extension (1.6).

"Filename expansion" uses the metacharacters `*', `?' and `[' and `]' to provide a convenient mechanism for naming files. Using "filename expansion" it is easy to name all the files in the current directory, or all files which have a common root name. Other "filename expansion" mechanisms use the metacharacter `~' and allow files in other users' directories to be named easily (1.6, 4.2).

Many \s-2UNIX\s0 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 flag options, and by convention consist of one or more letters preceded by the character `-' (1.2). Thus the ls (list files) command has an option `-s' to list the sizes of files. This is specified ls -s The foreach 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 specified list (3.6, 4.1). 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 "foreground jobs" or "running in the foreground" . This is as opposed to background . Foreground jobs can be stopped by signals from the terminal caused by typing different control characters at the keyboard (1.8, 2.6). The shell has a command goto used in shell scripts to transfer control to a given label (3.7). The grep command searches through a list of argument files for a specified string. Thus grep bill /etc/passwd will print each line in the file "/etc/passwd" which contains the string `bill'. Actually, grep scans for "regular expressions" in the sense of the editors `ed (1)' and `ex (1)'. Grep stands for `globally find "regular expression" and print' (2.4). The head 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 head 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).

Head is also used to describe the part of a pathname before and including the last `/' character. The tail of a pathname is the part after the last `/'. The `:h' and `:t' modifiers allow the head or tail of a pathname stored in a shell variable to be used (3.6).

The history 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 "history list" where these commands are kept, and a history variable which controls how large this list is (2.3).

Each user has a "home directory" , which is given in your entry in the password file, /etc/passwd . This is the directory which you are placed in when you first login. The cd or chdir command with no arguments takes you back to this directory, whose name is recorded in the shell variable home . You can also access the "home directories" of other users in forming filenames using a "filename expansion" notation and the character `~' (1.6).

A conditional command within the shell, the if command is used in shell command scripts to make decisions about what course of action to take next (3.6). 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 set the ignoreeof variable if you wish in your .login file and then use the command logout to logout. This is useful if you sometimes accidentally type too many control-d characters, logging yourself off (2.2). Many commands on \s-2UNIX\s0 take information from the terminal or from files which they then act on. This information is called input . Commands normally read for input from their "standard input" which is, by default, the terminal. This "standard input" can be redirected from a file using a shell metanotation with the character `<'. Many commands will also read from a file specified as argument. Commands placed in pipelines will read from the output of the previous command in the pipeline . The leftmost command in a pipeline reads from the terminal if you neither redirect its input nor give it a filename to use as "standard input" . Special mechanisms exist for supplying input to commands in shell scripts (1.5, 3.8). An interrupt is a signal to a program that is generated by typing ^C. (On older versions of UNIX the \s-2RUBOUT\s0 or \s-2DELETE\s0 key were used for this purpose.) It causes most programs to stop execution. Certain programs, such as the shell and the editors, handle an interrupt 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 interrupts. The shell often wakes up when you hit interrupt because many commands die when they receive an interrupt (1.8, 3.9). One or more commands typed on the same input line separated by `|' or `;' characters are run together and are called a job . Simple commands run by themselves without any `|' or `;' characters are the simplest jobs. Jobs are classified as foreground , background , or suspended (2.6). The builtin functions that control the execution of jobs are called "job control" commands. These are "bg, fg, stop, kill" (2.6). When each job is started it is assigned a small number called a "job number" which is printed next to the job in the output of the jobs command. This number, preceded by a `%' character, can be used as an argument to "job control" commands to indicate a specific job (2.6). The jobs command prints a table showing jobs that are either running in the background or are suspended (2.6). A command which sends a signal to a job causing it to terminate (2.6). The file .login in your home directory is read by the shell each time you login to \s-2UNIX\s0 and the commands there are executed. There are a number of commands which are usefully placed here, especially set commands to the shell itself (2.1). The shell that is started on your terminal when you login is called your "login shell" . It is different from other shells which you may run (e.g. on shell scripts) in that it reads the .login file before reading commands from the terminal and it reads the .logout file after you logout (2.1). The logout command causes a login shell to exit. Normally, a login shell will exit when you hit control-d generating an end\f1-of\f1-file, but if you have set ignoreeof in you .login file then this will not work and you must use logout to log off the \s-2UNIX\s0 system (2.8). When you log off of \s-2UNIX\s0 the shell will execute commands from the file .logout in your home directory after it prints `logout'. The command lpr is the line printer daemon. The standard input of lpr spooled and printed on the \s-2UNIX\s0 line printer. You can also give lpr a list of filenames as arguments to be printed. It is most common to use lpr as the last component of a pipeline (2.3). The ls (list files) command is one of the most commonly used \s-2UNIX\s0 commands. With no argument filenames it prints the names of the files in the current directory. It has a number of useful flag arguments, and can also be given the names of directories as arguments, in which case it lists the names of the files in these directories (1.2). The mail program is used to send and receive messages from other \s-2UNIX\s0 users (1.1, 2.1), whether they are logged on or not. The make command is used to maintain one or more related files and to organize functions to be performed on these files. In many ways make is easier to use, and more helpful than shell command scripts (3.2). The file containing commands for make is called makefile or Makefile (3.2). The manual often referred to is the `\s-2UNIX\s0 manual'. It contains 8 numbered sections with a description of each \s-2UNIX\s0 program (section 1), system call (section 2), subroutine (section 3), device (section 4), special data structure (section 5), game (section 6), miscellaneous item (section 7) and system administration program (section 8). There are also supplementary documents (tutorials and reference guides) for individual programs which require explanation in more detail. An online version of the manual is accessible through the man command. Its documentation can be obtained online via man man If you can't decide what manual page to look in, try the apropos (1) command. The supplementary documents are in subdirectories of /usr/doc.

Many characters which are neither letters nor digits have special meaning either to the shell or to \s-2UNIX\s0. These characters are called metacharacters . If it is necessary to place these characters in arguments to commands without them having their special meaning then they must be quoted . An example of a metacharacter is the character `>' which is used to indicate placement of output into a file. For the purposes of the history mechanism, most unquoted metacharacters form separate words (1.4). The appendix to this user's manual lists the metacharacters in groups by their function.

The mkdir command is used to create a new directory. Substitutions with the history mechanism, keyed by the character `!' or of variables using the metacharacter `$', are often subjected to modifications, indicated by placing the character `:' after the substitution and following this with the modifier itself. The "command substitution" mechanism can also be used to perform modification in a similar way, but this notation is less clear (3.6). The program more writes a file on your terminal allowing you to control how much text is displayed at a time. More 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). The shell has a variable noclobber which may be set in the file .login to prevent accidental destruction of files by the `>' output redirection metasyntax of the shell (2.2, 2.5). The shell variable noglob is set to suppress the "filename expansion" of arguments containing the metacharacters `~', `*', `?', `[' and `]' (3.6). The notify command tells the shell to report on the termination of a specific "background job" at the exact time it occurs as opposed to waiting until just before the next prompt to report the termination. The notify variable, if set, causes the shell to always report the termination of background jobs exactly when they occur (2.6). The onintr command is built into the shell and is used to control the action of a shell command script when an interrupt signal is received (3.9). Many commands in \s-2UNIX\s0 result in some lines of text which are called their output. This output is usually placed on what is known as the "standard output" which is normally connected to the user's terminal. The shell has a syntax using the metacharacter `>' for redirecting the "standard output" of a command to a file (1.3). Using the pipe mechanism and the metacharacter `|' it is also possible for the "standard output" of one command to become the "standard input" of another command (1.5). Certain commands such as the line printer daemon p do not place their results on the "standard output" but rather in more useful places such as on the line printer (2.3). Similarly the write command places its output on another user's terminal rather than its "standard output" (2.3). Commands also have a "diagnostic output" where they write their error messages. Normally these go to the terminal even if the "standard output" has been sent to a file or another command, but it is possible to direct error diagnostics along with "standard output" using a special metanotation (2.5). The shell has a variable path 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 directories in the path variable, left to right. Since the normal definition of the path 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 path , you should issue the command rehash (2.2). A list of names, separated by `/' characters, forms a pathname. Each component, between successive `/' characters, names a directory in which the next component file resides. Pathnames which begin with the character `/' are interpreted relative to the root directory in the file system. Other pathnames are interpreted relative to the current directory as reported by pwd. The last component of a pathname may name a directory, but usually names a file. A group of commands which are connected together, the "standard output" of each connected to the "standard input" of the next, is called a pipeline. The pipe mechanism used to connect these commands is indicated by the shell metacharacter `|' (1.5, 2.3). The popd command changes the shell's "working directory" to the directory you most recently left using the pushd command. It returns to the directory without having to type its name, forgetting the name of the current "working directory" before doing so (2.7). The part of a computer system to which each terminal is connected is called a port . Usually the system has a fixed number of ports , some of which are connected to telephone lines for dial-up access, and some of which are permanently wired directly to specific terminals. The pr 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). The printenv command is used to print the current setting of variables in the environment (2.8). An instance of a running program is called a process (2.6). \s-2UNIX\s0 assigns each process a unique number when it is started - called the "process number" . "Process numbers" can be used to stop individual processes using the kill or stop commands when the processes are part of a detached background job. Usually synonymous with command ; a binary file or shell command script which performs a useful function is often called a program . Many programs will print a prompt on the terminal when they expect input. Thus the editor `ex (1)' will print a `:' when it expects input. The shell prompts for input with `% ' and occasionally with `? ' when reading commands from the terminal (1.1). The shell has a variable prompt which may be set to a different value to change the shell's main prompt . This is mostly used when debugging the shell (2.8). The pushd command, which means `push directory', changes the shell's "working directory" and also remembers the current "working directory" before the change is made, allowing you to return to the same directory via the popd command later without retyping its name (2.7). The ps 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 \s-2CPU\s0 time it has used so far. The command is identified by printing some of the words used when it was invoked (2.6). Shells, such as the csh you use to run the ps command, are not normally shown in the output. The pwd command prints the full pathname of the current "working directory" . The dirs builtin command is usually a better and faster choice. The quit signal, generated by a control-\e, is used to terminate programs which are behaving unreasonably. It normally produces a core image file (1.8). The process by which metacharacters are prevented their special meaning, usually by using the character `\' in pairs, or by using the character `\e', is referred to as quotation (1.7). The routing of input or output from or to a file is known as redirection of input or output (1.3). The rehash command tells the shell to rebuild its internal table of which commands are found in which directories in your path . This is necessary when a new program is installed in one of these directories (2.8).

A pathname which does not begin with a `/' is called a "relative pathname" since it is interpreted relative to the current "working directory" . The first component of such a pathname refers to some file or directory in the "working directory" , and subsequent components between `/' characters refer to directories below the "working directory" . Pathnames that are not relative are called "absolute pathnames" (1.6).

The repeat command iterates another command a specified number of times. The directory that is at the top of the entire directory structure is called the root directory since it is the `root' of the entire tree structure of directories. The name used in pathnames to indicate the root is `/'. Pathnames starting with `/' are said to be absolute since they start at the root directory. Root is also used as the part of a pathname that is left after removing the extension . See filename for a further explanation (1.6). The \s-2RUBOUT\s0 or \s-2DELETE\s0 key is often used to erase the previously typed character; some users prefer the \s-2BACKSPACE\s0 for this purpose. On older versions of \s-2UNIX\s0 this key served as the \s-2INTR\s0 character. Files whose names begin with a `#' are referred to as "scratch files" , 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). Sequences of shell commands placed in a file are called shell command scripts . It is often possible to perform simple tasks using these scripts without writing a program in a language such as C, by using the shell to selectively run other programs (3.3, 3.10). The builtin set 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 set command the behavior of the shell can be affected (2.1). Variables in the environment `environ (5)' can be changed by using the setenv builtin command (2.8). The printenv command can be used to print the value of the variables in the environment. A shell is a command language interpreter. It is possible to write and run your own shell , as shells are no different than any other programs as far as the system is concerned. This manual deals with the details of one particular shell , called csh. See script (3.3, 3.10). A signal in \s-2UNIX\s0 is a short message that is sent to a running program which causes something to happen to that process. Signals are sent either by typing special control characters on the keyboard or by using the kill or stop commands (1.8, 2.6). The sort program sorts a sequence of lines in ways that can be controlled by argument flags (1.5). The source command causes the shell to read commands from a specified file. It is most useful for reading files such as .cshrc after changing them (2.8).

See metacharacters and the appendix to this manual.

We refer often to the "standard input" and "standard output" of commands. See input and output (1.3, 3.8). A command normally returns a status when it finishes. By convention a status of zero indicates that the command succeeded. Commands may return non-zero status to indicate that some abnormal event has occurred. The shell variable status is set to the status returned by the last command. It is most useful in shell command scripts (3.6). The stop command causes a background job to become suspended (2.6). A sequential group of characters taken together is called a string . Strings can contain any printable characters (2.2). The stty program changes certain parameters inside \s-2UNIX\s0 which determine how your terminal is handled. See `stty (1)' for a complete description (2.6). The shell implements a number of substitutions where sequences indicated by metacharacters are replaced by other sequences. Notable examples of this are history substitution keyed by the metacharacter `!' and variable substitution indicated by `$'. We also refer to substitutions as expansions (3.4). A job becomes suspended after a \s-2STOP\s0 signal is sent to it, either by typing a control -z at the terminal (for foreground jobs) or by using the stop command (for background jobs). When suspended , a job temporarily stops running until it is restarted by either the fg or bg command (2.6). The switch command of the shell allows the shell to select one of a number of sequences of commands based on an argument string. It is similar to the switch statement in the language C (3.7). When a command which is being executed finishes we say it undergoes termination or terminates. Commands normally terminate when they read an end\f1-of\f1-file from their "standard input" . It is also possible to terminate commands by sending them an interrupt or quit signal (1.8). The kill program terminates specified jobs (2.6). The then command is part of the shell's `if-then-else-endif' control construct used in command scripts (3.6). The time command can be used to measure the amount of \s-2CPU\s0 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). The tset 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 .login file (2.1). The word tty is a historical abbreviation for `teletype' which is frequently used in \s-2UNIX\s0 to indicate the port to which a given terminal is connected. The tty command will print the name of the tty or port to which your terminal is presently connected. The unalias command removes aliases (2.8). \s-2UNIX\s0 is an operating system on which csh runs. \s-2UNIX\s0 provides facilities which allow csh to invoke other programs such as editors and text formatters which you may wish to use. The unset command removes the definitions of shell variables (2.2, 2.8).

See variables and expansion (2.2, 3.4).

Variables in csh hold one or more strings as value. The most common use of variables is in controlling the behavior of the shell. See path , noclobber , and ignoreeof for examples. Variables such as argv are also used in writing shell programs (shell command scripts) (2.2). The verbose 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 verbose variable is set by the shell's -v command line option (3.10). The wc program calculates the number of characters, words, and lines in the files whose names are given as arguments (2.6). The while builtin control construct is used in shell command scripts (3.7). A sequence of characters which forms an argument to a command is called a word . Many characters which are neither letters, digits, `-', `.' nor `/' form words all by themselves even if they are not surrounded by blanks. Any sequence of characters may be made into a word by surrounding it with `\'' characters except for the characters `\'' and `!' which require special treatment (1.1). This process of placing special characters in words without their special meaning is called quoting .

At any given time you are in one particular directory, called your "working directory" . This directory's name is printed by the pwd command and the files listed by ls are the ones in this directory. You can change "working directories" using chdir .

The write command is an obsolete way of communicating with other users who are logged in to \s-2UNIX\s0 (you have to take turns typing). If you are both using display terminals, use talk(1), which is much more pleasant.