Variables (UNIX in a Nutshell: System V Edition) 5.3. Variables This section describes the following: Variable substitution Variable modifiers Predefined shell variables Example .cshrc file Environment variables 5.3.1. Variable Substitution In the following substitutions, braces ({}) are optional, except when needed to separate a variable name from following characters that would otherwise be a part of it. ${var}The value of variable var. ${var[i]}Select word or words in position i of var. i can be a single number, a range m–n, a range -n (missing m implies 1), a range m- (missing n implies all remaining words), or * (select all words). i can also be a variable that expands to one of these values. ${#var}The number of words in var. ${#argv}The number of arguments. $0Name of the program. (Usually not set in interactive shells.) ${argv[n]}Individual arguments on command line (positional parameters). n = 1–9. ${n}Same as ${argv[n]}. ${argv[*]}All arguments on command line. $*Same as $argv[*]. $argv[$#argv]The last argument. ${?var}Return 1 if var is set; 0 if var is not set. $$Process number of current shell; useful as part of a filename for creating temporary files with unique names. $?0Return 1 if input filename is known; 0 if not. $<Read a line from standard input. 5.3.1.1. ExamplesSort the third through last arguments (files) and save the output in a unique temporary file: sort $argv[3-] > tmp.$$ Process .cshrc commands only if the shell is interactive (i.e., the prompt variable must be set): if ($?prompt) then set commands, alias commands, etc. endif 5.3.2. Variable Modifiers Except for $?var, $$, $?0, and $<, the previous variable substitutions may be followed by one of the following modifiers. When braces are used, the modifier goes inside them. :rReturn the variable's root. :eReturn the variable's extension. :hReturn the variable's header. :tReturn the variable's tail. :grReturn all roots. :geReturn all extensions. :ghReturn all headers. :gtReturn all tails. :qQuote a wordlist variable, keeping the items separate. Useful when the variable contains filename metacharacters that should not be expanded. :xQuote a pattern, expanding it into a wordlist. 5.3.2.1. Examples using pathname modifiers This table shows the use of pathname modifiers on the following variable: set aa=(/progs/num.c /book/chap.ps) Variable PortionSpecificationOutput Result Normal variableecho $aa/progs/num.c /book/chap.ps Second rootecho $aa[2]:r/book/chap Second headerecho $aa[2]:h/book Second tailecho $aa[2]:tchap.ps Second extensionecho $aa[2]:eps Rootecho $aa:r/progs/num /book/chap.ps Global rootecho $aa:gr/progs/num /book/chap Headerecho $aa:h/progs /book/chap.ps Global headerecho $aa:gh/progs /book Tailecho $aa:tnum.c /book/chap.ps Global tailecho $aa:gtnum.c chap.ps Extensionecho $aa:ec /book/chap.ps Global extensionecho $aa:gec ps 5.3.2.2. Examples using quoting modifiers% set a="[a-z]*" A="[A-Z]*" % echo "$a" "$A" [a-z]* [A-Z]* % echo $a $A at cc m4 Book Doc % echo $a:x $A [a-z]* Book Doc % set d=($a:q $A:q) % echo $d at cc m4 Book Doc % echo $d:q [a-z]* [A-Z]* % echo $d[1] +++ $d[2] at cc m4 +++ Book Doc % echo $d[1]:q [a-z]* 5.3.3. Predefined Shell Variables Variables can be set in one of two ways, by assigning a value: set var=value or by simply turning them on: set var In the following table, variables that accept values are shown with the equals sign followed by the type of value they accept; the value is then described. (Note, however, that variables such as argv, cwd, or status are never explicitly assigned.) For variables that are turned on or off, the table describes what they do when set. The C shell automatically sets the variables argv, cwd, home, path, prompt, shell, status, term, and user. VariableDescription argv=(args)List of arguments passed to current command; default is (). cdpath=(dirs)List of alternate directories to search when locating arguments for cd, popd, or pushd. cwd=dirFull pathname of current directory. echoRedisplay each command line before execution; same as csh -x command. fignore=(chars)List of filename suffixes to ignore during filename completion (see filec). filecIf set, a filename that is partially typed on the command line can be expanded to its full name when the Escape key is pressed. If more than one filename matches, type EOF to list possible completions. hardpathsTell dirs to display the actual pathname of any directory that is a symbolic link. histchars=abA two-character string that sets the characters to use in history-substitution and quick-substitution (default is !^). history=n Number of commands to save in history list. home=dirHome directory of user, initialized from HOME. The ~ character is shorthand for this value. ignoreeof Ignore an end-of-file (EOF) from terminals; prevents accidental logout. mail=(n file) One or more files checked for new mail every five minutes or (if n is supplied) every n seconds. nobeep Don't ring bell for ambiguous file completion (see filec). noclobberDon't redirect output to an existing file; prevents accidental destruction of files. noglob Turn off filename expansion; useful in shell scripts. nonomatch Treat filename metacharacters as literal characters; e.g., vi ch* creates new file ch* instead of printing “No match.” notifyNotify user of completed jobs right away, instead of waiting for the next prompt. path=(dirs) List of pathnames in which to search for commands to execute. Initialized from PATH. SVR4 default is ( . /usr/ucb /usr/bin ). On Solaris, the default path is ( /usr/bin . ). However, the standard start-up scripts then change it to ( /bin /usr/bin /usr/ucb /etc . ). prompt='str' String that prompts for interactive input; default is %. savehist=nNumber of history commands to save in ~/.history upon logout; they can be accessed at the next login. shell=filePathname of the shell program currently in use; default is /bin/csh. status=nExit status of last command. Built-in commands return 0 (success) or 1 (failure). term=IDName of terminal type, same as TERM. time='n %c'If command execution takes more than n CPU seconds, report user time, system time, elapsed time, and CPU percentage. Supply optional %c flags to show other data. user=nameLogin name of user, initialized from USER. verboseDisplay a command after history substitution; same as the command csh -v. 5.3.4. Example .cshrc File # PREDEFINED VARIABLES set path=(~ ~/bin /usr/ucb /bin /usr/bin . ) set mail=(/var/mail/tom) if ($?prompt) then # Settings for interactive use set echo set filec set noclobber ignoreeof set cdpath=(/usr/lib /var/spool/uucp) # Now I can type cd macros # instead of cd /usr/lib/macros set fignore=.o # Ignore object files for filec set history=100 savehist=25 set prompt='tom \!% ' # Includes history number set time=3 # MY VARIABLES set man1="/usr/man/man1" # Lets me do cd $man1, ls $man1 set a="[a-z]*" # Lets me do vi $a set A="[A-Z]*" # Or grep string $A # ALIASES alias c "clear; dirs" # Use quotes to protect ; or | alias h "history | more" alias j jobs -l alias ls ls -sFC # Redefine ls command alias del 'mv \!* ~/tmp_dir'# A safe alternative to rm endif 5.3.5. Environment Variables The C shell maintains a set of environment variables, which are distinct from shell variables and aren't really part of the C shell. Shell variables are meaningful only within the current shell, but environment variables are automatically exported, making them available globally. For example, C shell variables are accessible only to a particular script in which they're defined, whereas environment variables can be used by any shell scripts, mail utilities, or editors you might invoke. Environment variables are assigned as follows: setenv VAR value By convention, environment variable names are all uppercase. You can create your own environment variables, or you can use the following predefined environment variables. These environment variables have a corresponding C shell variable: HOME Home directory; same as home. These may be changed independently of each other. PATH Search path for commands; same as path. Changing either one updates the value stored in the other. TERM Terminal type; same as term. Changing term updates TERM, but not the other way around. USER Username; same as user. Changing user updates USER, but not the other way around. Other environment variables include the following: EXINITA string of ex commands similar to those found in the startup .exrc file (e.g., set ai). Used by vi and ex. LOGNAME Another name for the USER variable. MAILThe file that holds mail. Used by mail programs. This is not the same as the C shell mail variable, which only checks for new mail. PWD The current directory; the value is copied from cwd. SHELLUndefined by default; once initialized to shell, the two are identical. 5.2. Syntax5.4. Expressions Copyright © 2003 O'Reilly & Associates. All rights reserved.