Variables (UNIX in a Nutshell: System V Edition)

4.3.1. Variable Substitution

ksh93 provides structured variables, such as pos.x and pos.y. To create either one, pos must already exist, and braces must be used to retrieve their values. Names beginning with .sh are reserved for use by ksh.

No spaces should be used in the following expressions. The colon (:) is optional; if it's included, var must be nonnull as well as set.

`var``=value` ...	Set each variable var to a value.
`${``var``}`	Use value of var; braces are optional if var is separated from the following text. They are required in `ksh93` if a variable name contains periods.
`${``var``:-value}`	Use var if set; otherwise, use value.
`${``var``:=value}`	Use var if set; otherwise, use value and assign value to var.
`${``var``:?value}`	Use var if set; otherwise, print value and exit (if not interactive). If value isn't supplied, print the phrase “parameter null or not set.”
`${``var``:+value}`	Use value if var is set; otherwise, use nothing.

In the Korn shell:

`${#``var``}`	Use the length of var.
`${#*}` `${#@}`	Use the number of positional parameters.
`${``var``#pattern}`	Use value of var after removing pattern from the left. Remove the shortest matching piece.
`${``var``##pattern}`	Same as `#`pattern, but remove the longest matching piece.
`${``var``%pattern}`	Use value of var after removing pattern from the right. Remove the shortest matching piece.
`${``var``%%pattern}`	Same as `%`pattern, but remove the longest matching piece.

In ksh93:

`${!``prefix``}` `${!``prefix`*`@}`	List of variables whose names begin with prefix.
`${``var``:pos}` `${``var``:pos:len}`	Starting at position pos (0-based) in variable var, extract len characters, or rest of string if no len. pos and len may be arithmetic expressions.
`${``var``:pos:len}`
`${``var``/pat/repl}`	Use value of var, with first match of pat replaced with repl.
`${``var``/pat}`	Use value of var, with first match of pat deleted.
`${``var``//pat/repl}`	Use value of var, with every match of pat replaced with repl.
`${``var``/#pat/repl}`	Use value of var, with match of pat replaced with repl. Match must occur at beginning of the value.
`${``var``/%pat/repl}`	Use value of var, with match of pat replaced with repl. Match must occur at end of the value.

In ksh93, indirect variables allow you to “alias” one variable name to affect the value of another. This is accomplished using typeset -n:

$ greet="hello, world"			Create initial variable
$ typeset -n friendly_message=greet	Set up alias
$ echo $friendly_message		Access old value through new name
hello, world
$ friendly_message="don't panic"	Change the value
$ echo $greet				Old variable is changed
don't panic

4.3.1.1. Examples

$ u=up d=down blank=		Assign values to three variables (last is null)
$ echo ${u}root			Braces are needed here
uproot
$ echo ${u-$d}                   Display value of u or d; since u is set, it's printed
up
$ echo ${tmp-`date`}	          If tmp is not set, the date command is executed
Thu Feb  4 15:03:46 EST 1993
$ echo ${blank="no data"}        blank is set, so it is printed (a blank line)
$ echo ${blank:="no data"}	blank is set but null, so the string is printed
no data	
$ echo $blank			blank now has a new value 
no data

4.3.1.2. Korn shell example

tail='${PWD##*/}'	Take the current directory name and remove the longest character string 
                        ending with /, which removes the leading pathname and leaves the tail

4.3.3. Other Shell Variables

The following variables are not automatically set by the shell. They are typically used in your .profile file, where you can define them to suit your needs. Variables can be assigned values by issuing commands of the form:

variable=value

This list includes the type of value expected when defining these variables. Those that are specific to the Korn shell are marked as (K). Those that are specific to ksh93 are marked (K93).

`CDPATH=``dirs`	Directories searched by `cd`; allows shortcuts in changing directories; unset by default.
`COLUMNS=``n`	(K) Screen's column width; used in line edit modes and `select` lists.
`EDITOR=``file`	(K) Pathname of line edit mode to turn on (can end in `emacs` or `vi`); used when VISUAL is not set.
`ENV=``file`	(K) Name of script that gets executed at startup; useful for storing alias and function definitions. For example, `ENV=$HOME/.kshrc` (like C shell's `.cshrc`).
`FCEDIT=``file`	(K) Editor used by `fc` command (default is `/bin/ed`). Obsoleted in `ksh93` by HISTEDIT.
`FIGNORE=``pattern`	(K93) Pattern describing the set of filenames to ignore during pattern matching.
`FPATH=``dirs`	(K) Directories to search for function definitions; undefined functions are set via `typeset -fu`; FPATH is searched when these functions are first referenced. (`ksh93` also searches PATH.)
`HISTEDIT=``file`	(K93) Editor used by `hist` command, if set. Overrides the setting of FCEDIT.
`HISTFILE=``file`	(K) File in which to store command history (must be set before `ksh` is started); default is `$HOME/.sh_history`.
`HISTSIZE=``n`	(K) Number of history commands available (must be set before `ksh` is started); default is 128.
`HOME=``dir`	Home directory; set by `login` (from `/etc/passwd` file).
`IFS='``chars``'`	Input field separators; default is space, tab, and newline.
`LANG=``dir`	Directory to use for certain language-dependent programs.
`LC_ALL=``locale`	(K93) Current locale; overrides LANG and the other LC_* variables.
`LC_COLLATE=``locale`	(K93) Locale to use for character collation (sorting order).
`LC_CTYPE=``locale`	(K93) Locale to use for character class functions. (See the earlier Section 4.2.2.)
`LC_NUMERIC=``locale`	(K93) Locale to use for the decimal-point character.
`LINES=``n`	(K) Screen's height; used for `select` lists.
`MAIL=``file`	Default file in which to receive mail; set by `login`.
`MAILCHECK=``n`	Number of seconds between mail checks; default is 600 (10 minutes).
`MAILPATH=``files`	One or more files, delimited by a colon, in which to receive mail. Along with each file, you may supply an optional message that the shell prints when the file increases in size. Messages are separated from the file name by a separator character. The Korn shell separator is `?`, and the default message is `You have mail in $_`. `$_` is replaced with the name of the file. The Bourne shell separator is `%`, and the default message is `You have mail`. For example, for `ksh`, you might have: MAILPATH="$MAIL?Ring! Candygram!:/etc/motd?New Login Message"
`PATH=``dirlist`	One or more pathnames, delimited by colons, in which to search for commands to execute. Default for SVR4 is `/bin:/usr/bin`. On Solaris, the default is `/usr/bin:`. However, the standard start-up scripts change it to: /usr/bin:/usr/ucb:/etc:. `ksh93`: PATH is also searched for function definitions for undefined functions.
`PS1=``string`	Primary prompt string; default is `$`.
`PS2=``string`	Secondary prompt (used in multiline commands); default is `>`.
`PS3=``string`	(K) Prompt string in `select` loops; default is `#?`.
`PS4=``string`	(K) Prompt string for execution trace (`ksh -x` or `set -x`); default is `+`.
`SHACCT=``file`	“Shell account”; file in which to log executed shell scripts. Not in Korn shell.
`SHELL=``file`	Name of default shell (e.g., `/bin/sh`).
`TERM=``string`	Terminal type.
`TMOUT=``n`	(K) If no command is typed after n seconds, exit the shell.
`VISUAL=``path`	(K) Same as EDITOR, but VISUAL is checked first.

4.3.1.1. Examples

4.3.1.2. Korn shell example

4.3. Variables

4.3.1. Variable Substitution

4.3.2. Built-in Shell Variables

4.3.3. Other Shell Variables

4.3.4. Arrays

4.3.5. Discipline Functions (ksh93 only)