## 4.3. Variables

This section describes the following:

- Variable substitution

- Built-in shell variables

- Other shell variables

- Arrays (Korn shell only)

- Discipline functions (`ksh93` only)

### 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

### 4.3.2. Built-in Shell Variables

Built-in variables are automatically set by the shell and are typically used inside shell scripts. Built-in variables can make use of the variable substitution patterns shown previously. Note that the `$` is not actually part of the variable name, although the variable is always referenced this way.

`$#` | Number of command-line arguments.
`$-` | Options currently in effect (arguments supplied to `sh` or to `set`).
`$?` | Exit value of last executed command.
`$$` | Process number of current process.
`$!` | Process number of last background command.
`$0` | First word; that is, command name. This will have the full path name if it was found via a PATH search.
`$``n` | Individual arguments on command line (positional parameters). The Bourne shell allows only nine parameters to be referenced directly (n = 1–9); the Korn shell allows n to be greater than 9 if specified as `${``n``}`.
`$*`, `$@` | All arguments on command line (`$1 $2` ...).
`"$*"` | All arguments on command line as one string (`"$1 $2..."`).
`"$@"` | All arguments on command line, individually quoted (`"$1" "$2"` ...).

The Korn shell automatically sets these additional variables:

`$_` | Temporary variable; initialized to pathname of script or program being executed. Later, stores the last argument of previous command. Also stores name of matching MAIL file during mail checks.
`LINENO` | Current line number within the script or function.
`OLDPWD` | Previous working directory (set by `cd`).
`OPTARG` | Name of last option processed by `getopts`.
`OPTIND` | Numerical index of OPTARG.
`PPID` | Process number of this shell's parent.
`PWD` | Current working directory (set by `cd`).
`RANDOM`[`=``n`] | Generate a new random number with each reference; start with integer n, if given.
`REPLY` | Default reply, used by `select` and `read`.
`SECONDS`[`=``n`] | Number of seconds since the shell was started, or, if n is given, number of seconds + n since the shell started.

`ksh93` automatically sets these additional variables. Variables whose names contain “.” must be enclosed in braces when referenced, e.g., `${.sh.edchar}`.

`HISTCMD` | The history number of the current command.
`.sh.edchar` | The character(s) entered when processing a `KEYBD` trap. Changing it replaces the characters that caused the trap.
`.sh.edcol` | The position of the cursor in the most recent `KEYBD` trap.
`.sh.edmode` | Will be equal to ESCAPE if in a `KEYBD` trap in `vi` mode, otherwise empty.
`.sh.edtext` | The characters in the input buffer during a `KEYBD` trap.
`.sh.name` | The name of the variable running a discipline function.
`.sh.subscript` | The subscript of the variable running a discipline function.
`.sh.value` | The value of the variable inside the `set` and `get` discipline functions.
`.sh.version` | The version of `ksh93`.

### 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.

### 4.3.4. Arrays

The Korn shell supports one-dimensional arrays of up to 1024 elements. The first element is numbered 0. An array name can be initialized as follows:

>     set -A name value0 value1 ...

where the specified values become elements of name. Declaring arrays is not required, however. Any valid reference to a subscripted variable can create an array.

When referencing arrays, use the `${` ... `}` syntax. This isn't needed when referencing arrays inside `(( ))` (the form of `let` that does automatic quoting). Note that `[` and `]` are typed literally (i.e., they don't stand for optional syntax).

`${``name``[`i`]}` | Use element i of array name. i can be any arithmetic expression as described under `let`. The expression must return a value between 0 and 1023.
`${``name``}` | Use element 0 of array name.
`${``name``[*]}` `${``name``[@]}` | Use all elements of array name.
`${#``name``[*]}` `${#``name``[@]}` | Use the number of elements in array name.

`ksh93` provides associative arrays, where the indices are strings instead of numbers (as in `awk`). In this case, `[` and `]` act like double quotes. Associative arrays are created with `typeset -A`. A special syntax allows assigning to multiple elements at once:

>     data=([joe]=30 [mary]=25)

The values would be retrieved as `${data[joe]}` and `${data[mary]}`.

### 4.3.5. Discipline Functions (ksh93 only)

Along with structured variables, `ksh93` introduces discipline functions. These are special functions that are called whenever a variable's value is accessed or changed. For a shell variable named `x`, you can define the following functions:

`x.get` | Called when `x`'s value is retrieved (`$x`).
`x.set` | Called when `x`'s value is changed (`x=2`).
`x.unset` | Called when `x` is unset (`unset x`).

Within the discipline functions, special variables provide information about the variable being changed:

`.sh.name` | The name of the variable being changed.
`.sh.subscript` | The subscript of the array element being changed.
`.sh.value` | The value of the variable being assigned or returned. Changing it within the discipline function changes the value that is actually assigned or returned.

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