How to Use Korn Shell (ksh) Variables in Linux

Introducing Korn Shell Variables

A variable refers to a temporary storage area in memory. Variables contain:

Information needed for customizing the shell
Information needed by other processes to function properly

The shell enables you to store values in variables. Korn shell programming uses two types of variables:

Variables that are exported to subprocesses
Variables that are not exported to subprocesses

The table below describes the Korn shell commands used to set, unset, and view variables.

Action	Command
To set a variable	VAR=value export VAR=value
To unset a variable	unset VAR
To display all variables	set, env, or export
To display values stored in variables	echo $VAR

When a shell variable follows the $ sign, the shell interprets that the value stored inside that variable is to be substituted at that point.

Referencing Values in Variables

You can use the echo command to display the value that is stored inside a shell variable. For example:

$ echo $SHELL
/bin/ks

Displaying Variables

To list all shell variables with their current values, perform the set command. For example:

$ set
DISPLAY=:0.0
EDITOR=/usr/bin/vi
ERRNO=13
FCEDIT=/bin/vi
HELPPATH=/usr/openwin/lib/locale:/usr/openwin/lib/help
HOME=/export/home/user1
HZ=100
IFS=
LANG=C
LINENO=1
LOGNAME=user1
MAIL=/var/mail/user1
MAILCHECK=600
MANPATH=/usr/man:/usr/openwin/share/man
OLDPWD=/export/home/user1
OPENWINHOME=usr/openwin
PATH=/usr/openwin/bin:/bin:/usr/bin:/usr/ucb:/usr/sbin
PPID=596
PS1=’$ ’
PS2=’> ’
PS3=’#? ’
PS4=’+ ’
PWD=/tmp
SHELL=/bin/ksh
TERM=dtterm
TERMINAL_EMULATOR=dtterm
TMOUT=0
TZ=MET
USER=user1
_=set
office=/export/home/user1/office
private=/export/home/user1/private

To make the value of a variable known to a sub-shell, export it using the export command. To view a list of all these variables and their current values, perform the export command:

$ export
DISPLAY=:0.0
EDITOR=/usr/bin/vi
HELPPATH=/usr/openwin/lib/locale:/usr/openwin/lib/help
HOME=/export/home/user1
LANG=C
LINENO=1
LOGNAME=user1
MAIL=/var/mail/user1
MANPATH=/usr/openwin/share/man:/usr/man
OPENWINHOME=usr/openwin
PATH=/usr/openwin/bin:/bin:/usr/bin:/usr/ucb:/usr/sbin
PWD=/etc
SHELL=/bin/ksh
TERM=dtterm
TERMINAL_EMULATOR=dtterm
TZ=MET
USER=user1
_=set
office=/export/home/user1/office

Setting and Unsetting Shell Variables

A variable is set and a value is assigned with the following syntax:

var=value

VAR=value

There is no space on either side of the equal sign (=). For example:

$ private=/export/home/user1/private
$ set| grep private
private=/export/home/user1/private
$ cd $private; pwd
/export/home/user1/private

To make the value of a variable known to a sub-shell, use the following command syntax:

$ export VAR

$ export var

For example:

$ export office=/export/home/user1/office
$ echo $office
/export/home/user1/office

You can delete the values stored in shell variables with the following command syntax:

$ unset VAR

$ unset var

For example, to unset the variable private, perform the following commands:

$ unset private
$ echo $private
$

The output of the echo command is a blank line.

NOTE: Usually, uppercase variables are used for environment settings, and lowercase variables are used for settings local to the shell.

Displaying Default Variables

The table below describes variables that are assigned default values by the shell on login.

Variable	Meaning
EDITOR	Defines the default editor for the shell.
FCEDIT	Defines the editor for the fc command. Used with the history mechanism for editing previously executed commands.
HOME	Sets the directory to which the cd command changes when no argument is supplied in the command line.
LOGNAME	Sets the login name of the user.
PATH	Specifies a colon-delimited list of directories to be searched when the shell needs to find a command to be executed.
PS1	Specifies the primary Korn shell prompt ($).
PS2	Specifies the secondary command prompt (>).
SHELL	Specifies the name of the shell (that is, /bin/ksh).

Customizing Korn Shell Variables

This section describes how to customize Korn shell variables.

The PS1 Prompt Variable

The shell prompt string is stored in the shell variable PS1, and you can customize it according to your preferences.

$ PS1="$LOGNAME@`uname -n` \$PWD $ "
user1@host1: $

In this example, the prompt displays the login name of the user the system’s hostname and the current working directory. The username is read from the variable LOGNAME, and the hostname comes from the output of the ‘uname -n’ command. This prompt displays the correct information even when the user logs in on different hosts. The back quotation marks delimit an imbedded command string The following example shows how the value of another variable is used for prompt deﬁnition.

NOTE: Please be aware that your shell prompt changes from “$” to “I like UNIX >”.

$ ILU=”I like UNIX”
$ PS1=”$ILU >”
I Like UNIX > echo $ILU
I like UNIX
I like UNIX >

NOTE: To make the new shell prompt appear in every shell, it must be included in the user’s Korn shell initialization ﬁle (usually the .kshrc ﬁle).

The PATH Variable

The PATH variable contains a list of directory path names, separated by colons. When you perform a command on the command line, the shell searches these directories from left to right to locate that command. The shell executes the ﬁrst command that it ﬁnds. If the shell does not ﬁnd the command in any of the listed directories, it displays the following error message:

ksh: command_name : not found

The shell restricts its search to the directories speciﬁed in the PATH variable. Sometimes when the shell is unable to ﬁnd a particular command, the command may reside in a directory that has not been speciﬁed in the PATH variable. When the shell is unable to ﬁnd a particular command, the user can type the absolute path name of the directory in which the command resides. For example:

$ /usr/bin/id

If the shell is able to successfully execute the command when you specify the absolute path name of the directory, check the PATH variable to ensure that the directory exists in the search path. If the directory exists in the search path, check that it has been named correctly. For example:

$ echo $PATH
/usr/dt/bin:/usr/openwin/bin:/usr/bin:/usr/ucb
$

Extending the PATH Variable

To extend the PATH variable to include the home directory of the user, perform the following commands.

$ echo $PATH
/usr/dt/bin:/usr/openwin/bin:/usr/bin:/usr/ucb
$
$ PATH=$PATH:~
$
$ echo $PATH
/usr/dt/bin:/usr/openwin/bin:/usr/bin:/usr/ucb:/export/home/user1

The PATH variable passes the value automatically to the sub-shells.

Using the Quoting Characters

Quoting is a process that instructs the shell to mask, or ignore, the special meaning of metacharacters. The quoting characters are single forward quotation marks (’’), double quotation marks (""), backslash (\), and parentheses ($( command )). Quotation marks around a string of metacharacters prevent the shell from interpreting the special meaning of the metacharacters. There are two types of quotation marks that mask the special meaning of metacharacters: single forward quotation marks (’’) and double quotation marks ("").

Single forward quotation marks instruct the shell to ignore all enclosed metacharacters. Double quotation marks instruct the shell to ignore all enclosed metacharacters, except for the following three characters:

1. The single backward quotation marks (‘) – Single backward quotation marks instruct the shell to execute and display the output for a UNIX system command. 2. The backslash (\) – A backslash (\) character in front of a metacharacter prevents the shell from interpreting the next character as a metacharacter. 3. The dollar sign ($) – You can also use parentheses () to perform command substitution. Parentheses ($( command )) instructs the shell to execute and display the output for the command enclosed within. The output of the nested command, command2, is substituted when the command1 command is executed. The syntax for this is:

command1 $(command2)

To ignore the special meaning of the dollar sign metacharacter, perform the following command:

$ echo ’$SHELL’
$SHELL

NOTE: The echo utility writes arguments to the standard output.

To interpret the special meaning of the dollar sign metacharacter, perform the following command:

$ echo "$SHELL"
/bin/ksh

To ignore the special meaning of the dollar sign metacharacter when you use double quotes, perform the following command:

$ echo "\$SHELL"
$SHELL

To display the output for the date command, perform the following command:

$ echo “Today’s date is ‘date‘”
Today’s date is Wed Dec 15 15:52:56 GMT 2004
$

To execute the pwd command using parentheses, perform the following command:

$ echo “The user is currently in the $(pwd) directory.”
The user is currently in the /export/home/user1 directory.