7.6. Variables

There are two types of variables: local and environment. Some variables are created by the user and others are special shell variables.

7.6.1 Local Variables

Local variables are given values that are known only to the shell in which they are created. Variable names must begin with an alphabetic or underscore character. The remaining characters can be alphabetic, decimal digits 0 to 9, or an underscore character. Any other characters mark the termination of the variable name. When assigning a value, there can be no whitespace surrounding the equal sign. To set the variable to null, the equal sign is followed by a newline.^[1]

^[1] A variable set to some value or to null will be displayed by using the set command, whereas an unset variable will not.

A dollar sign is used in front of a variable to extract the value stored there.

Setting Local Variables

See Example 7.18 for a demonstration on setting a local variable

Example 7.18.

1   $ round=world

    $ echo $round

    world

2   $ name="Peter Piper"

    $ echo $name

    Peter Piper

3   $ x=

    $ echo $x



4   $ file.bak="$HOME/junk"

    file.bak=/home/jody/ellie/junk: not found

EXPLANATION

The Scope of Local Variables

A local variable is known only to the shell in which it was created. It is not passed on to subshells. The double dollar sign variable is a special variable containing the PID of the current shell.

Example 7.19.

1   $ echo $$

    1313

2   $ round=world

    $ echo $round

    world

3   $ sh           # Start a subshell

4   $ echo $$

    1326

5   $ echo $round



6   $ exit         # Exits this shell, returns to parent shell

7   $ echo $$

    1313

8   $ echo $round

    world

EXPLANATION

Setting Read-Only Variables

A read-only variable cannot be redefined or unset.

Example 7.20.

1   $ name=Tom

2   $ readonly name

    $ echo $name

    Tom

3   $ unset name

    name: readonly

4   $ name=Joe

    name: readonly

EXPLANATION

7.6.2 Environment Variables

Environment variables are available to the shell in which they are created and any subshells or processes spawned from that shell. By convention, environment variables are capitalized. Environment variables are variables that have been exported.

The shell in which a variable is created is called the parent shell. If a new shell is started from the parent shell, it is called the child shell. Some of the environment variables, such as HOME, LOGNAME, PATH, and SHELL, are set before you log on by the /bin/login program. Normally, environment variables are defined and stored in the .profile file in the user's home directory. See Table 7.3 for a list of environment variables.

Table 7.3. Bourne Shell Environment Variables

Variable	Value
PATH	The search path for commands.
HOME	Home directory; used by cd when no directory is specified.
IFS	Internal field separators, normally space, tab, and newline.
LOGNAME	The user's login name.
MAIL	If this parameter is set to the name of a mail file and the MAILPATH parameter is not set, the shell informs the user of the arrival of mail in the specified file.
MAILCHECK	This parameter specifies how often (in seconds) the shell will check for the arrival of mail in the files specified by the MAILPATH or MAIL parameters. The default value is 600 seconds (10 minutes). If set to zero, the shell will check before issuing each primary prompt.
MAILPATH	A colon-separated list of filenames. If this parameter is set, the shell informs the user of the arrival of mail in any of the specified files. Each filename can be followed by a percent sign and a message that will be printed when the modification time changes. The default message is You have mail.
PWD	Present working directory.
PS1	Primary prompt string, which is a dollar sign by default.
PS2	Secondary prompt string, which is a right angle bracket by default.
SHELL	When the shell is invoked, it scans the environment for this name. The shell gives default values to PATH, PS1, PS2, MAILCHECK, and IFS. HOME and MAIL are set by login.

Setting Environment Variables

To set environment variables, the export command is used either after assigning a value or when the variable is set. (Do not use the dollar sign on a variable when exporting it.)

Example 7.21.

1   $ TERM=wyse

    $ export TERM

2   $ NAME="John Smith"

    $ export NAME

    $ echo $NAME

    John Smith

    $ echo $$

    319              # pid number for parent shell

4   $ sh            # Start a subshell

5   $ echo $$

    340              # pid number for new shell

6   $ echo $NAME

    John Smith

7   $ NAME="April Jenner"

    $ export NAME

    $ echo $NAME

    April Jenner

8   $ exit           # Exit the subshell and go back to parent shell

9   $ echo $$

    319              # pid number for parent shell

10  $ echo $NAME

    John Smith

EXPLANATION

7.6.3 Listing Set Variables

There are two built-in commands that print the value of a variable: set and env. The set command prints all variables, local and global. The env command prints just the global variables.

Example 7.22.

1   $ env     (Partial list)

    LOGNAME=ellie

    TERMCAP=sun–cmd

    USER=ellie

    DISPLAY=:0.0

    SHELL=/bin/sh

    HOME=/home/jody/ellie

    TERM=sun–cmd

    LD_LIBRARY_PATH=/usr/local/OW3/lib

    PWD=/home/jody/ellie/perl



2   $ set

    DISPLAY=:0.0

    FMHOME=/usr/local/Frame–2.1X

    FONTPATH=/usr/local/OW3/lib/fonts

    HELPPATH=/usr/local/OW3/lib/locale:/usr/local/OW3/lib/help

    HOME=/home/jody/ellie

    HZ=100

    IFS=

    LANG=C

    LD_LIBRARY_PATH=/usr/local/OW3/lib

    LOGNAME=ellie

    MAILCHECK=600

    MANPATH=/usr/local/OW3/share/man:/usr/local/OW3/man:/

    usr/local/man:/usr/local/doctools/man:/usr/man

    OPTIND=1

    PATH=/home/jody/ellie:/usr/local/OW3/bin:/usr/ucb:/usr/local/

    doctools/bin:/usr/bin:/usr/local:/usr/etc:/etc:/usr/spool/

    news/bin:/home/jody/ellie/bin:/usr/lo

    PS1=$

    PS2=>

    PWD=/home/jody/ellie/kshprog/joke

    SHELL=/bin/sh

    TERM=sun–cmd

    TERMCAP=sun–cmd:te=\E[>4h:ti=\E[>4l:tc=sun:

    USER=ellie

    name=Tom

    place="San Francisco"

EXPLANATION

7.6.4 Unsetting Variables

Both local and environment variables can be unset by using the unset command, unless the variables are set as read-only.

Example 7.23.

unset name; unset TERM

EXPLANATION

The unset command removes the variable from the shell's memory.

7.6.5 Printing the Values of Variables: The echo Command

The echo command prints its arguments to standard output and is used primarily in Bourne and C shells. The Korn shell has a built-in print command. There are different versions of the echo command; for example, the Berkeley (BSD) echo is different from the System V echo. Unless you specify a full pathname, you will use the built-in version of the echo command. The built-in version will reflect the version of UNIX you are using. The System V echo allows the use of numerous escape sequences, whereas the BSD version does not. Table 7.4 lists the BSD echo option and escape sequences.

Example 7.24.

1   $ echo The username is $LOGNAME.

    The username is ellie.

2   $ echo "\t\tHello there\c"   # System V

         Hello there$

3   $ echo -n "Hello there"      # BSD

    Hello there$

Table 7.4. BSD echo Option and System V Escape Sequences

Option	Meaning
BSD
–n	Suppress newline at the end of a line of output
System V
\b	Backspace
\c	Print the line without a newline
\f	Form feed
\n	Newline
\r	Return
\t	Tab
\v	Vertical tab
\\	Backslash

EXPLANATION

7.6.6 Variable Expansion Modifiers

Variables can be tested and modified by using special modifiers. The modifier provides a shortcut conditional test to check if a variable has been set, and then assigns a value to the variable based on the outcome of the test. See Table 7.5 for a list of Bourne shell variable modifiers.

Table 7.5. Bourne Shell Variable Modifiers

Modifier	Value
${variable:–word}	If variable is set and is non-null, substitute its value; otherwise, substitute word.
${variable:=word}	If variable is set or is non-null, substitute its value; otherwise, set it to word. The value of variable is substituted permanently. Positional parameters may not be assigned in this way.
${variable:+word}	If parameter is set and is non-null, substitute word; otherwise, substitute nothing.
${variable:?word}	If variable is set and is non-null, substitute its value; otherwise, print word and exit from the shell. If word is omitted, the message parameter null or not set is printed.

Using the colon with any of the modifiers (–, =, +, ?) checks whether the variable is not set or is null; without the colon, a variable set to null is considered to be set.

Example 7.25.

(Assigning temporary default values)

1   $ fruit=peach

2   $ echo ${fruit:–plum}

    peach

3   $ echo ${newfruit:–apple}

    apple

4   $ echo $newfruit



5   $ echo $EDITOR       # More realistic example



6   $ echo ${EDITOR:-/bin/vi}

    /bin/vi

7   $ echo $EDITOR



8   $ name=

    $ echo ${name-Joe}



9   $ echo ${name:-Joe}

    Joe

EXPLANATION

Example 7.26.

(Assigning permanent default values)

1   $ name=

2   $ echo  ${name:=Patty}

    Patty

3   $ echo $name

    Patty

4   $ echo ${EDITOR:=/bin/vi}

    /bin/vi

5   $ echo $EDITOR

    /bin/vi

EXPLANATION

Example 7.27.

(Assigning temporary alternate values)

1   $ foo=grapes

2   $ echo ${foo:+pears}

    pears

3   $ echo $foo

    grapes

    $ 

EXPLANATION

Example 7.28.

(Creating error messages based on default values)

1   $ echo ${namex:?"namex is undefined"}

    namex: namex is undefined

2   $ echo ${y?}

    y: parameter null or not set

EXPLANATION

7.6.7 Positional Parameters

Normally, the special built-in variables, often called positional parameters, are used in shell scripts when passing arguments from the command line, or used in functions to hold the value of arguments passed to the function. See Table 7.6. The variables are called positional parameters because they are denoted by their position on the command line. The Bourne shell allows up to nine positional parameters. The name of the shell script is stored in the $0 variable. The positional parameters can be set and reset with the set command.

Example 7.29.

1   $ set tim bill ann fred

    $ echo $*      # Prints all the positional parameters.

    tim bill ann fred

2   $ echo $1      # Prints the first positional parameter.

    tim

3   $ echo $2 $3   # Prints the second and third positional parameters.

    bill ann

4   $ echo $#      # Prints the total number of positional parameters.

    4

5   $ set a b c d e f g h i j k l m

    $ echo  $10    # Prints the first positional parameter followed by a zero.

    a0

6   $ echo  $*

    a b c d e f g h i j k l m

7   $ set file1 file2 file3

    $ echo  \$$#

    $3

8   $ eval echo  \$$#

    file3

Table 7.6. Bourne Shell Positional Parameters

Positional Parameter	Meaning
$0	References the name of the current shell script
$1–$9	Denotes positional parameters 1 through 9
$#	Evaluates to the number of positional parameters
$*	Evaluates to all the positional parameters
$@	Means the same as $*, except when double quoted
"$*"	Evaluates to "$1 $2 $3"
"$@"	Evaluates to "$1" "$2" "$3"

EXPLANATION

7.6.8 Other Special Variables

The shell has special variables consisting of a single character. The dollar sign preceding the character allows you to access the value stored in the variable. See Table 7.7.

Example 7.30.

1   $ echo The pid of this shell is $$

    The pid of this shell is 4725

2   $ echo The options for this shell are $–

    The options for this shell are s

3   $ grep dodo /etc/passwd

    $ echo $?

    1

4   $ sleep 25&

    4736

    $ echo $!

    4736

Table 7.7. Bourne Shell Special Variables

Variable	Meaning
$	The PID of the shell
–	The sh options currently set
?	The exit value of last executed command
!	The PID of the last job put in the background

EXPLANATION