CHAPTER 9 Shell Programming

9.4 Variables

These shell variables are:

Shell Variables

Variable	Usage	sh	csh
$#	number of arguments on the command line	x
$-	options supplied to the shell	x
$?	exit value of the last command executed	x
$$	process number of the current process	x	x
$!	process number of the last command done in background	x
$n	argument on the command line, where n is from 1 through 9, reading left to right	x	x
$0	the name of the current shell or program	x	x
$*	all arguments on the command line ("$1 $2 ... $9")	x	x
$@	all arguments on the command line, each separately quoted ("$1" "$2" ... "$9")	x
$argv[n]	selects the nth word from the input list		x
${argv[n]}	same as above		x
$#argv	report the number of words in the input list		x

We can illustrate these with some simple scripts. First for the Bourne shell the script will be:

#!/bin/sh

echo "$#:" $#

echo '$#:' $#

echo '$-:' $-

echo '$?:' $?

echo '$$:' $$

echo '$!:' $!

echo '$3:' $3

echo '$0:' $0

echo '$*:' $*

echo '$@:' $@

When executed with some arguments it displays the values for the shell variables, e.g.:

$ ./variables.sh one two three four five

5: 5

$#: 5

$-:

$?: 0

$$: 12417

$!:

$3: three

$0: ./variables.sh

$*: one two three four five

$@: one two three four five

As you can see, we needed to use single quotes to prevent the shell from assigning special meaning to $. The double quotes, as in the first echo statement, allowed substitution to take place.

Similarly, for the C shell variables we illustrate variable substitution with the script:

#!/bin/csh -f

echo '$$:' $$

echo '$3:' $3

echo '$0:' $0

echo '$*:' $*

echo '$argv[2]:' $argv[2]

echo '${argv[4]}:' ${argv[4]}

echo '$#argv:' $#argv

which when executed with some arguments displays the following:

% ./variables.csh one two three four five

$$: 12419

$3: three

$0: ./variables.csh

$*: one two three four five

$argv[2]: two

${argv[4]}: four

$#argv: 5