CHAPTER 9 Shell Programming

9.5 Parameter Substitution

$parameter substitute the value of parameter for this string

${parameter} same as above. The brackets are helpful if there's no separation between this parameter and a neighboring string.

$parameter= sets parameter to null.

${parameter-default} if parameter is not set, then use default as the value here. The parameter is not reset.

${parameter=default} if parameter is not set, then set it to default and use the new value

${parameter+newval) if parameter is set, then use newval, otherwise use nothing here. The parameter is not reset.

${parameter?message} if parameter is not set, then display message. If parameter is set, then use its current value.

There are no spaces in the above operators. If a colon (:) is inserted before the -, =, +, or ? then a test if first performed to see if the parameter has a non-null setting.

The C shell has a few additional ways of substituting parameters:

$list[n] selects the nth word from list

${list[n]} same as above

$#list report the number of words in list

$?parameter return 1 if parameter is set, 0 otherwise

${?parameter} same as above

$< read a line from stdin

The C shell also defines the array, $argv[n] to contain the n arguments on the command line and $#argv to be the number of arguments, as noted in Table 9.1.

To illustrate some of these features we'll use the test script below.

#!/bin/sh

param0=$0

test -n "$1" && param1=$1

test -n "$2" && param2=$2

test -n "$3" && param3=$3

echo 0: $param0

echo "1: ${param1-1}: \c" ;echo $param1

echo "2: ${param2=2}: \c" ;echo $param2

echo "3: ${param3+3}: \c" ;echo $param3

In the script we first test to see if the variable exists, if so we set a parameter to its value. Below this we report the values, allowing substitution.

In the first run through the script we won't provide any arguments:

$ ./parameter.sh

0: ./parameter.sh # always finds $0

1: 1: # substitute 1, but don't assign this value

2: 2: 2 # substitute 2 and assign this value

3: : # don't substitute

In the second run through the script we'll provide the arguments:

$ ./parameter one two three

0: ./parameter.sh # always finds $0

1: one: one # don't substitute, it already has a value

2: two: two # don't substitute, it already has a value

3: 3: three # substitute 3, but don't assign this value