Fedora™ Unleashed, 2008 edition

var=$test	bash
set var = $test	tcsh

The backslash before the dollar sign ($) signals the shell to interpret the $ as any other ordinary character and not to associate any special meaning to it. You could also use single quotes (') around the $test variable to get the same result.

Using the Backtick to Replace a String with Output

You can use the backtick (`) character to signal the shell to replace a string with its output when executed. This special character can be used in shell programs when you want the result of the execution of a command to be stored in a variable. For example, if you want to count the number of lines in a file called test.txt in the current directory and store the result in a variable called var, you can use the following command:

Command	Environment
var=`wc -l test.txt`	bash
set var = `wc -l test.txt`	tcsh

Comparison of Expressions in bash

Comparing values or evaluating the differences between similar bits of data — such as file information, character strings, or numbers — is a task known as comparison of expressions. Comparison of expressions is an integral part of using logic in shell programs to accomplish tasks. The way the logical comparison of two operators (numeric or string) is done varies slightly in different shells. In bash, a command called test can be used to achieve comparisons of expressions. In tcsh, you can write an expression to accomplish the same thing.

The bash shell syntax provides a command named test to compare strings, numbers, and files. The syntax of the test command is as follows:

test expression

or

[ expression ]

Both forms of the test commands are processed the same way by bash. The test commands support the following types of comparisons:

> String comparison

> Numeric comparison

> File operators

> Logical operators

String Comparison

The following operators can be used to compare two string expressions:

> = — To compare whether two strings are equal

> != — To compare whether two strings are not equal

> -n — To evaluate whether the string length is greater than zero

> -z — To evaluate whether the string length is equal to zero

Next are some examples using these operators when comparing two strings, string1 and string2, in a shell program called compare1:

#!/bin/sh
string1='abc'
string2='abd'
if [ $string1 = $string2 ]; then
 echo 'string1 equal to string2'
else
 echo 'string1 not equal to string2'
fi
if [ $string2 != string1 ]; then
 echo 'string2 not equal to string1'
else
 echo 'string2 equal to string2'
fi
if [ $string1 ]; then
 echo 'string1 is not empty'
else
 echo 'string1 is empty'
fi
if [ -n $string2 ]; then
 echo 'string2 has a length greater than zero'
else
 echo 'string2 has length equal to zero'
fi
if [ -z $string1 ]; then