Linux System Administration

bash Beginnings

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Variables

bash is a programming language, and programming languages have common fea-

tures. One of the most basic is the variable: a symbol that contains a value. bash vari-

ables are strings unless you specify otherwise with a

declare statement. You don’t

need to declare or define bash variables before you use them, unlike with many other

languages.

A variable’s name is a string starting with a letter and containing letters, numbers, or

underscores (

_). A variable’s value is obtained by putting a $ character before the

variable’s name. Here’s a shell script that assigns a string value to the variable

hw,

then prints it:

#!/bin/bash

hw="hello world"

echo $hw

The variable hw is created by the assignment in line 2. In line 3, the contents of the

variable

hw will replace the $hw reference. Because bash and other shells treat

whitespace characters (spaces and tabs) as command argument separators rather

than normal argument characters, to preserve them you must surround the whole

string with double quote (

") or single quote (') characters. The difference is that shell

variables (and other special shell syntax) are expanded within double quotes and

treated literally within single quotes. Look at the difference in output from the two

echo commands in the following script:

admin@server1:~$ cat hello2

#!/bin/bash

hw="hello world"

echo "$hw"

echo '$hw'

admin@server1:~$ ./hello2

hello world

$hw

admin@server1:~$ ...