discussed in the following sections.
Using Double Quotes to Resolve Variables in Strings with Embedded Spaces
If a string contains embedded spaces, you can enclose the string in double quotes (') so that the shell interprets the whole string as one entity instead of more than one.
For example, if you assigned the value of abc def (abc followed by one space, followed by def) to a variable called x in a shell program as follows, you would get an error because the shell would try to execute def as a separate command:
| Command | Environment |
|---|---|
x=abc def | bash |
set x=abc def | tcsh |
The shell executes the string as a single command if you surround the string in double quotes, as follows:
| Command | Environment |
|---|---|
x='abc def' | bash |
set x='abc def' | tcsh |
The double quotes resolve all variables within the string. Here is an example for bash:
var='test string'
newvar='Value of var is $var'
echo $newvar
If you execute a shell program containing the preceding three lines, you get the following result:
Value of var is test string
Using Single Quotes to Maintain Unexpanded Variables
You can surround a string with single quotes (') to stop the shell from expanding variables and interpreting special characters. When used for the latter purpose, the single quote is an escape character, similar to the backslash, which you learn about in the next section. Here, you learn how to use the single quote to avoid expanding a variable in a shell script. An unexpanded variable maintains its original form in the output.
In the following examples, the double quotes in the preceding examples have been changed to single quotes:
var='test string'
newvar='Value of var is $var'
echo $newvar
If you execute a shell program containing these three lines, you get the following result:
Value of var is $var
As you can see, the variable var maintains its original format in the results, rather than having been expanded.
Using the Backslash As an Escape Character
As you learned earlier, the backslash () serves as an escape character that stops the shell from interpreting the succeeding character as a special character. Imagine that you want to assign a value of $test to a variable called var. If you use the following command, the shell reads the special character $ and interprets $test as the value of the variable test. No value has been assigned to test; a null value is stored in var as follows:
| Command | Environment |
|---|---|
var=$test | bash |
set var=$test | tcsh |
Unfortunately, this assignment might work for bash, but it returns an error of undefined variable if you use it with tcsh. Use the following commands to correctly store $test in var:
| Command | Environment |
|---|---|
