Shell Script Tutorial

Variable

Let's look at some examples that show when to use each method of quoting.

USER="alan"

var1='Good Morning $USER !'
echo $var1

Result: Good Morning $USER !

var2="Good Morning $USER !"
echo $var2

Good Morning alan !

In the first case, the value of the 'var1' variable would probably not be what you wanted. The single quotation marks caused Bash to not treat $USER as a variable. In the second case, the results look much better. The double quotation marks allowed Bash to substitute the value of $USER, which is set automatically when you log in, in the string.

Here's another example that demonstrates a common error:

msg="Price is $5.00"
echo $msg

Actual result: Price is .00

We thought enough to quote the string, but the dollar sign tells Bash to use the value in the $5 variable, which is not what we wanted.. We can easily solve the problem by prefixing the dollar sign with a backslash, as shown here:

msg="Price is \$5.00"
echo $msg

Actual result: Price is $5.00

· Single quotation marks, as in the preceding example, will always get you exactly what's inside the quotation marks--any characters that might otherwise have special meaning to the shell (like the dollar sign or the backslash) are treated literally.

· Use double quotation marks when you want to assign a string that contains special characters the shell should act on.

· The backslash is used to escape (treat literally) a single character (such as $ or *) that might otherwise be treated as a special character by the shell.