5.1. Quoting Variables

When referencing a variable, it is generally advisable to enclose its name in double quotes. This prevents reinterpretation of all special characters within the quoted string -- the variable name [1] -- except $, ` (backquote), and \ (escape). [2] Keeping $ as a special character within double quotes permits referencing a quoted variable ("$variable"), that is, replacing the variable with its value (see Example 4-1, above).

Use double quotes to prevent word splitting. [3] An argument enclosed in double quotes presents itself as a single word, even if it contains whitespace separators.

List="one two three" for a in $List # Splits the variable in parts at whitespace. do echo "$a" done # one # two # three echo "---" for a in "$List" # Preserves whitespace in a single variable. do # ^ ^ echo "$a" done # one two three

A more elaborate example:

variable1="a variable containing five words" COMMAND This is $variable1 # Executes COMMAND with 7 arguments: # "This" "is" "a" "variable" "containing" "five" "words" COMMAND "This is $variable1" # Executes COMMAND with 1 argument: # "This is a variable containing five words" variable2="" # Empty. COMMAND $variable2 $variable2 $variable2 # Executes COMMAND with no arguments. COMMAND "$variable2" "$variable2" "$variable2" # Executes COMMAND with 3 empty arguments. COMMAND "$variable2 $variable2 $variable2" # Executes COMMAND with 1 argument (2 spaces). # Thanks, Stéphane Chazelas.

Tip

Enclosing the arguments to an echo statement in double quotes is necessary only when word splitting or preservation of whitespace is an issue.

Example 5-1. Echoing Weird Variables

#!/bin/bash # weirdvars.sh: Echoing weird variables. var="'(]\\{}\$\"" echo $var # '(]\{}$" echo "$var" # '(]\{}$" Doesn't make a difference. echo IFS='\' echo $var # '(] {}$" \ converted to space. Why? echo "$var" # '(]\{}$" # Examples above supplied by Stephane Chazelas. exit 0

Single quotes (' ') operate similarly to double quotes, but do not permit referencing variables, since the special meaning of $ is turned off. Within single quotes, every special character except ' gets interpreted literally. Consider single quotes ("full quoting") to be a stricter method of quoting than double quotes ("partial quoting").

Note

Since even the escape character (\) gets a literal interpretation within single quotes, trying to enclose a single quote within single quotes will not yield the expected result.
echo "Why can't I write 's between single quotes" echo # The roundabout method. echo 'Why can'\''t I write '"'"'s between single quotes' # |-------| |----------| |-----------------------| # Three single-quoted strings, with escaped and quoted single quotes between. # This example courtesy of Stéphane Chazelas.

Notes

[1]

It also has side-effects on the value of the variable (see below)

[2]

Encapsulating "!" within double quotes gives an error when used from the command line. This is interpreted as a history command. Within a script, though, this problem does not occur, since the Bash history mechanism is disabled then.

Of more concern is the apparently inconsistent behavior of "\" within double quotes.

bash$ echo hello\! hello! bash$ echo "hello\!" hello\! bash$ echo -e x\ty xty bash$ echo -e "x\ty" x y 

What happens is that double quotes normally escape the "\" escape character, so that it echoes literally. However, the -e option to echo changes that. It causes the "\t" to be interpreted as a tab.

(Thank you, Wayne Pollock, for pointing this out, and Geoff Lee for explaining it.)

[3]

"Word splitting", in this context, means dividing a character string into a number of separate and discrete arguments.