Difference between single and double quotes in Bash

Question

In Bash, what are the differences between single quotes ('') and double quotes ("")?

Answer 1

Single quotes won't interpolate anything, but double quotes will. For example: variables, backticks, certain \ escapes, etc.

Example:

$ echo "$(echo "upg")" upg $ echo '$(echo "upg")' $(echo "upg") 

The Bash manual has this to say:

3.1.2.2 Single Quotes

Enclosing characters in single quotes (') preserves the literal value of each character within the quotes. A single quote may not occur between single quotes, even when preceded by a backslash.

3.1.2.3 Double Quotes

Enclosing characters in double quotes (") preserves the literal value of all characters within the quotes, with the exception of $, `, \, and, when history expansion is enabled, !. The characters $ and ` retain their special meaning within double quotes (see Shell Expansions). The backslash retains its special meaning only when followed by one of the following characters: $, `, ", \, or newline. Within double quotes, backslashes that are followed by one of these characters are removed. Backslashes preceding characters without a special meaning are left unmodified. A double quote may be quoted within double quotes by preceding it with a backslash. If enabled, history expansion will be performed unless an ! appearing in double quotes is escaped using a backslash. The backslash preceding the ! is not removed.

The special parameters * and @ have special meaning when in double quotes (see Shell Parameter Expansion).

Answer 2

The accepted answer is great. I am making a table that helps in quick comprehension of the topic. The explanation involves a simple variable a as well as an indexed array arr.

If we set

a=apple      # a simple variable arr=(apple)  # an indexed array with a single element 

and then echo the expression in the second column, we would get the result / behavior shown in the third column. The fourth column explains the behavior.

#	Expression	Result	Comments
1	"$a"	apple	variables are expanded inside ""
2	'$a'	$a	variables are not expanded inside ''
3	"'$a'"	'apple'	'' has no special meaning inside ""
4	'"$a"'	"$a"	"" is treated literally inside ''
5	'\''	invalid	can not escape a ' within ''; use "'" or $'\'' (ANSI-C quoting)
6	"red$arocks"	red	$arocks does not expand $a; use ${a}rocks to preserve $a
7	"redapple$"	redapple$	$ followed by no variable name evaluates to $
8	'\"'	\"	\ has no special meaning inside ''
9	"\'"	\'	\' is interpreted inside "" but has no significance for '
10	"\""	"	\" is interpreted inside ""
11	"*"	*	glob does not work inside "" or ''
12	"\t\n"	\t\n	\t and \n have no special meaning inside "" or ''; use ANSI-C quoting
13	"`echo hi`"	hi	`` and $() are evaluated inside "" (backquotes are retained in actual output)
14	'`echo hi`'	`echo hi`	`` and $() are not evaluated inside '' (backquotes are retained in actual output)
15	'${arr[0]}'	${arr[0]}	array access not possible inside ''
16	"${arr[0]}"	apple	array access works inside ""
17	$'$a\''	$a'	single quotes can be escaped inside ANSI-C quoting
18	"$'\t'"	$'\t'	ANSI-C quoting is not interpreted inside ""
19	'!cmd'	!cmd	history expansion character '!' is ignored inside ''
20	"!cmd"	cmd args	expands to the most recent command matching "cmd"
21	$'!cmd'	!cmd	history expansion character '!' is ignored inside ANSI-C quotes

---

See also:

• ANSI-C quoting with $'' - GNU Bash Manual
• Locale translation with $"" - GNU Bash Manual
• A three-point formula for quotes