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