Bash
StackOverflow
a=apple # a simple variable
arr=(apple) # an indexed array with a single element
# | 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 ""
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 ""
14 | '`echo hi`' | `echo hi` | `` and $() are not evaluated inside ''
15 | '${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 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"
---+-------------+-------------+--------------------------------------------------------------------