From the bash manual:
220.127.116.11 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.
18.104.22.168 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).
Here are some practical demonstrations of the above:
- Double quotes allow evaluation of parameter expansions, but single quotes do not:
$ echo "$var"
$ echo '$var'
- Double quotes allow evaluation of backslash escapes, but single quotes do not:
$ echo "\\"
$ echo '\\'
- Double quotes allow evaluation of command substitutions (
`), but single quotes do not:
$ echo "$(echo bar)"
$ echo '$(echo bar)'