What is the difference between double and single square brackets [ and [[ as well as equal signs = and ==?

Question

I cannot find the difference between:

[ $foo = b* ]
[ $foo == b* ]

[[ $foo = b* ]]
[[ $foo == b* ]]

Answer 1

[] vs [[ ]] in general was covered at Is [[ ]] preferable over [ ] in bash scripts? so let's not touch it.

= vs ==:

Inside [ ]

• = is POSIX and checks if strings are equal
• == is a Bash defined alias to it, see man bash.

So only use = as it is both more portable and shorter.

Inside [[ ]]

[[ ]] is not POSIX, so we refer only to man bash.

It says under CONDITIONAL EXPRESSIONS that = and == are the same inside [[ ]] and that both do pattern matching as described at "Compound Commands".

Then "Compound Commands" is the same pattern used for glob expansion, e.g.:

[[ 'abcde' = ?b[cC]* ]] && echo true

possibly with extglob extensions.

Note that you must not quote the pattern: it will not glob expand to files as usual. If you quote it, you lose the pattern matching magic and get literal characters.

Answer 2

Difference between [] and [[]] check this

Difference between = and ==

= is assignment operator

== equality check operator OR conditional operator