Difference between [ and [[ in if statements [duplicate]

Question

The following code

if [ $a == "apple" ];
then
    echo "True"
else
    echo "False"
fi

outputs "True" ("False") if a="apple" (a="plum"). The comparison fails if one uses wildcards:

if [ $a == "appl"* ];

and is fixed if one replaces [ by [[:

if [[ $a == "appl"* ]];

What is the difference between [ and [[ in if statements?

Answer 1

• [ is a command (basically a variant of the test command). [[ is a builtin in many shells.
• When you write foo* inside [...] filename expansion (aka globbing) occurs; while inside [[...]] pattern matching occurs.
• [[ is more powerful and capable than [ but should not be used if portability is a concern.
• [ and [[ are not part of the if syntax, you can use them as in [ "$exit" = 'yes' ] && exit.
• Inside [...] you should prefer = instead of ==. As far as I know, the second one is accepted in many shells but is not POSIX-compliant.

By the way, I recommend you to double-quote your variables even if you really know how word splitting will behave.