if statement - Difference between single and double square brackets in Bash

Question

I'm reading bash examples about if but some examples are written with single square brackets:

if [ -f $param ]
then
  #...
fi

others with double square brackets:

if [[ $? -ne 0 ]]
then
    start looking for errors in yourlog
fi

What is the difference?

Answer 1

Single [] are posix shell compliant condition tests.

Double [[]] are an extension to the standard [] and are supported by bash and other shells (e.g. zsh, ksh). They support extra operations (as well as the standard posix operations). For example: || instead of -o and regex matching with =~. A fuller list of differences can be found in the bash manual section on conditional constructs.

Use [] whenever you want your script to be portable across shells. Use [[]] if you want conditional expressions not supported by [] and don't need to be portable.