bashif-statementsyntax

Are double square brackets [[ ]] preferable over single square brackets [ ] in Bash?


A coworker claimed recently in a code review that the [[ ]] construct is to be preferred over [ ] in constructs like

if [ "`id -nu`" = "$someuser" ] ; then
     echo "I love you madly, $someuser"
fi

He couldn't provide a rationale. Is there one?


Solution

  • [[ has fewer surprises and is generally safer to use. But it is not portable - POSIX doesn't specify what it does and only some shells support it (beside bash, I heard ksh supports it too). For example, you can do

    [[ -e $b ]]
    

    to test whether a file exists. But with [, you have to wrap $b in double-quotes, because otherwise it splits the argument and expands things like "a*" (where [[ takes it literally). That has also to do with how [ can be an external program and receives its argument just normally like every other program (although it can also be a builtin, but then it still has not this special handling).

    [[ also has some other nice features, like regular expression matching with =~ along with operators like they are known in C-like languages. Here is a good page about it: What is the difference between test, [ and [[ ? and Bash Tests