Shell Programming and Scripting

To elaborate further, while, as already mentioned, [ and test are synonyms,
the [[ version introduces some important improvements. Notice that the double bracket
syntax is not standard and is not supported by some shells.

So, some of the differences are that in [[ ... ]] context:

1. No shell word splitting and path expansion (globbing) are performed:

Code:

bash-4.1.9(1)[t]$ touch aa
bash-4.1.9(1)[t]$ x='a* b'
bash-4.1.9(1)[t]$ set -x;[ $x == 'a b' ];set +x
+ '[' aa b == 'a b' ']'
bash: [: too many arguments
+ set +x
bash-4.1.9(1)[t]$ set -x;[[ $x == 'a b' ]];set +x
+ [[ a* b == \a\ \b ]]
+ set +x

2. More readable(?) boolean operators are supported.

For example, you can use:

Code:

[[ 1 -gt 0 && 2 -gt 1 ]]

Instead of:

Code:

[ 1 -gt 0  -a  2 -gt 1 ]

3. Some shells support an additional binary operator, =~.
When it is used, the string to the right of the operator
is considered an extended regular expression:

Code:

bash-4.1.9(1)[t]$ [[ aa =~ ^a+ ]] && echo matches
matches

It may help your understanding further to realise that [ is actually a binary. See this dialogue from my Debian Lenny system :

Code:

mega:~# cd /usr/bin
mega:/usr/bin# ls -l '['
-rwxr-xr-x 1 root root 34504 2008-04-04 15:22 [
mega:/usr/bin# ls -l test
-rwxr-xr-x 1 root root 22852 2008-04-04 15:22 test

As an historical note, on an old SVR0 Unix box I once administered, [ and test were hard linked, i.e. they were the same binary. As you can see from the output above, this is not the case in Linux.

Regards
Neil

Although most shells use regular shell-builtins to overrule the binaries and speed things up nowadays....