Is there any difference in bash shell in [ .. ] and [[ .. ]] ?

Shell Programming and Scripting
1

Could someone tell me if there is any difference in using square brackets single or doubled pair?

So, do those two some how could be treted by shell differently:

if [ <condition> ] ; then <statements;> else <statements;> fi

and

if [[ <condition> ]] ; then <statements;> else <statements;> fi

Maybe any one could provide with any additional shell usage compare to another?

Thank you,
Alex

2

If you use "[.....]" you are actually using /bin/test (/bin/[). Try man [ and you will get the test(1) manpage.

If you use [[.....]] you are using bash's builtin test . Using a builtin is faster and more efficient since no fork/exec is required.

3

These days they are both built-ins:

$ echo $BASH_VERSION
3.2.39(19)-release
$ type [
[ is a shell builtin
$ type [[
[[ is a shell keyword
$

[ is included mostly for backward compatibility to Bourne shell and the /usr/bin/test that fpmurphy mentioned. I usually prefer the syntax of [[.

4

Thank you, guys for reply!
So, from user point of view there is no difference in usage [ ... ] or [[ ... ]]?
Is it correct?

Having by today both as a part of shell (buildin and keyword) now it is really hard to say witch one could be more efficient, right?

5

See The UNIX Forums - View Single Post - Shell basics

6

For portability use [.....] . For efficiency, use [[.....]]