# The builtin test operator



In [bash](http://www.gnu.org/software/bash) (and other shells) you
can use the [[ construct

From the bash manpage:

	[[ expression ]]
              Return a status of 0 or 1 depending on  the  evaluation  of  the
              conditional  expression expression.  Expressions are composed of
              the primaries described  below  under  CONDITIONAL  EXPRESSIONS.

              Word  splitting  and pathname expansion are not performed on the

              words between the [[ and  ]];  tilde  expansion,  parameter  and
              variable  expansion, arithmetic expansion, command substitution,
              process substitution, and quote removal are  performed.   Condi‐
              tional operators such as -f must be unquoted to be recognized as
              primaries.

So no *word splitting* and *pathname expansion*.

# A little example

First we have some setting up to do

    $ mkdir ~/tmp/zz
    $ touch ~/tmp/zz/111
    $ echo ~/tmp/zz/*
    /home/miekg/tmp/zz

Well, lets test [[

    $ if [[  tmp/zz/* = tmp/zz/111 ]]; then echo OK; else echo NOK; fi
    NOK

NOK - that's correct, because [[ will not do pathname expansion.

The other way around

    $ if [[  tmp/zz/111 = tmp/zz/* ]]; then echo OK; else echo NOK; fi
    OK

Huh, `OK`?

 Why
To see why bash behaves as it does, the best explanation comes from
the `ksh` manual. Yes, the [K shell](http://www.kornshell.com/). 
In the section 'Conditional Expressions' of ksh's manual page

    string = pattern        True, if string matches pattern.

Aha, so the right side of a `=` test is a *pattern* and a such
it *is* expanded *even though* we put it in a [[ construct.

In the bash manual we just have

    string1 == string2
	True if the strings are equal.  = may be used
	in place of == for strict POSIX compliance.

Which is not *entirely* correct.

In [zsh](http://http://www.zsh.org/) (which is of course *the* shell) in the section
'CONDITIONAL EXPRESSIONS' it is also correct

    string = pattern
    string == pattern
        true if string matches pattern.  The ‘==’ form is the  preferred
        one.   The  ‘=’ form is for backward compatibility and should be
        considered obsolete.

So this might be something worth considering.