Re: [[ and [

[Peter Stephenson <Peter.Stephenson@xxxxxxx>]

On Tue, 6 Mar 2012 15:05:41 +0400
sergio <mailbox@xxxxxxxxxxxxx> wrote:
> On 03/06/2012 10:05 AM, Bart Schaefer wrote:
> 
> What was the reason to implement [[ ]]? What advantages it gives? As
> I see [[ ]] and [ ] provides same features.

[[ ... ]] is specially parsed: this gives better error checking and an
assurance that each argument means what you think it does.

Consider:

  foo=
  [ $foo -eq "" ]

With normal shell behaviour, this turns into the arguments "[", "-eq",
"]", which confuses the test command.  This is why you see all those
"x"s and double quotes in configure scripts.  However,

  [[ $foo -eq "" ]]

does exactly what it looks like it does because "[[" is special to the
shell and it looks at the raw arguments to parse the syntax.

Generally, I get the impression Bourne shell syntax was something of a
leap in the dark at the time; if you were designing it from scratch now,
it would look considerably different in a lot of ways.

-- 
Peter Stephenson <pws@xxxxxxx>            Software Engineer
Tel: +44 (0)1223 692070                   Cambridge Silicon Radio Limited
Churchill House, Cambridge Business Park, Cowley Road, Cambridge, CB4 0WZ, UK


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