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[Paul Howarth]

Dan Track wrote:
> On 4/7/06, Stuart Sears <stuart at sjsears.com> wrote:
>> -----BEGIN PGP SIGNED MESSAGE-----
>> Hash: SHA1
>>
>> Dan Track wrote:
>>> On 4/7/06, Paul Howarth <paul at city-fan.org> wrote:
>>>> Dan Track wrote:
>>>>> Hi
>>>>>
>>>>> Simple question, I'm running a command `command`, what I'd like to do
>>>>> is check to see if the response is empty then exit. Does anyone know
>>>>> how I can perform that check?
>>>> Exit if "command" produces no output:
>>>> [ -z "$(command)" ] && exit
>>>>
>>>> Paul.
>>> Hi
>>>
>>> Incidentally. What do you have $() instead of just ().
>> $(command) == `command`
>>
>> otherwise you would be testing the length of the literal string '(command)'
>>
>> this way we are running command and testing the length of its output.
>>
>> Regards
>>
>> Stuart
>> - --
> 
> Hi
> Thanks for that. Although never having used it I thought "(command)"
> meant execute the command in a child shell. I thought the pranthesis
> represented anew shell. I take it I'm definitely wrong. If I am wrong
> then what is the difference between (command)=="" and `command`==""

(command)
does indeed mean execute the command in a child shell.

$(command)
is equivalent to
`command`
and represents the standard output from running command

"(command)"
is the literal text "(command)"

"$(command)" is equivalent to
$(command)
but the quotes guard against spaces in the output from running command 
being used by the shell to split the output into multiple words, as
[ -z $(command) ]
would likely generate syntax errors if there were spaces in the output 
of running command.

Paul.