bash null conditional

[Bill Davidsen]

Craig White wrote:
> On Mon, 2009-03-30 at 13:33 -0500, Justin Willmert wrote:
>> Craig White wrote:
>>> I'm in my bash book and looking on web but can't seem to resolve this
>>> simple problem.
>>>
>>> $ if [ -n "grep A121 myfile.csv" ]; then echo "null"; fi
>>> null
>>>
>>> $ if [ -n "grep A125 myfile.csv" ]; then echo "null"; fi
>>> null
>>>
>>> A125 definitely is null when I just run the grep command in the quotes
>>> but A121 definitely is not null.
>>>
>>> What am I missing on the if/null operator here?
>>>
>>> Craig
>>>
>> Have you tried
>>     $ if [ -n $(grep A121 myfile.csv) ]; then echo "null"; fi
>>     $ if [ -n $(grep A125 myfile.csv) ]; then echo "null"; fi
>>
>> The double quotes make a string, but it looks like you which to be 
>> executing grep which is accomplished by using the $() notation.  You 
>> could also use a pair of backticks surrounding the command.
> ----
> I started with backticks...
> 
> $ if [ -n `grep A125 ARdebtorsmaster.csv` ]; then echo "null"; fi
> null
> $ if [ -n `grep A121 ARdebtorsmaster.csv` ]; then echo "null"; fi
> bash: [: too many arguments
> 
> which interestingly is also the same problem that I get with your first
> suggestion...
> 
> $ if [ -n $(grep A125 ARdebtorsmaster.csv) ]; then echo "null"; fi
> null
> $ if [ -n $(grep A121 ARdebtorsmaster.csv) ]; then echo "null"; fi
> bash: [: too many arguments
> 
> and then of course, light bulb goes off...I have to figure out how to
> get a basic exit code from grep, but seemingly the -s and the -q aren't
> the answer.
> 
They are. You want
   if ! grep -q pat file; then echo null; fi
or my personal choich
   grep -q pat file || echo null

-- 
Bill Davidsen <davidsen at tmr.com>
   "We have more to fear from the bungling of the incompetent than from
the machinations of the wicked."  - from Slashdot