Re: Something Odd - "top" Delivers Nada When Run By Crontab

On 7/11/26 00:34, Rich wrote:
> c186282 <[email protected]> wrote:
>> Wrote a short Python script that uses os.system() to run 'top' ...
>> redirecting the output to a file.  We then clean-up the weird chars
>> in the file and append it to an ongoing log file.
>>
>> Run the script from the CL and it works perfectly.
>>
>> Run it via root crontab though and the top redirect file exists, but
>> is EMPTY.
>>
>> ...
>>
>> The line :
>>    os.system("top -n 1 > topp.txt")
>> We then clean up weird chars and append it to the real log file.  DID
>> check if there was any output BEFORE the clean-ups.  Nada.
>>
>> I've tried firming up the path - but I do an os.chdir() anyway using
>> sys.argv[0] at the top of the script anyhow.  Regardless, the 'top'
>> output delivers NOTHING to the dest file.  DOES create it, but ...
>>
>> So ... why doesn't this work ???
> 
> Very likely because you overlooked the "batch" CLI argument:
> 
>      -b  :Batch-mode operation
>          Starts top in Batch mode, which could be useful for sending
>          output from top to other programs or to a file.  In this mode,
>          top will not accept input and runs until the iterations limit
>          you've set with the `-n' command-line option or until killed.


   I'll try it ! :-)

   Still weird the CL -vs- Crontab behavior is SO different
   for such an old basic utility.


>> 'top' is 'ubiquitous', nothing new there.
> 
> Top also expects (in normal operation mode) to be connected to a tty so
> it can receive input from a user.  When run from a crontab there is no
> tty, and top likely aborts when it finds that it's file descriptor zero
> is not a tty.  The batch mode argument is described in the man page in
> such a way that it implies it turns off the "check that fd[0] is a tty"
> test.
> 
>> The different behavior from the CL vs crontab is puzzling.  Been
>> screwing with this all day, trying lots of little variants.
> 
> It is not at all puzzling for anyone who understands the difference
> between programs that expect tty's and those that don't.

   I do know there's a diff - but didn't expect it HERE.

   BTW, tried both a conventional crontab entry AND one
   with a "&" at the end - which usually launches an
   independent process.

   'Top' is USUALLY used as something that's constantly
   running kinda like 'ping'. However the "-n" param lets
   it do just ONE scan and then exit. Yea, there are other,
   really grungy, ways to get similar info, but why re-invent
   what's basically already there ???