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

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.

> '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.