Re: writing to /dev/stdout fails in cron script

[Tony van der Hoff <tony@vanderhoff.org>]

On 22/08/14 18:02, Sven Hartge wrote:
> Tony van der Hoff <tony@vanderhoff.org> wrote:
>> On 22/08/14 15:57, Sven Hartge wrote:
>>> Tony van der Hoff <tony@vanderhoff.org> wrote:
>>>  
>>>> Cron task is invoked from a simple crontab entry:
>>>> # /home/tony/crontab -- crontab for user tony
>>>> MAILTO=tony@vanderhoff.org
>>>> SHELL=/bin/sh
>>>> <snip>
>>>> * * * * *       /home/tony/scripts/test
>>>
>>>> tony@tony-fr:~$ ls -al /dev/stdout
>>>> lrwxrwxrwx 1 root root 15 Aug 21 17:30 /dev/stdout -> /proc/self/fd/1
>>>
>>>> tony@tony-fr:~$ ls -ald /dev
>>>> drwxr-xr-x 15 root root 3340 Aug 21 17:30 /dev
>>>
>>> At least you don't have a multi-gigabyte /dev/stdout-file on your system
>>> filled with the output of numerous scripts from years ago :)
>>>
>>> I'd guess /proc/self/fd/1 is different for programs invoked from cron,
>>> because cron redirects stdout to catch any output from programs.
>>>
>> Well, indeed. The output from cron scripts normally goes to mail in this
>> case. I'm rather hoping that is where the output from stdout would go.
> 
> If you want to have an output which is selectable between debug and
> no-debug output it would be better to write a small shell function and
> use that instead of "echo" to print things. Much more flexible, you can
> even easily add a timestamp on front of every messagen etc.
> 
>>> Please add something like this:
>>>
>>>   ls -al /proc/self/fd/1 /dev/stdout 2>&1 > /tmp/testoutput
>>>
>>> to your script and provide the output. Maybe this will be helpful.
>>>
>> lrwxrwxrwx 1 root root 15 Aug 21 17:30 /dev/stdout -> /proc/self/fd/1
>> l-wx------ 1 tony tony 64 Aug 22 16:39 /proc/self/fd/1 -> /tmp/testoutput
> 
>> Not sure how that helps :(
> 
> No, but this was my fault. "/proc/self/fd/1" is magic, it always points
> to the stdout of current program. If you redirect it, like I asked you
> to do, then of course /proc/self/fd/1 points to the new target.
> 
> So please only use "ls -al /proc/self/fd/1 /dev/stdout" and paste the
> output from the mail you get.
>

Thanks Sven. FWIW that yields:
lrwxrwxrwx 1 root root 15 Aug 21 17:30 /dev/stdout -> /proc/self/fd/1
lrwx------ 1 tony tony 64 Aug 23 10:35 /proc/self/fd/1 ->
/tmp/tmpfVNGK0G (deleted)

However, from reading Karl Jorgensen's post, I now think I understand
what's happening, and I can't explicitly send stuff to stdout from a
cron script. Ah, well...

Thanks for your help.

-- 
Tony van der Hoff  | mailto:tony@vanderhoff.org
Ariège, France     |