Bash script : parent / child process issue ?

[mohicann]

Hello there,

I managed to write a bash script that, among other functionalities, launches another sub-process which is not another bash script but a binary (which is supposed to be run only by bash). When I launch directly my "parent" script, everything works perfectly, subprocess included. But once it starts automatically through cron, the binary subprocess doesn't work anymore.

My crontab command was the following, in order to check and restart only if necessary my script:

crontab -l | { cat; echo "*/1 * * * * /bin/bash pgrep -f ~/Documents/Linux/checker > /dev/null || ~/Documents/Linux/checker"; } | crontab -

Is there anything (or a lot of things) I've done incorrectly? Thanks in advance!

[TheFu]

~/ doesn't work in cron. Most environment variables aren't set.

[mohicann]

Thank you for your reply. Actually, I tried to write and run some small test loop scripts with the same crontab command in order to figure out where the issue was coming from, and nonetheless it worked. But they didn't have child processes. That's why I was focusing on this possibility.

PS: I probably got your idea. I need to check a few more things indeed, for instance the relative paths in my "parent" script.

[TheFu]

Compare the output of

env > /home/mohicann/env.interactive

to running it from a crontab

env > /home/mohicann/env.cron

You'll see what is and isn't set that way.

[mohicann]

Thank for you reply. There are exactly the same. I assume it doesn't come from relative and absolute paths?

[papibe]

Hi mohicann.

You don't need to use /bin/bash on your crontab line. This should work:

*/1 * * * * pgrep -f ~/Documents/Linux/ch...

~ works depending on the shell used by the distribution. crontab uses /bin/sh. In Ubuntu /bin/sh is a link to /bin/dash. dash expands the ~ to your home directory

Thus, in Ubuntu ~ works in crontab. However, if you want portability, use absolute paths instead.

Hope it helps. Let us know how it goes.
Regards.

[TheFu]

Thank for you reply. There are exactly the same. I assume it doesn't come from relative and absolute paths?

I suspect you aren't doing it correct then. Let me check mine. Ok ... that ran.

$ wc -l  env*
  16 env.cron
  74 env.int

The interactive env is VERY DIFFERENT from the env under cron.
Oddly, HOME and PWD are set - those are both new to me in a crontab. Very unexpected. It wasn't that way 10 yrs ago.

SHELL=/bin/sh
PATH=/usr/bin:/bin
PWD=/home/thefu

But a bunch of other variables are missing and the PATH is certainly tiny under cron. USER is not set, but LOGNAME is. Interesting.

Much of my cron knowledge comes from UNIX systems, so some things are a little different.

[mohicann]

Thank you for your replies and information.

I've checked with this crontab command line:

crontab -l | { cat; echo "30 * * * * /home/mohicann/Documents/Linux/checker"; } | crontab -

It starts but my subprocess still doesn't work.

I've checked my script with bash -x and it runs perfectly, and when it is started manually, everything works fine. There really should be a trick somewhere between cron and bash or cron and my subprocess that I still don't get. Moreover, if I fill in the crontab command with my script command line (absolute path)) that starts my subprocess, it doenst' work either while it works perfectly from a bash console. It seems it can only work through my script alone or a bash console.

[TheFu]

Simplify and test.
What, exactly, is the line in the crontab? I know what you think it is, but what is it really?
Which crontab, exactly, is being used?
What are the file permissions for the script?
Does the script depend on any interactive environment variables? I bet it does.

[mohicann]

I've added absolute paths in my script and a chmod 777 to my subprocess (as my script was already).

I'm using my user crontab but I've noticed something: when my script is launched manually (double click), its parent is dolphin, and when it's launched by crontab, its parent is sh. My subprocess (when it works) always has my script as parent. However, when my subprocess is directly started from a console as a main process, it has bash as parent. Indeed, there could be something set that doesn't work, such as an absolute path of bash command inside my script, that was supposed to start my subprocess. Moreover, if I start my script from a bash console, its parent is still bash but the subprocess works.

PS: if I specify /bin/bash to run my subprocess in my script, it doesn't work either.