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About bash : Cannot-run-bash-script-from-crontab-when-it-works-from-command-line-bash

Question Detail

I have a strange problem of being to able to run a bash script from commandline but not from the crontab entry for root. I am running Ubuntu 12.04.

* * * * 1-5 root /home/xxxxxx/jmeter/VerificationService-0.0.1-SNAPSHOT/jmeter-cron-randomise.sh >> /home/xxxxxxx/jmeter/VerificationService-0.0.1-SNAPSHOT/cron.log

If I run the script from the cmd line using bash, it works fine but sh fails with following error:

> jmeter-cron-randomise.sh: 7: jmeter-cron-randomise.sh: arithmetic
> expression: expecting primary: "  % 1 "

Having googled the problem, it seems like standard shell doesn’t have the same math operators, like % (modulus), as bash. I’m Not sure why the cron job is failing in the script? I am assuming it is because it’s not using the bash shell? It’s definitely being fired by the cron daemon (can see it in /var/log/syslog). Any help much appreciated.

Question Answer

You likely need to tell cron that the shell to use is the bash shell as it defaults to sh. You can do that for all crontab entries by putting this line in your crontab:

SHELL=/bin/bash

Note that this will cause all scripts in the crontab to be run under bash which may not be what you want. If you want to change the crontab line itself to just run bash, change it to this:

* * * * 1-5 root /bin/bash /home/xxxxxx/jmeter/VerificationService-0.0.1-SNAPSHOT/jmeter-cron-randomise.sh >> /home/xxxxxxx/jmeter/VerificationService-0.0.1-SNAPSHOT/cron.log 2>&1

Note that I have also caused stderr to be written to the cron.log file (2>&1) which may not be what you want but is pretty common practice. This may help you further diagnose errors from the script.

In case this helps anyone: for me this appeared to be because I had ended up with “DOS” line endings (CR-LF) instead of “unix” line endings (LF). This can be checked using od or your favourite hex dump tool, e.g.:

od -c <script_file>

… and look for \r\n instead of just \n.

It seems (and this article supports it) that the CR character stops the “shebang” from working because it’s interpreted as part of the shell executable’s filename.

(The line endings themselves appeared because the file came from a git repository and was transferred via a Windows machine).

I also encountered this problem trying to schedule a database backup as root and it made me pull my hair out! I was working on a CentOS 7 box.
Whenever I would check /var/spool/mail/root I would see a log:
sh: root: command not found, yet the command would run perfectly in the terminal.

This is what worked for me:

  1. I created the crontab entry using crontab -e while logged in as root.

  2. Using the command above as an example:

    * * * * 1-5 root /home/xxxxxx/jmeter/VerificationService-0.0.1-SNAPSHOT/jmeter-cron-randomise.sh >> /home/xxxxxxx/jmeter/VerificationService-0.0.1-SNAPSHOT/cron.log

I deleted the root user entry like:
* * * * 1-5 /home/xxxxxx/jmeter/VerificationService-0.0.1-SNAPSHOT/jmeter-cron-randomise.sh >> /home/xxxxxxx/jmeter/VerificationService-0.0.1-SNAPSHOT/cron.log

That solved my problem.

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