Special escaping for crontab

Question

I have the following user crontab entry on a RHEL 6 machine (sensitive values have been replaced):

[email protected]
0 5 * * * ~/bin/app_state.sh host-arg 9200 > ~/state/app-state-$(hostname)-$(date +%F).json

Which produces this entry in /var/log/cron:

Apr 23 05:00:08 host CROND[13901]: (dbjobs) CMD (~/bin/app_state.sh host-arg 9200 > ~/state/app-state-$(hostname)-$(date +)

But no file.

After changing the statement to:

43 5 * * * ~/bin/app_state.sh host-arg 9200 > ~/state/app-state-static.json

I get a better log entry and the file is created at ~/state/app-state-static.json

I'm sure there's some issue with not escaping the +%F but can't for the life of me find details of how I should be escaping it. I could wrap the filename generation inside another shell script but this is more easy to read for people coming looking for the file.

Answer 1

As crontab tag wiki says, percent characters are problematic in crontabs; % gets converted to a newline:

[..] > ~/state/app-state-$(hostname)-$(date +%F).json

would run command as

[..] > ~/state/app-state-$(hostname)-$(date +\nF).json

“Escaping” percent characters is possible, but the escape character gets executed in the command. A job like following would run the command with \ in front of percent character.

0 5 * * * ~/bin/app_state.sh host-arg 9200 > ~/state/app-state-$(hostname)-$(date +%F).json

One way to circumvent this problem is to have the command in a script and execute that as a cron job:

/usr/local/bin/app_state_cron.sh:

#!/bin/sh

~/bin/app_state.sh host-arg 9200 > ~/state/app-state-$(hostname)-$(date +%F).json

And in crontab:

0 5 * * * /bin/sh /usr/local/bin/app_state_cron.sh