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Debugging cron with `env`

Sometimes cronjobs don’t work as expected. One of the reasons for this is that cron commands run with a different set of environment variables. While you can redirect a script’s output to a file in the crontab, I find it easier to run the command myself and see the output directly.

Quick & easy 

The quick and easy way to simulate a cron command is to use env -i, which creates a completely empty environment:

$ env -i <your command to run>

To illustrate the difference:

$ printenv | wc -l
54
$ env -i printenv | wc -l
0

A more precise environment 

env -i will be sufficient for most cases. However, cron’s environment isn’t completely empty. If you want to run a command with exactly the same parameters, you’ll first have to export the parameters to a file in a cronjob. For example:

* * * * * /usr/bin/env | sort > /tmp/cron_env_vars

You’ll have to wait a minute for this, unfortunately. If you know a better way, let me know!

Once the file is created, you can use env again to load the parameters from it:

$ env -i $(cat /tmp/cron_env_vars | xargs) <your command to run>

Using printenv again to illustrate this:

$ env -i $(cat /tmp/cron_env_vars | xargs) printenv
HOME=/home/mark
LANG=en_US.UTF-8
LANGUAGE=en_GB:en
LOGNAME=mark
PATH=/usr/bin:/bin
PWD=/home/mark
SHELL=/bin/sh