This is a text-only version of the following page on https://raymii.org:

This is a text-only version of the following page on https://raymii.org:
---
Title : Better Cron env and shell control with the SHELL variable
Author : shtylman
Date : 08-03-2013
URL : https://raymii.org/s/tutorials/Better_cron_env_and_shell_control_with_the_SHELL_variale.html
Format : Markdown/HTML
---

> original article from: <http://shtylman.com/post/cron-shell-power/> \-
archived for importance.

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If you don't know what [cron][2] is this post is not for you.

Using the `SHELL` variable in cron is more powerful than you may realize.

## typical crontab

Most people will have this type of setup in their crontab.

NODE_ENV=production
OTHER_VAR=foo

*/10 * * * * /path/to/node /path/to/my/script.js

If you don't want to repeat `/path/to/node` (or your runtime) over and over, you
will add a `PATH` variable to go with the other variables.

But what happens if you want to use something like [nvm][3] or [rvm][4] or
[virtualenv][5], etc? It is not uncommon to have the above change to something
like the following

*/10 * * * * /path/to/my/launcher.sh
*/10 * * * * /path/to/my/launcher_another.sh

Now you have several shell scripts which invoke the required commands to setup
the environment and then run whatever program.

## enter SHELL

There is a little known special env variable for cron: `SHELL`. Most people know
this variable can be used to change the shell your scripts run run (i.e.
`SHELL=/bin/bash`), but it can actually run any file!

So lets say I use nvm and want to setup my environment. Instead of making custom
launchers for each command, I can simply do the following:

SHELL=/path/to/setup/cron.bash

*/10 * * * * node $HOME/foo.js

Now lets look at what `cron.bash` might look like:

#!/bin/bash
set -e

source /etc/environment
source /etc/profile

# setup any env variables you want here
export NODE_ENV=production

# I use node so I want to add node path stuff via npm
# $HOME is available, but not many other env vars are by default
source $HOME/nvm/nvm.sh

# restore SHELL env var for cron
SHELL=/bin/bash
# execute the cron command in an actual shell
exec /bin/bash --norc "$@"

For the most part it looks just like any other shell script. The important
magical parts are the last 4 lines. These lines put back the SHELL variable to
`/bin/bash` and then execute a bash shell to run the cronline command (the stuff
for the specific cronjob).

SHELL=/full/path/to/cron.bash
[email protected]

# For more information see the manual pages of crontab(5) and cron(8)
#
# m h dom mon dow command

# dummy cron command to print the environment variables ever minute
* * * * * env

# node scripts can simply be run using `node` now
* * * * * node /path/to/script/.js

Now our cron files have a consistent environment setup and we can simply run
whatever commands we need without further PATH tricks or nonsense.

Go forth and update your dirty crontabs!

_[\- home -][6]_

[1]: https://www.digitalocean.com/?refcode=7435ae6b8212
[2]: http://en.wikipedia.org/wiki/Cron
[3]: https://github.com/creationix/nvm
[4]: https://rvm.io/
[5]: https://pypi.python.org/pypi/virtualenv
[6]: http://shtylman.com/

---

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