Editing Creating Python Cron Job On Synology

[[Category:Synology]][[Category:Python]]
== Objective ==

To install a python script on Synology NAS and run it as a cron job. The python script relies on packages distributed on both PyPI and GitHub.

== Prerequisites ==

=== Git ===

Git is needed if python packages are installed from GitHub.

'''DSM''' > '''Package Center''' > '''Utilities''' > '''Third-Party''' > '''GIT Server''' > Install

=== python ===

Install Python3 using the Package Center app in the DSM.

=== pip ===

SSH to the DiskStation and install `pip` with

<syntaxhighlight lang="bash">
$ wget https://bootstrap.pypa.io/get-pip.py 
$ sudo python3 get-pip.py
$ rm get-pip.py
</syntaxhighlight>

This will install `pip` at `/volume1/@appstore/py3k/usr/local/bin/`, but the `pip` command isn't automatically available.

<syntaxhighlight lang="bash">
$ pip
-sh: pip: command not found
$ sudo ln -s /volume1/@appstore/py3k/usr/local/bin/pip /usr/local/bin/pip
$pip --version
pip 9.0.3 from /volume1/@appstore/py3k/usr/local/lib/python3.5/site-packages (python 3.5)
</syntaxhighlight>

=== virtualenv ===

Once `pip` is installed, use it to install `virtualenv`, and make sure the command is available from the command line:

<syntaxhighlight lang="bash">
$ sudo pip install virtualenv
$ sudo ln -s /volume1/@appstore/py3k/usr/local/bin/virtualenv /usr/local/bin/virtualenv
</syntaxhighlight>

Create a virtual environment for the python script:

<syntaxhighlight lang="bash">
$ virtualenv -p /volume1/@appstore/py3k/usr/local/bin/python3 venv
</syntaxhighlight>

=== SSH key for GitHub ===

* [https://help.github.com/articles/generating-a-new-ssh-key-and-adding-it-to-the-ssh-agent/#platform-linux Generate a SSH key]
* [https://help.github.com/articles/adding-a-new-ssh-key-to-your-github-account/#platform-linux Add the SSH key to the GitHub acccount]
** The instructions on that page say to use `apt-get` to install `xclip`. `apt-get` is not available on Synology. <ref>[[Synology Package Managers]]</ref> `xclip` is being installed to copy the contents of the ssh key file from the command line, however `xclip` doesn't seem to be readily available for Synology, so open up the key file and manually copy its contents instead.

== Installing python packages ==

I experienced an issue on DS213 where at least one PyPI package (`lxml`) could not be compiled. Something to the effect that there was no gcc or cc compiler available. Not sure why a python package needs that, but I spent a while trying to install the various things that were reported missing without ultimately succeeding.

The more recent DiskStation on the other hand was able to install `lxml` right away with pip.

For instructions on how to install python packages from GitHub see wiki article on [[Python_Packaging#Installation_from_GitHub|Installing Python Packages from GitHub]].

== Scheduled task ==

=== Crontab format ===

Edit `/etc/crontab` to add task.

Columns are '''minute''', '''hour''', '''day of month'', '''month''', '''day of week'''.

`*` runs at every interval, e.g. every minute of the hour, or every day of the month.

`*/4` runs every fourth interval, e.g. every four hours, or every four days.

`9,12,15` specifies specific values, e.g. 9, 12, and 15 minutes past the hour, or 9 am, 12 pm, and 3 pm.

For days of the week, 1 - 6 stand for Monday through Saturday. 0 and 7 both equal Sunday.

=== Running a script in a virtual environment ===

Cron runs a `sh` shell, not a `bash` shell, so `source` is not available to invoke the virtual environment. Use `.` instead.

Full paths are required for all files and commands since cron runs as root.

Output from the command, such as errors, should be redirected to a log file for fixing errors. 

To run the `python` command, run it from within the virtualenv path, e.g.:

<syntaxhighlight lang="bash">
. /var/services/homes/username/scriptdir/venv/bin/activate && /var/services/homes/username/scriptdir/venv/bin/python /var/services/homes/username/scriptdir/run.py > /var/services/homes/username/scriptdir/cron.log
</syntaxhighlight>

* `. /var/services/homes/username/scriptdir/venv/bin/activate` runs the virtual environment.
* `&&` runs python within the virtual environment.
* Full path to python within the virtual environment.
* Full path to python script to run.
* Redirect output to log file, using full path.

=== Docker dependent tasks ===

Docker containers must be restarted after a system reboot. 

* '''DSM''' > '''Package Center''' > '''Installed''' > '''Docker''' > '''Open''' button
* '''Container''' tab > '''Settings''' button

All the Docker containers are displayed and can be restarted from this window.

== Notes ==

=== References ===
<references />

=== See also ===

* [https://crontab.guru/every-5-minutes Every 5 Minutes] - Crontab Guru (lists syntax for specifying values within each column)
* [https://www.lifewire.com/crontab-linux-command-4095300 How To Edit the Crontab File to Schedule Jobs] - Lifewire
* [https://stackoverflow.com/questions/18919151/crontab-day-of-the-week-syntax Crontab Day of Week Syntax] - StackOverflow