Running Perl thru crontab and perlbrew
Perlbrew is amazing! It allows you to install different Perl versions within your home directory, tune such installation (viacpanm) and be free to use modern Perl and related libraries even on those systems you don’t have control over and that runs, as an example, Perl 5.16-something!
For me, Perlbrew has been a real changer because now I can develop and run a lot of automated tasks written in one of my favourite language without any worry or need to ask a system administrator to install, upgrade and tune something for me.
But then I want to push it further, and schedule Perl task execution via
crontab(5).
That should sound easy, but it is not as it may seem. The fact is that Perlbrew requires some trick to instrument your shell to run the version of Perl you want to, and
crontab(5) does not always respect your login shell artifacts and tuning.
perlbrew exec as a way to quickly setup a crontab(5) task
Perlbrew includes a particular command, named exec that allows you to run a program using the Perlbrew configuration (i.e., the Perl you have switched to). Therefore, my crontab could look like the following one:
ORACLE_HOME=$HOME/instantclient_18_3
SHELL=/bin/bash
# min hour dom month dow
35 22 * * 1-6 $HOME/perl5/perlbrew/bin/perlbrew exec perl $HOME/my_script_perl.pl -r -v -a /tmp
as you can see, in the beginning of the crontab I set some variables, most notably the
ORACLE_HOME because I could need to run scripts that use DBD::Oracle, but that’s an example.
The important thing to note is the absolute path to
perlbrew, then the exec command and the full command line to the script I want to execute, including various options.
Who is running what?
When the scheduler will execute, you are going to see two (or more) processes that seems the same:$ ps
...
luca 2292 1.1 0.2 157504 11948 ? Ss 12:49 0:00 /usr/bin/perl /home/luca/perl5/perlbrew/bin/perlbrew exec perl /home/luca/my_script_perl.pl -r -v -a /tmp
luca 2293 1.8 0.6 395008 25344 ? S 12:49 0:00 perl /home/luca/my_script_perl.pl -r -v -a /tmp
The first process is the execution of
perlbrew (note the usage of the default perl). The second process is the sub-process run by perlbrew exec.
Conclusions
With a little effort, it is possible to avoid wrapping scripts and useperlbrew directly into crontab(5).
Now, it possible to automate pretty much everything with the power of Perl at our fingertips!