Controlling Ubuntu's and Fedora's Upstart (the init replacement)
Controlling Startup Events
Juliet Kemp
Wednesday, May 13, 2009 02:00:48 PM
In the previous article in this series I looked at how init (the
system we've all been using for years to start up our services) works, and how
upstart, the new replacement for init, works instead. This
article looks at upstart and its scripts in more detail.
Moving over
The first piece of very good news is that it's possible to move from
init to upstart without having to shift all of your services
at once. There's a default set of example scripts available from the
upstart website which includes a job which runs the init
scripts. For example, here's the rc3 job:
start on runlevel 3
stop on runlevel
console output
script
set $(runlevel --set 3 || true)
if [ "$1" != "unknown" ]; then
PREVLEVEL=$1
RUNLEVEL=$2
export PREVLEVEL RUNLEVEL
fi
exec /etc/init.d/rc 3
end script
The first two lines define what events this job should react to, and how it
should react. So on the occurrence of a 'runlevel 3' event (probably
triggered by the telinit compatability wrapper), the job starts. On
the occurrence of any runlevel event, it stops (so in fact a runlevel 3
event will stop it, then start it again – this matches how init
works). The script section does the actual work of running the
job.
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Writing an upstart job
But what about setting up a particular job for yourself? You keep your job
definitions in /etc/init/jobs.d, and they should be plain text files,
and not executable. Your job needs to have either an exec line:
exec /bin/echo "ping"
giving a path to a binary, and the arguments to pass to it; or a
script section, as in the runlevel 3 job description above, which has
a shell script to be run with /bin/sh. These define what will be run
when the job is triggered. Jobs can be set to respawn with the line:
respawn
There's an automatic limit set to this: if a process is respawned more than 10
times within 5 seconds, it will be stopped and not restarted. You can alter
this default with:
respawn limit 20 5
(this would set the limit at 20 times in 5 seconds).
So, we could have a very basic job that looked like this:
start on startup
exec /bin/echo "ping"
which would run that echo line on startup. There's no point in
putting a stopping stanza here, as it's not a daemon to start/stop,
and similarly no need for respawning. Next: Scripts Can Run Before or After a Job »
