Note 2014-12-16: I’m leaving this here for historical reasons, and since some older OS versions still need properly-written init scripts. Since starting to use Arch Linux on my laptop, I’ve become a convert to the systemd world. I know this is a hot topic and has sparked a lot of controversy. While I agree with some of the arguments in principal, I strongly feel that systemd provides the interface that Linux needs in modern times, and provides a unified solution to many problems that were previously solved in myriad ways inside init scripts. In short, if your distro supports systemd, I’d recommend to skip past this page and go ahead and write a unit file.
I’ve been deploying some new software lately (specifically selenesse, which combines Selenium and fitnesse, xvfb). None of these seem to come with init scripts to run as daemons, and the quality of the few Fedora/RedHat/CentOS init scripts I was able to find was quite poor. The Fedora project has a Specification for SysV-style Init Scripts in their Packaging wiki, which specifies what a Fedora/RedHat/CentOS init script should look like, in excruciating detail. What follows is an overview of the more important points, which I’m using to develop or modify the scripts I’m currently working on.
- Scripts must be put in
/etc/rc.d/init.d, not in the
/etc/init.dsymlink. They should have 0755 permissions.
- Scripts must have a Fedora-style chkconfig header (“chkconfig:”, “description:” lines), and may have an LSB-style header (BEGIN INIT INFO/END INIT INFO). See Initscript template.
- Scripts must make use of a lockfile in
/var/lock/subsys/, and the name of the lockfile must be the same as the name of the init script. (There is a technical reason for this relating to how sysv init terminates daemons at shutdown). The lockfile should be touched when the daemon successfully starts, and removed when it successfully stops.
- Init scripts should not depend on any environment variables set
outside the script. They should operate gracefully with an
empty/uninitialized environment (or only LANG and TERM set and a CWD
/, as enforced by
service(8), or with a full environment if they are called directly by a user.
- all of the following actions are required, and have specific definitions:
- start: starts the service
- stop: stops the service
- restart: stop and restart the service if the service is already running, otherwise just start the service
- condrestart (and try-restart): restart the service if the service is already running, if not, do nothing
- reload: reload the configuration of the service without actually stopping and restarting the service (if the service does not support this, do nothing)
- force-reload: reload the configuration of the service and restart it so that it takes effect
- status: print the current status of the service
- usage: by default, if the initscript is run without any action, it should list a “usage message” that has all actions (intended for use)
- They must “behave sensibly”. I’ve found this to be one of the
biggest problems with homegrown init scripts. If
servicename startis called while the service is already running, it should simply exit 0. Likewise if the service is already stopped. Init scripts must not kill unrelated processes. I don’t know how many times I’ve seen scripts that kill every java or python process on a machine.
I intend to use this as a quick checklist when developing or evaluating init scripts for RedHat/Fedora based systems. In my experience, the biggest problems with most init scripts revolve around poor handling of PID files and lockfiles, mainly:
- Killing processes other than the one that the script started (i.e. killing all java or python processes), usually because the PID isn’t tracked at start
- Starting a second instance of the subsystem because lockfiles aren’t used, or the status function is broken.
- improper exit codes
- either explicitly relying on environment variables (and therefore
breaking when called through
service(8)), or conversely, not cleaning/resetting environment variables that are used by dependent code or processes.