Systemd's solution to large init scripts

Journal phantomfive's Journal: Systemd's solution to large init scripts 1

Journal by phantomfive on Monday February 16, 2015 @02:55AM

(Note: When you write code, you're making a UI for programmers. Learn to do it well.)

Below you can see a traditional unix init script. It's long, but if you're familiar with shell-script you can figure out what is going on. There is a lot of redundancy here, most init scripts have a switch that runs an option based on the first command-line parameter, for example. One solution is to put common code in a function, but Poettering decided to use config files.

Let's examine the general tradeoff between putting code in a function, and using config files (a form of declarative programming). Config files are fine as long as there aren't too many special cases. If there are too many special cases, you end up with so many options and keywords that it would have been easier to just use a scripting language.

The good side is systemd saves a lot of typing. Way down at the bottom, is a unit file for the same init script. It's clearly shorter, and easier to type.

The bad side is it has arcane keywords which are are not discoverable merely by looking at the file. This is a pattern that repeats itself over and over in systemd, things are easier if you know how to do them, but the system itself is inscrutable without arcane knowledge.

Ideal systems fulfill the requirements while making it easy for those who want to dig deeper. The system opens like the petals of a rose.

#!/bin/bash # Starts the abrt daemon # # chkconfig: 35 82 16 # description: Daemon to detect crashing apps # processname: abrtd ### BEGIN INIT INFO # Provides: abrt # Required-Start: $syslog $local_fs # Required-Stop: $syslog $local_fs # Default-Stop: 0 1 2 6 # Default-Start: 3 5 # Short-Description: start and stop abrt daemon # Description: Listen to and dispatch crash events ### END INIT INFO # Source function library. . /etc/rc.d/init.d/functions ABRT_BIN="/usr/sbin/abrtd" LOCK="/var/lock/subsys/abrtd" OLD_LOCK="/var/lock/subsys/abrt" RETVAL=0 # # Set these variables if you are behind proxy # #export http_proxy= #export https_proxy= # # See how we were called. # check() { # Check that we're a privileged user [ "`id -u`" = 0 ] || exit 4 # Check if abrt is executable test -x $ABRT_BIN || exit 5 } start() { check # Check if it is already running if [ ! -f $LOCK ] && [ ! -f $OLD_LOCK ]; then echo -n $"Starting abrt daemon: " daemon $ABRT_BIN RETVAL=$? [ $RETVAL -eq 0 ] && touch $LOCK echo fi return $RETVAL } stop() { check echo -n $"Stopping abrt daemon: " killproc $ABRT_BIN RETVAL=$? [ $RETVAL -eq 0 ] && rm -f $LOCK [ $RETVAL -eq 0 ] && rm -f $OLD_LOCK echo return $RETVAL } restart() { stop start } reload() { restart } case "$1" in start) start ;; stop) stop ;; reload) reload ;; force-reload) echo "$0: Unimplemented feature." RETVAL=3 ;; restart) restart ;; condrestart) if [ -f $LOCK ]; then restart fi # update from older version if [ -f $OLD_LOCK ]; then restart fi ;; status) status abrtd RETVAL=$? ;; *) echo $"Usage: $0 {start|stop|status|restart|condrestart|reload|force-reload}" RETVAL=2 esac exit $RETVAL

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[Unit] Description=Daemon to detect crashing apps After=syslog.target [Service] ExecStart=/usr/sbin/abrtd Type=forking [Install] WantedBy=multi-user.target

Reference to the examples.

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