quickshiftin quickshiftin -5 years ago 228
Linux Question

RedHat daemon function usage

I'm working on an init script for Jetty on RHEL. Trying to use the

function provided by the init library (

I found this terse documentation, and an online example (I've also been looking at other init scripts on the system for examples).

Look at this snippet from online to start the daemon

daemon --user="$DAEMON_USER" --pidfile="$PIDFILE" "$DAEMON $DAEMON_ARGS &"
pid=`ps -A | grep $NAME | cut -d" " -f2`
pid=`echo $pid | cut -d" " -f2`
if [ -n "$pid" ]; then
echo $pid > "$PIDFILE"

Why bother looking up the
and writing it to the
by hand? I guess I'm wondering what the point of the
option to the
function is.

Answer Source

This answer is incorrect because it assumed daemon was the libslack tool. It is actually an independent shell function defined by the RHEL init system. Please go with jnas' answer instead.

Based on the code we can backtrack what happened.

Someone thought they had to make the program invoked by daemon background itself, so they added &. This was wrong.

daemon examins the command string to look for shell metacharacters, and if so, runs the command as an argument to sh -c. This is well intentioned, but bad design because it's unpredictable and error prone. In any case, daemon now ends up executing sh -c 'something &'. daemon puts the pid of sh in the pidfile.

sh, of course, immediately exits, because its only job was to background a command.

The person who wrote this example noticed that they couldn't control the daemon, because the pid file referred to the pid of now dead sh instead of their daemon.

They decided to "fix" this by manually getting extracting the pid from ps. This is where the pid= lines comes from.

However, they found that this didn't always work. This is because there's a race condition between the daemon starting and ps command running.

They decided to "fix" this by only writing the pid if it was find, and leaving the invalid pid otherwise. This is where the if [ -n "$pid" ] condition comes from.

To summarize,

  1. User mistakenly added & in the daemon command.
  2. User discovered pid file was broken and added a workaround.
  3. User discovered workaround was broken and added a workaround.

The way to fix this is simply to not have the daemon background itself, and let the daemon command do that. The rest of the code is now useless.

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