Friday, April 19, 2013

Understanding SysV-style Initscripts in Red Hat Linux

It is often needed to write your own init start/stop script, the following is the minimum requirement for your script to behave as expected. The discussion is based on Red Hat Linux family, other distributions like Debian use LSB (Linux Standard Base Specification) Init Scripts.

Location of the script

/etc/init.d is the well known location, but actually /etc/rc.d/init.d is the real original location. Since /etc/init.d is a hard link to /etc/rc.d/init.d, it makes no difference.

Header of the script

It needs at least 3 lines.  The shell script interpreter (/bin/sh, /bin/bash .. etc), the chkconfig header and script description
#   chkconfig: 345 56 10
#   description: Startup/shutdown script for the Common UNIX 

Body of the script

Obviously, it need  to accept parameter “start”, which  /etc/rc3.d/S* will call  on OS startup and accept parameter “stop”, which /etc/rc0.d/K* script will call on OS shutdown.
The lockfile is often overlooked, it is used to check the existence of the daemon on OS shutdown, otherwise the stop action won’t be called. If you found an issue that a script started on OS startup but never stop properly on shutdown, you need to create lockfile. note: lockfile is not pidfile which contains PID of the process, lockfile is usually a blank file.
lockfile=/var/lock/subsys/$(basename $0)
case $1 in
  [ $? = 0 ] && touch $lockfile
  [ $? = 0 ] && rf –f  $lockfile


It is recommended to import functions in /etc/rc.d/init.d/functions to use ‘daemon’ to startup your application or killproc to shutdown your application instead of reinventing the wheel.

LSB headers

You may see something like this in an init script.
 # Provides: boot_facility_1 [ boot_facility_2 ...]
 # Required-Start: boot_facility_1 [ boot_facility_2 ...]
 # Required-Stop: boot_facility_1 [ boot_facility_2 ...]
 # Should-Start: boot_facility_1 [ boot_facility_2 ...]
 # Should-Stop: boot_facility_1 [ boot_facility_2 ...]
 # Default-Start: run_level_1 [ run_level_2 ...]
 # Default-Stop: run_level_1 [ run_level_2 ...]
 # Short-Description: short_description
 # Description: multiline_description
They are LSB(Linux Standard Base) headers, they are supported by default in Debian and SUSE Linux.
Red Hat Linux supports this by additional package “redhat-lsb” and it is not installed by default, Be warned,50+ dependences need to installed as well.