The document discusses Linux services and run levels. It explains that Linux uses scripts in /etc/rc.d and /etc/init.d directories to start and stop services during boot up and shutdown. These scripts are executed in different run levels which determine the system configuration and available processes. The document also provides details on the structure and contents of different run level directories, a skeleton for service scripts, and how to use update-rc.d to automatically start scripts on boot.

1. Gaurav Mishra
<gmishx@gmail.com>
Linux - 4
/etc/rc.d 26-02-2018

2. Gaurav Mishra <gmishx@gmail.com>
Services in Linux
• Linux environment support services which are daemons running in background.
• These services can be make to start at boot.
• Linux distros have directories /etc/rc.d and /etc/init.d which hold scripts to run
these services.
• Every script in these directories is invoked at the boot time.
• When the system boots, it looks for scripts in /etc/rc.d directory followed by
/etc/init.d, run them and setup the system for use.
• When the system shutdowns, it first halts scripts in /etc/init.d then moves to
/etc/rc.d
26-02-2018

3. Gaurav Mishra <gmishx@gmail.com>
Run level in Unix like systems
• A runlevel is a preset operating state on a Unix-like operating system.
• A system can be booted into (i.e., started up into) any of several runlevels, each of which is
represented by a single digit integer.
• Each runlevel designates a different system configuration and allows access to a different
combination of processes (i.e., instances of executing programs).
• The are differences in the runlevels according to the operating system. Seven runlevels are
supported in the standard Linux kernel (i.e., core of the operating system). They are:
0 - System halt; no activity, the system can be safely powered down.
1 - Single user; rarely used.
2 - Multiple users, no NFS (network filesystem); also used rarely.
3 - Multiple users, command line (i.e., all-text mode) interface; the standard runlevel for
most Linux-based server hardware.
4 - User-definable
5 - Multiple users, GUI (graphical user interface); the standard runlevel for most Linux-
based desktop systems.
6 - Reboot; used when restarting the system.
26-02-2018

4. Gaurav Mishra <gmishx@gmail.com>
/etc/rc.d structure
Structure for Debian based distros
26-02-2018

5. Gaurav Mishra <gmishx@gmail.com>
Contents of directories
The scripts in this directory are executed each time the system enters
single-user mode.
Their purpose is to stop all services and thus to put the system in
single-user mode.
• The scripts in this directory are executed once when entering
runlevel 0 (shutdown).
• The scripts are all symbolic links whose targets are located in
/etc/init.d/.
• Their purpose is to stop all services and to make the system
ready for shutdown.
/etc/rc2.d/
• Run level 6 for reboot.
• The purpose of the scripts in this directory is to stop all services
and to make the system ready for reboot.
• Multi-user mode with GUI.
• Their purpose is to start all services in their GUI mode for
multi-user.
/etc/rc6.d/
/etc/rc0.d/ /etc/rc1.d/

6. Gaurav Mishra <gmishx@gmail.com>
Contents of directories
Contains scripts used by the System V init tools (SysVinit).
Files in /etc/init.d are shell scripts that respond to start, stop, restart
commands to manage a particular service.
These scripts can be invoked directly or (most commonly) via some
other trigger (typically the presence of a symbolic link in /etc/rc?.d/).
• The scripts in this directory are executed once when booting the
system, even when booting directly into single user mode.
/etc/init/
• Contains configuration files used by Upstart.
• Files in /etc/init are configuration files telling Upstart how and
when to start, stop, reload the configuration, or query the
status of a service.
/etc/rcS.d/ /etc/init.d/

7. Gaurav Mishra <gmishx@gmail.com>
Skeleton of service script
#! /bin/sh
### BEGIN INIT INFO
# Provides: skeleton
# Required-Start: $remote_fs $syslog
# Required-Stop: $remote_fs $syslog
# Default-Start: 2 3 4 5
# Default-Stop: 0 1 6
# Short-Description: Example initscript
# Description: This file should
be used to construct scripts to be
# placed in
/etc/init.d.
### END INIT INFO
do_start()
{
# Return
# 0 if daemon has been started
# 1 if daemon was already running
# 2 if daemon could not be started
start-stop-daemon --start --quiet --
pidfile $PIDFILE --exec $DAEMON --
$DAEMON_ARGS || return 2
}
do_stop()
{
# Return
# 0 if daemon has been stopped
# 1 if daemon was already stopped
# 2 if daemon could not be stopped
26-02-2018

8. Gaurav Mishra <gmishx@gmail.com>
Skeleton of service script
# other if a failure occurred
start-stop-daemon --stop --quiet --
retry=TERM/30/KILL/5 --pidfile
$PIDFILE --name $NAME
rm -f $PIDFILE
return "$?"
}
case "$1" in
start)
do_start
;;
stop)
do_stop
;;
status)
status_of_proc "$DAEMON" "$NAME" &&
exit 0 || exit $?
;;
restart|force-reload)
do_stop
do_start
;;
*)
#echo "Usage: $SCRIPTNAME
{start|stop|restart|reload|force-
reload}" >&2
echo "Usage: $SCRIPTNAME
{start|stop|status|restart|force-
reload}" >&2
exit 3
;;
esac
26-02-2018

9. Gaurav Mishra <gmishx@gmail.com>
Auto start script
1. Create the service script in /etc/init.d/ with a
unique name.
2. Insert links
1. update-rc.d syntax:
update-rc.d [-n] name start|stop NN runlevel
[runlevel]... . start|stop NN runlevel
[runlevel]... . ...
update-rc.d [-n] name defaults|disable|enable [
S|2|3|4|5 ]
2. Using the defaults:
update-rc.d foobar defaults
3. Equivalent command using explicit
argument sets:
update-rc.d foobar start 20 2 3 4 5 . stop 20 0 1 6
.
4. Insert links at default runlevels when B
requires A
update-rc.d script_for_A defaults 80 20
update-rc.d script_for_B defaults 90 10
5. Remove all links for a script:
update-rc.d foobar remove
6. Example of a command for installing a
system initialization-and-shutdown
script:
update-rc.d foobar start 45 S . stop 31 0 6 .
Example links created using defaults:
Adding system startup for /etc/init.d/foobar
...
/etc/rc0.d/K20foobar -> ../init.d/foobar
/etc/rc1.d/K20foobar -> ../init.d/foobar
/etc/rc6.d/K20foobar -> ../init.d/foobar
/etc/rc2.d/S20foobar -> ../init.d/foobar
/etc/rc3.d/S20foobar -> ../init.d/foobar
/etc/rc4.d/S20foobar -> ../init.d/foobar
/etc/rc5.d/S20foobar -> ../init.d/foobar
26-02-2018