init/docs/services.md

Service Files

Services that can be started and managed by init are described by service description files stored in /usr/share/init.

The init process actually reads from /etc/init.d which contains symlinks to the actual service files.

Enabling a service means adding a symlink, disabling means removing a symlink.

Service descriptions can be parameterized. The arguments are extracted from the name of the symlink. Currently only 1 parameter is supported. The argument value is separated from the service name by an '@' character in the symlink name.

Below is an annotated example for a simple, service description for a generic, parameterized agetty service:

#
# The text that init should print out when the status of the
# service changes.
#
# The '%0' is replaced with the first argument extracted from the
# symlink name.
#
description "agetty on %0"

#
# How to run the service. 'respawn' means restart the service when it
# terminates, 'once' means run it only once and continue with other
# services in the mean while, 'wait' means run it once, but block until
# it exits.
#
type respawn

#
# When to start the service. 'boot' means when booting the system. Other
# options are 'reboot', 'shutdown' and 'ctrlaltdel'. The system always
# starts into the 'boot' target and then later transitions to one of the
# others.
#
target boot

#
# A list of service names that must be started before this service can
# be run, i.e. this services needs to be started after those.
#
# This can only refer to generic names, not specific instances. For
# instance, you can say "after getty" to make sure a service comes up after
# all gettys are started, but you cannot specify "after agetty@tty1".
#
# Similar to 'after', there is also a 'before' keyword for specifying
# dependencies.
#
after sysinit

#
# The 'tty' directive specifies a file to which all I/O of the process is
# redirected. The specified device file is used as a controlling tty for
# the process and a new session is created with the service process as
# session leader.
#
# In this example, we derive the controlling tty from the service
# description argument.
#
tty "/dev/%0"

#
# The 'exec' directive specifies the command to execute in order to start
# the service. See in the example below on how to run multiple commands.
#
# Again we use the argument to specify what terminal our getty
# should run on.
#
exec agetty %0 linux

As can be seen in this simple example, each line in a service description is made up of a keyword, followed by one or more arguments and terminated by a line break.

Blank lines are ignored and shell-style comments can be used.

Arguments are separated by space. Quotation marks can be used to treat something containing spaces or comment character as a single argument.

In between quotation marks, C-style escape sequences can be used.

Argument substitution (arguments derived from the symlink name) can be done using a '%' sign, followed by the argument index. A '%' sign can be escaped by writing '%%'.

If a service should sequentially run multiple commands, they can be grouped inside braces as can be seen in the following, abbreviated example:

description "mount /var"
type wait
target boot
before vfs
exec {
    mount -t tmpfs none /var
    mkdir /var/log -m 0755
    mkdir /var/spool -m 0755
    mkdir /var/lib -m 0755
    mkdir /var/tmp -m 0755
    mount --bind /cfg/preserve/var_lib /var/lib
}