System V style init and systemd in practice

Dejan Radic, 123RF

Who's Boss

Harald Zisler

Both init and systemd can be used to start, monitor, and shut down services on Linux systems, but these utilities differ in terms of configuration and operation.

When it comes to managing system services, a Linux system administrator can choose between the classic init [1] and the relatively new systemd [2]. These utilities form a meeting ground for mature technology and new concepts.

Post-Boot

Directly after booting the kernel, a service will start and then accompany the rest of the system's ongoing operation and shut down. Throughout, this service is assigned the Process ID 1.

You can use ps on a running system to determine which program executes the administration of services. (See the instructions in the "Determining Process 1" box.)Whatever the result, you may be misled because of a possibly existing downward-compatible invocation, which may have convinced you that init is performing this task when it is indeed systemd.

Determining Process 1

Using the correct query, you can determine whether the system uses the classic init or systemd. By means of the -ax option of the ps command, you will get the following output even on a computer managed by systemd.

~$ ps -ax | head -2
  PID TTY      STAT   TIME COMMAND
    1 ?        Ss     0:01 /sbin/init

To avoid being misled, you should use the -e option so that you get the correct results:

~$ ps -e | head -2
  PID TTY          TIME CMD
    1 ?        00:00:01 systemd

On a system using init, you will get the following output:

~$ ps -e | head -2
  PID TTY          TIME CMD
    1 ?        00:00:00 init

Using pstree also gives you the correct output.

System States (init)

On Linux and other related systems, a runlevel defines a certain system state, which may include services that are running (daemons).

In Table 1, you will find a list of the possible system states. Except for S , all of the other states are indicated with a numerical value running from 0 to 6. The runlevels between 2 and 5 are not used uniformly by the various distributions.

Table 1

Runlevel

Action	Runlevel	Command
Shutdown, powering down, writing buffered data to the disk, terminating network connections, synchronization and unmounting the data storage devices from the directory tree (sync, umount), possibly disconnecting the power supply	0	shutdown -h now , init 0
Single user for root , no network access, computer access only via the console, used for system administration	S	init s
Single user without network access, computer access only via the console	1	init 1
Multi-user operation with or without network access, with or without graphical user interface, depends on settings of the distribution used	2-5	init [2-5] , exit in s or 1
Reboot, reload computer, restart, write buffered data to the disks, disconnect network connections, synchronize, and unmount data storage devices from the directory tree (sync, umount), restart computer with appropriate boot procedure	6	shutdown -r now , reboot , init 6

You can determine what the default runlevel at system start is by looking at the beginning of the /etc/inittab file. Listing 1 shows a relevant snippet. The standard runlevel of the system, initdefault , is indicated in the line beginning with id . The default runlevel is 2 in the example.

Listing 1

Default runlevel in inittab

01 .
02 .
03 .
04 # The default runlevel.
05 id:2:initdefault:
06
07 # Boot-time system configuration/initialization script.
08 # This is run first except when booting in emergency (-b) mode.
09 si::sysinit:/etc/init.d/rcS
10
11 # What to do in single-user mode.
12 ~~:S:wait:/sbin/sulogin
13 .
14 .
15 .

The single user mode (si: ) must have previously finished, except in emergency mode due to interruptions of the boot process. There's an important entry at the end of the listing. init executes an automatic login for root as a necessary precondition for operating the system in single user mode.

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