Init and init in linux

Init process on UNIX and Linux systems

Init is the parent of all processes, executed by the kernel during the booting of a system. Its principle role is to create processes from a script stored in the file /etc/inittab. It usually has entries which cause init to spawn gettys on each line that users can log in. It controls autonomous processes required by any particular system.

After reading this file, how the system should be set up in each runlevel is determined by init and also set default runlevel. Init starts all background process after setting default runlevel for the system.

Runlevels

Runlevel, a software configuration of the system which allows only a selected group of processes to exist. The processes produced by init for each of these runlevels are defined in the /etc/inittab file.

Init can be in one of these eight runlevels: 0-6 and S or s. The runlevel can be changed by having a privileged user run telinit, which sends appropriate signals to init, telling it which runlevel to change to.

S or s are same.

7-9 are valid runlevels. though not really documented as «traditional» Unix variants don’t use them. runlevels S and s are in fact the same. Internally they are aliases for the same runlevel.

Runlevels	Functions
0	To halt the system
1	To get the system down into single user mode
2	To get multiuser mode without networking
3	To get multiuser mode with networking
4	Not used
5	To get multiuser mode with networking and X windows
6	To reboot the system
S or s	Not used directly.

Booting

After invoking init as the last step of the kernel boot sequence, it sees if an entry of the type initdefault is present in the file /etc/inittab. The initdefault entry determines the initial runlevel of the system. If no such entry (or no /etc/inittab at all) is present there, a runlevel must be entered at the system console.

Changing Runlevels

After specifying all the processes, init waits for one of its descendant processes to die, a powerfail signal, or until it is signaled by telinit to change the system’s runlevel. It re-examines the /etc/inittab file, when one of the above three conditions occurs.

init still waits for one of the above three conditions to occur. For providing an instantaneous response, the telinit Q or q command can wake up init to re-examine the/etc/inittab file.

If init is not in single user mode and receives a powerfail signal (SIGPWR), it reads the file /etc/powerstatus. Then it starts a command based on the contents of this file −

Tag	Description
F(AIL)	Power is failing, UPS is providing the power. Execute the powerwait and powerfail entries.
O(K)	Power has been restored, execute the powerokwait entries.
L(OW)	The power is failing and the UPS has a low battery. Execute the powerfailnow entries.

If /etc/powerstatus contains anything else then the letters F, O or L or doesn’t exist, init will behave as if it has read the letter F.

Usage of SIGPWR and /etc/powerstatus is discouraged. If Someone wanting to interact with init should use the /dev/initctl control channel — see the source code of the sysvinit package for more documentation about this.

When init is requested to change the runlevel, it sends the warning signal SIGTERM to all processes that are undefined in the new runlevel. Then It waits 5 seconds before forcibly terminating these processes via the SIGKILL signal.

Init assumes that all these processes and their descendants remain in the same process group which init originally created for them. It will not receive these signals, if any process changes its process group affiliation. Such processes need to be terminated separately.

TELINIT

/sbin/telinit is linked to /sbin/init which takes a one-character argument and signals init to perform the appropriate action. The following arguments serve as directives to telinit −

Tag	Description
0,1,2,3,4,5 or 6	tell init to switch to the specified run level.
a, b, c	tell init to process only those /etc/inittab file entries having runlevel a, b or c.
Q or q	tell init to re-examine the /etc/inittab file.
S or s	tell init to switch to single user mode.
U or u	tell init to re-execute itself (preserving the state). No re-examining of/etc/inittab file happens. Request would be silently ignored, if Run level is not one of Ss12345.

It can also tell init how long it should wait between sending processes the SIGTERM and SIGKILL signals. 5 seconds is the default, but this can be changed with the -t sec option.

Only by users with appropriate privileges can invoke telinit.

By looking at its process id, init binary checks if it is init or telinit; the real init’s process id is always 1. So, instead of calling telinit one can also just use init instead as a shortcut.

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The Story Behind ‘init’ and ‘systemd’: Why ‘init’ Needed to be Replaced with ‘systemd’ in Linux

I am subscribed to several mailing lists related to various Linux Distributions and Applications just to keep myself updated with what’s going on where. What are the new bugs? What are the Patches Released? What is expected in next release? and a whole lot of other stuffs. These days the mailing list is heavily populated with “Choose your side on Linux Divide”, mainly on Debian Mailing list along with a few other.

systemd replaces init

What “Choose your side on Linux Divide” is all about?

The init daemon is going to be replaced with daemon systemd on some of the Linux Distributions, while a lot of them have already implemented it. This is/will be creating a huge gap between traditional Unix/Linux Guard and New Linux Guard – programmers and System Admins.

In this article, we will discuss and solve following all queries one-by-one.

1. What init is?
2. What is systemd?
3. Why init needed to be replaced?
4. What features systemd will own.

What is init?

In Linux, init is a abbreviation for Initialization. The init is a daemon process which starts as soon as the computer starts and continue running till, it is shutdown. In-fact init is the first process that starts when a computer boots, making it the parent of all other running processes directly or indirectly and hence typically it is assigned “pid=1“.

If somehow init daemon could not start, no process will be started and the system will reach a stage called “Kernel Panic“. init is most commonly referred to as System V init. System V is the first commercial UNIX Operating System designed and usages of init on most of the Linux Distribution of today is identical with System V OS with a few exception like Slackware using BSD-style and Gentoo using custom init.

The need to replace init with something more perfect was felt from a long time and several alternatives were developed from time-to-time, some of which became distribution’s native init replacement, some of which are:

1. Upstart – A init replacement daemon implemented in Ubuntu GNU/Linux and designed to start process asynchronously.
2. Epoch – A init replacement daemon built around simplicity and service management, designed to start process single-threaded.
3. Mudar – A init replacement daemon written in Python, implemented on Pardus GNU/Linux and designed to start process asynchronously.
4. systemd – A init replacement daemon designed to start process in parallel, implemented in a number of standard distribution – Fedora, OpenSuSE, Arch, RHEL, CentOS, etc.

What is systemd?

A systemd is a System Management Daemon named with UNIX convention to add ‘d‘ at the end of daemon. So, that they can be easily recognized. Initially it was released under GNU General Public License, but now the releases are made under GNU Lesser General Public License. Similar to init, systemd is the parent of all other processes directly or indirectly and is the first process that starts at boot hence typically assigned a “pid=1“.

A systemd, may refer to all the packages, utilities and libraries around daemon. It was designed to overcome the shortcomings of init. It itself is a background processes which is designed to start processes in parallel, thus reducing the boot time and computational overhead. It has a lot other features as compared to init.

Why there was a need to replace init?

A init process starts serially i.e., one task starts only after the last task startup was successful and it was loaded in the memory. This often resulted into delayed and long booting time. However, systemd was not designed for speed but for getting the things done neatly which in turns avoid all the UN-necessary delay.

Features of systemd

1. Clean, stateforward and efficient design.
2. Simpler boot process.
3. Concurrent and parallel processing at boot.
4. Better API.
5. Simple Unit Syntax.
6. Ability to remove optional components.
7. Low memory footprints.
8. Improved technique to express dependencies.
9. Initialization instruction written in config file and not in shell script.
10. Make use of Unix Domain Socket.
11. Job Scheduling using systemd Calendar Timers.
12. Event Logging with journald.
13. Choice of logging System events with systemd as well as syslog.
14. Logs are stored in binary file.
15. systemd state can be preserved to be called later in future.
16. Track process using kernel’s cgroup and not PID.
17. Users login managed by systemd-logind.
18. Better integration with Gnome for interoperability.

Bottlenecks systemd

1. Everything at one place.
2. Not POSIX standard.

Systemd and Distro Integration

Linux Distribution	Integration
Fedora	Yes, first distro to adopt systemd
Arch	Yes
RedHat	Yes
CentOS	Yes
Debian	Yes, Debian 8 codename Jessie will have systemd by default
Gentoo	Yes, but needs to be downloaded, installed and configure side with custom init
OpenSUSE	Yes
Slack	No (Though it has not been adopted till now in slackware, Patric Volkerding has not shown any indication if it will be adopted or not)
Ubuntu	Yes, needs to be installed and configured with Upstream.

Controversy

Linus Torvalds, Chief architect of Linux kernel, feels attitude of key developer of systemd towards users and bug reports do not seems ok. It was also reported that systemd philosophy is weird and a foreign way to control system processes. The same has been recorded from Patric Volkerding and other notable Linux Users and Developers as well as over online forum, time-to-time.

systemd vs init

Features	init	systemd
DBus Dependency – Mandatory	No	Yes
Device based Activation	No	Yes
Device dependency configuration with udev	No	Yes
Timer based Activation	Cron/at	Proprietary
Quota Management	No	Yes
Automatic Service Dependency Handling	No	Yes
Kills users Process at logout	No	Yes
Swap Management	No	Yes
SELinux integration	No	Yes
Support for Encrypted HDD	No	Yes
Static kernle module loading	No	Yes
GUI	No	Yes
List all the child processes	No	Yes
Sysv compatible	Yes	Yes
Interactive booting	No	Yes
Portable to non x86	Yes	No
Adopted on	Several Distro	Several Distro
Parallel service startup	No	Yes
Resource limit per service	No	Yes
Easy extensible startup script	Yes	No
Separate Code and Configuration File	Yes	No
Automatic dependency calculation	No	Yes
Verbose debug	Yes	No
Version	N/A	V44+
Size	560 KB	N/A
Number of Files	75 files	900 files + glib + DBus
Lines of code – LOC	15000 (Approx)	224000 (Approx) (inc Codes, comments and white space) 125000 (Approx) (acctual code)

Conclusion

Anything running as pid=1 must not break, must not be mess and must be controlled by users effectively and efficiently. Many-a-user believes that replacing init for systemd is nothing more than reinventing the wheel everytime as a side effect of Linux. But this is the diverse nature of Linux. This is because Linux is that much powerful. Change is good and we must appreciate it if it is for a good reason.

That’s all for now. I’ll be here again with another Interesting article you people will love to read. Till then stay tuned and connected to Tecmint. Don’t forget to provide us with your valuable feedback in the comments below.

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