What starts the init process 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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What starts the init process in linux

Init is the first process started during system boot. It is a daemon process that continues running until the system is shut down. Init is the direct or indirect ancestor of all other processes, and automatically adopts all orphaned processes. It is started by the kernel using a hard-coded filename; if the kernel is unable to start it, panic will result. Init is typically assigned process identifier 1.

The init scripts (or rc) are launched by the init process to guarantee basic functionality on system start and shutdown. This includes (un)mounting of file systems and launching of daemons. A service manager takes this one step further by providing active control over launched processes, or process supervision. An example is to monitor for crashes and restart processes accordingly.

These components combine to the init system. Some inits include the service manager in the init process, or have init scripts in close relation to them. These inits are below referred to as integrated, though entries in different categories may explicitly depend on each other.

Contents

Inits (integrated)

• anopa — Init system built around the s6 supervision suite.

https://jjacky.com/anopa/ || anopaAUR

• GNU Shepherd — Init system written in Guile.

https://www.gnu.org/software/shepherd/ || shepherdAUR

• OpenRC — Dependency-based init system.

https://www.gentoo.org/proj/en/base/openrc/ || openrcAURopenrc-arch-services-gitAUR

• systemd — Dependency-based init system with aggressive parallelization, process supervision using cgroups, and the ability to depend on a given mount point or dbus service.

https://freedesktop.org/wiki/Software/systemd/ || systemd

Inits

• BusyBox — Utilities for rescue and embedded systems.

https://busybox.net/ || busybox

• sinit — Simple init initially based on Rich Felker’s minimal init.

https://core.suckless.org/sinit || sinitAUR

• SysVinit — Traditional System V init.

https://savannah.nongnu.org/projects/sysvinit || sysvinitAUR

Init scripts

• initscripts-fork — Maintained fork of SysVinit scripts in Arch Linux.

https://bitbucket.org/TZ86/initscripts-fork/overview || initscripts-forkAUR

• minirc — Minimal init script designed for BusyBox.

https://github.com/hut/minirc/ || minirc-gitAUR

• kisslinux-init — Init framework of KISS Linux.

https://github.com/kisslinux/init || kisslinux-initAUR

Service managers

• daemontools — Collection of tools for managing UNIX services.

https://cr.yp.to/daemontools.html || daemontoolsAUR

• Monit — Monit is a process supervision tool for Unix and Linux. With monit, system status can be viewed directly from the command line, or via the native HTTP(S) web server.

https://mmonit.com/monit/ || monit

• perp — Persistent process (service) supervisor and management framework for UNIX.

http://b0llix.net/perp/ || perpAUR

• runit — UNIX init scheme with service supervision, a replacement for SysVinit, and other init schemes.

http://smarden.org/runit/ || busybox

• s6 — Small suite of programs for UNIX, designed to allow service supervision in the line of daemontools and runit.

https://skarnet.org/software/s6/ || s6AUR

• Supervisor — A system that allows its users to monitor and control processes on UNIX-like operating systems.

https://supervisord.org/ || supervisor

Configuration

Migrate running services

To run daemons under the new init, save a list of running daemons:

and configure the #Init scripts accordingly. See also [2].

logind

logind requires systemd to be the init process. [3] As such, local sessions and other functionality is not available.

Add users to respective user groups for device access and reboot. Current group membership should first be checked with id user .

As Xorg.wrap does not check if logind is active [5], root rights for Xorg need be enabled manually:

Scheduled tasks

Arch uses timer files instead of cron by default. See archlinux-cronjobs for basic cron jobs.

This article or section needs expansion.

User instances of dbus-daemon are launched by systemd/User [6]. When requiring IPC between desktop applications, restore 30-dbus.sh :

Tips and tricks

systemd-nspawn

systemd-nspawn is a tool for systemd systems. Since Linux 2.6.19 it is however possible to run systemd on a non-systemd system by using PID namespace. For it, the kernel needs to be configured with CONFIG_PID_NS and CONFIG_NAMESPACES ).

The PID namespace creates a new hierarchy of processes starting with PID 1. In addition to this, systemd requires a chrooted root filesystem to be mounted. Hence, you have to at least make a bind mount, because otherwise some services will fail with

as systemd tries to remount the root with private option.

To setup a chroot with a new PID namespace you can use jchroot.[7] [8]. Make sure not to mount /proc inside the new root before chrooting, otherwise systemd will detect the chroot environment. You can mount it later once systemd is running.

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All You Need To Know About Processes in Linux [Comprehensive Guide]

In this article, we will walk through a basic understanding of processes and briefly look at how to manage processes in Linux using certain commands.

A process refers to a program in execution; it’s a running instance of a program. It is made up of the program instruction, data read from files, other programs or input from a system user.

Types of Processes

There are fundamentally two types of processes in Linux:

• Foreground processes (also referred to as interactive processes) – these are initialized and controlled through a terminal session. In other words, there has to be a user connected to the system to start such processes; they haven’t started automatically as part of the system functions/services.
• Background processes (also referred to as non-interactive/automatic processes) – are processes not connected to a terminal; they don’t expect any user input.

What is Daemons

These are special types of background processes that start at system startup and keep running forever as a service; they don’t die. They are started as system tasks (run as services), spontaneously. However, they can be controlled by a user via the init process.

Linux Process State

Creation of a Processes in Linux

A new process is normally created when an existing process makes an exact copy of itself in memory. The child process will have the same environment as its parent, but only the process ID number is different.

There are two conventional ways used for creating a new process in Linux:

• Using The System() Function – this method is relatively simple, however, it’s inefficient and has significantly certain security risks.
• Using fork() and exec() Function – this technique is a little advanced but offers greater flexibility, speed, together with security.

How Does Linux Identify Processes?

Because Linux is a multi-user system, meaning different users can be running various programs on the system, each running instance of a program must be identified uniquely by the kernel.

And a program is identified by its process ID (PID) as well as it’s parent processes ID (PPID), therefore processes can further be categorized into:

• Parent processes – these are processes that create other processes during run-time.
• Child processes – these processes are created by other processes during run-time.

The Init Process

Init process is the mother (parent) of all processes on the system, it’s the first program that is executed when the Linux system boots up; it manages all other processes on the system. It is started by the kernel itself, so in principle it does not have a parent process.

The init process always has process ID of 1. It functions as an adoptive parent for all orphaned processes.

You can use the pidof command to find the ID of a process:

Find Linux Process ID

To find the process ID and parent process ID of the current shell, run:

Find Linux Parent Process ID

Starting a Process in Linux

Once you run a command or program (for example cloudcmd – CloudCommander), it will start a process in the system. You can start a foreground (interactive) process as follows, it will be connected to the terminal and a user can send input it:

Start Linux Interactive Process

Linux Background Jobs

To start a process in the background (non-interactive), use the & symbol, here, the process doesn’t read input from a user until it’s moved to the foreground.

Start Linux Process in Background

You can also send a process to the background by suspending it using [Ctrl + Z] , this will send the SIGSTOP signal to the process, thus stopping its operations; it becomes idle:

To continue running the above-suspended command in the background, use the bg command:

To send a background process to the foreground, use the fg command together with the job ID like so:

Linux Background Process Jobs

States of a Process in Linux

During execution, a process changes from one state to another depending on its environment/circumstances. In Linux, a process has the following possible states:

• Running – here it’s either running (it is the current process in the system) or it’s ready to run (it’s waiting to be assigned to one of the CPUs).
• Waiting – in this state, a process is waiting for an event to occur or for a system resource. Additionally, the kernel also differentiates between two types of waiting processes; interruptible waiting processes – can be interrupted by signals and uninterruptible waiting processes – are waiting directly on hardware conditions and cannot be interrupted by any event/signal.
• Stopped – in this state, a process has been stopped, usually by receiving a signal. For instance, a process that is being debugged.
• Zombie – here, a process is dead, it has been halted but it’s still has an entry in the process table.

How to View Active Processes in Linux

There are several Linux tools for viewing/listing running processes on the system, the two traditional and well known are ps and top commands:

1. ps Command

It displays information about a selection of the active processes on the system as shown below:

List Linux Active Processes

2. top – System Monitoring Tool

List Linux Running Processes

Read this for more top usage examples: 12 TOP Command Examples in Linux

3. glances – System Monitoring Tool

glances is a relatively new system monitoring tool with advanced features:

Glances – Linux Process Monitoring

There are several other useful Linux system monitoring tools you can use to list active processes, open the link below to read more about them:

How to Control Processes in Linux

Linux also has some commands for controlling processes such as kill, pkill, pgrep and killall, below are a few basic examples of how to use them:

Control Linux Processes

To learn how to use these commands in-depth, to kill/terminate active processes in Linux, open the links below:

Note that you can use them to kill unresponsive applications in Linux when your system freezes.

Sending Signals To Processes

The fundamental way of controlling processes in Linux is by sending signals to them. There are multiple signals that you can send to a process, to view all the signals run:

List All Linux Signals

To send a signal to a process, use the kill, pkill or pgrep commands we mentioned earlier on. But programs can only respond to signals if they are programmed to recognize those signals.

And most signals are for internal use by the system, or for programmers when they write code. The following are signals which are useful to a system user:

• SIGHUP 1 – sent to a process when its controlling terminal is closed.
• SIGINT 2 – sent to a process by its controlling terminal when a user interrupts the process by pressing [Ctrl+C] .
• SIGQUIT 3 – sent to a process if the user sends a quit signal [Ctrl+D] .
• SIGKILL 9 – this signal immediately terminates (kills) a process and the process will not perform any clean-up operations.
• SIGTERM 15 – this a program termination signal (kill will send this by default).
• SIGTSTP 20 – sent to a process by its controlling terminal to request it to stop (terminal stop); initiated by the user pressing [Ctrl+Z] .

The following are kill commands examples to kill the Firefox application using its PID once it freezes:

To kill an application using its name, use pkill or killall like so:

Changing Linux Process Priority

On the Linux system, all active processes have a priority and certain nice value. Processes with higher priority will normally get more CPU time than lower priority processes.

However, a system user with root privileges can influence this with the nice and renice commands.

From the output of the top command, the NI shows the process nice value:

List Linux Running Processes

Use the nice command to set a nice value for a process. Keep in mind that normal users can attribute a nice value from zero to 20 to processes they own.
Only the root user can use negative nice values.

To renice the priority of a process, use the renice command as follows:

Check out our some useful articles on how to manage and control Linux processes.

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