Recently I came to know about the runlevels in linux while going through initd. So I thought of writing about it and here it is what are runlevels in linux systems?
Run-levels are the mode in which you unix system boots up. These modes can be from single user with no networking to full fledge desktop like boot. There are 7 runlevels defined and these are numbered from 0-6.Following is the list of the runlevels.
These modes defines the features that your system can provide like if you boot up your systems in mode where networking is not enabled you won’t be able to connect your computer to other machines on the network.
What are runlevels in linux systems?
Here is a list of runlevels in linux:
| Run Level | Mode | What is does |
|---|---|---|
| 0 | Halt | System Shutdown |
| 1 | Single User | No network interfaces, start daemons, or allow non-root logins |
| 2 | Multi User | No configure network interfaces or start daemons. |
| 3 | Multi User with Networking | Starts the system normally.Like most of the servers with terminal |
| 4 | User defined | Not used/User-definable |
| 5 | Multi User with display | As most of the desktops are launched with display. |
| 6 | Reboot | Reboots the system |
These are standard runlevels and are used while init the system.
There is a runlevel 4, which is custom and user can define the capabilities that the machine will provide when launched in that particular run-level.
To see which runlevel your system is running in. Open terminal and type
runlevel
It will show the runlevel like below
These are used in initd to write startup init scripts. Initd is the first process that gets launched when you boot up your machine and the process ID is 1.
These runlevels are replace with target in systemd systems. You can read about them here.
https://www.learnsteps.com/what-are-targets-in-systemd-how-is-it-mapped-to-runlevels-in-system-v/
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