SysVinit runlevels
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Video Transcription
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>> Hey, Cybrarians.
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>> Welcome back to
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>> the Linux Plus course here at Cybrary.
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I'm your instructor Rob Goelz.
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In today's lesson, we're going to be
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covering system V runlevels.
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Upon completion of today's lesson,
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you are going to understand the purpose of
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runlevels in system V or with SysVinit,
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we're going to talk about the different runlevels
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that are used and
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how SysVinit interacts with /etc/inittab,
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rc.d, and rc.local.
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Then finally, we'll see how we can use the init or
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telinit command to change runlevels.
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In system V, a runlevel indicates
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a system states such as a multi-user environment,
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a graphical environment, or a rescue shell.
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This runlevel is used by the init process to determine
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what services or resources are available at that state.
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In other words, if you come
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up in a multi-user environment,
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do you need a graphical environment?
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No, you don't need a graphics,
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you don't need a display manager,
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it's just a multi-user environment with a shell.
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Really all of these runlevels are just sets
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of services that are grouped to run together.
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Runlevel 0, and we can compare these to systemd.
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Just like when we were looking at systemd,
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we compared it against SysV.
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Runlevel 0, shuts down the system.
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In systemd, that would be the poweroff.target.
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Runlevel 1 configures a rescue shell,
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that's the rescue.target in systemd.
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Then runlevel 2 is a non-graphical
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multi-user environment with networking disable.
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There's not really an equivalent for that in the systemd.
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Runlevel 3 is a non-graphical multi-user environment
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with networking enabled,
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and that is the multiuser target in systemd,
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that's where you come up with no graphics,
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but just a shell.
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Now runlevel 4 is something that is
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here in system V and SysVinit,
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but it's unused in systemd.
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Runlevel 5 is our graphic multi-user environment,
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that's the graphical target in systemd.
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Then finally runlevel 6,
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reboots the system, that's the reboot target in systemd.
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Now let's step back for a minute and think about this.
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We're at the initialization phase.
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How does this system know which runlevel
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to initialize with when we're talking about System V?
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Well, it uses /etc/inittab.
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The inittab is actually a script,
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and this script points to runlevel scripts and
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those runlevel scripts define which scripts be
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executed for each runlevel.
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/etc/inittab defines the scripts
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to be executed at each runlevel.
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Well, where are those scripts stored?
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Well, the runlevel scripts are stored in /etc/rc.d,
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that directory, and
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there's directories for each runlevel.
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We can see those on the right-hand side,
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rc 0 through six.
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Then the scripts themselves have an S or a K to
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indicate what happens to
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other services that start at this runlevel.
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If the script has an S in front of the service,
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that means that the service gets
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started when you go into this runlevel.
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But if the service has a K in front of it,
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when you go into this run level,
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it's going to kill that service,
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it's going to stop that service from running.
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Really this is just the way that it specifies
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which services start under the runlevel,
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and which services are stopped or
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killed when transferring it into that runlevel.
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Now, the rc.local file is
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another script and its purpose is to run at boot or
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whenever you change boot levels
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because it's going to contain
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the commands that are going to run
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after you boot or
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after you change into a different runlevel.
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Speaking of changing runlevels, how do you do that?
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Well, if you were in systemd,
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we saw how we could do that.
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We could change to a different run level
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called a target in
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systemd with the command system CTL isolate,
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and then the target that we want to go to.
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But how do you change runlevels in SysVinit?
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Well, you use the init or the telinit command,
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and really all that does is it calls
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the script /etc/rc.d/rc,
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and it only takes a one parameter,
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and that's the runlevel number.
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Then what happens is it runs the kill scripts and
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start scripts in that runlevel directory that we saw,
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to determine which services
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start and which services get
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killed when you go into that runlevel.
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But with that, we've reached the end of this lesson.
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In this lesson we covered
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the purposes of runlevels in SysVinit,
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we talked about the definitions of the runlevels 0-6.
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We also talked about how SysVinit
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interacts with runlevels looking
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at /etc/inittab, /etc/rc.d and /etc/rclocal.
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Then finally we talked about changing
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runlevels with init and telinit.
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Thanks so much for being here and I look
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forward to seeing you in our next lesson.
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