Managing
services with systemd:
Powerful
technology from 2008. How it works, what it does we can see here.
How are services controlled under Linux?
Man system uses init system. When system boots up it calls Kernel
which is wrote by Linus Torvalds. It itself don't do much. After kernel start
it starts init. Init will start various other services. It is used in Unix we
call sysvinit or System Five.
Drawbacks of SysVinit
It runs in serial order. Like script 1, script 2, and script 3. If
script 3 takes long time then it delays starting script 4.
In current situation, we constantly plug-in new devices like thumb
drive. In that case we need to re-run the scripts to look for missing hardware.
This is problem with sysvinit.
What SystemD does?
It is a system daemon which means any program that runs in the
background. It always stays online unlike SysVinit, so that it can detect
hardware changes. It can do things in parallel. It is faster and more stable.
It does lot of stuff in binary, while SysVinit allows you to change
configuration in a text file which many Linux users prefer.
SystemD reaches out to hard drive and find boots files, loads up the
Linux kernel. Linux Kernel launches init, in our case it is systemd. SystemD
will start everything else we interact with our system
How do we know whether we are using SystemD?
Major distro use SystemD by default. Sometimes people use upstart
which is modified version of SysVinit. Distro like Slackware, Devaun (variant
of Debian) still uses sysvinit.
How to verify if I have systemd or
sysvinit?
Init is our very first process. You can try firing ps aux if
you scroll up see the first process it shows what process it started first, it
should be systemd. Sometime it says /sbin/init which is SysVinit but you need
to dig deeper to check. ls -l
/sbin/init this may have symbolic link
to systemD. In these cases the ps output will still show init but under the
hood it is systemD. /sbin/init should be always present in the Linux system as per
the standard. So all of the recent distro would have symlinked to systemD.
SystemD starts right after the
Kernel, what happens next?
It uses unit file to start other services. It is
stored in /lib/systemd/system. There will be ton of service file with .service and
.target extension. .target is for GUI. .socket files are for network which
allows two application talk to each other via a network. .mount file contains
hard drive partition. We will still have /etc/fstab.
Example exploring sshd.service:
It starts with block [Unit]
After ->
will have dependencies. For sshd we need network services. So it will first
start network.services
Wants ->
Says what else to start along with sshd.service.
[Service] -> This block will specify actual service to be started.
EnvironmentFile -> Where we have environment variables
ExecStart -> Actual command that will start the service
ExecReload -> Usually kill command
KillMode -> Default is process
Restart ->
Says when to restart (on-failure) is default
RestartSec -> Wait time after failure
[Install] -> Specifies target
WantedBy=multi-user.target
When command line starts, it starts sshd. Most of the times it is
created.
How to modify the Unit files?
It is not modified directly. Don't anything
manually inside /lib. Go to /etc/systemd/system.
Sometime the folder won't exists we might need to create it.
Again in this folder you will see service, wants and target file. This will
override the files in /lib/systemd
SystemD binary is located @ /lib/systemd
Kernel -> Do all hardware interactions
SystemD -> Do all software interactions
Do we have to create new files
for Unit services?
Most of the software installation will come with Unit Files.
So while installing we get and while uninstalling it get removed.
If we want the service to start when system reboots we can
enable using below command:
sudo systemctl enable httpd
This will just create symlink to /etc folder because it takes
more precedence than /lib. You can also disable using below command:
sudo systemctl disable httpd
The command only disable starting of service after reboot.
You can still start manually.
How systemD keps track of
which service to run?
it is done using target files. .target files keep
track of what needs to run next. For example: If we need command line then multi-user.target will get triggered. Then it will see all the
entries inside the file which has other dependent target files.
If we edit, then it won't recognize unless we reload.
Another example: To Turn Off GUI
sudo systemctl isoloate multi-user.target
To Turn on GUI
sudo systemctl isoloate graphical.target
