Linux Boot Series – SystemD

boot process SystemD

Though SystemD will be compared to its predecessor SystemV for a long time , it has much more to offer in terms of System Management. It is a new way how Linux interact with the underline Objects such as hardware , sockets , application processes and many more.

Understand How SystemD works

systemd is a system and service manager for Linux, compatible with SysV and LSB init scripts.

Features
  1. Contrary to its predecessor SystemD handles processes parallel
  2. socket and D-Bus activation
    traditionally services will be configured to start on boot, but with systemd it is more event driven and you can configure to start when something connects to a specific port or a device get connected , this is called socket and d-bus activation.
  3. Offers on-demand starting of daemons, also keeps track of processes using Linux cgroups
  4. Supports snapshot and restoring of the system state
  5. Maintains mount and automount points
  6. Implements an elaborate transactional dependency-based service control logic.

systemD working

Concept of Units in SystemD

SystemD manages units, which are representations of system resources and services.

Type of Units in SystemD
  1. Service
    A Service unit is used to manage service, the unit file include instruction to start , stop , restart the service.
  2. Socket A network socket associated with a service.
  3. Device
    Unit file related to Device is used to manage Device , start , stop , auto etc.
  4. Mount Unit manages the Mount points via SystemD
  5. Automount Unit file mounts the file system on system boot. This feature might replace traditional fstab files in the near future.
  6. Swap Mounts swap space on the system
  7. Target Targets are much like Runlevel used previously to manage different services to start and stop at different
  8. Path
    A path for path-based activation. For example, you can start services based on the state of a certain path, such as whether it exists or not.
  9. Timer Timer unit is used similar to Crontab to schedule other units.
  10. Snapshot
    A snapshot of the current systemd state. Usually used to rollback after making temporary changes to systemd.
  11. Slice Restriction of resources through Linux Control Group nodes (cgroups).
  12. Scope Information from systemd bus interfaces. Usually used to manage external system processes.
systemctlcommand

is the primary tool to manage SystemD. It can be used for starting , stopping of services as well as enabling and disabling , this was previously performed with service and chkconfig commands in the previous versions.

Basic Service Management Syntax
Description: Command
Start the Service systemctl start foo
Stop the Service systemctl stop foo
Restart the Service systemctl restart foo
Status of the Service systemctl status foo
Enable Service to start at bootime systemctl enable foo
Disable the Service systemctl disable foo
Check if service is enabled
?
systemctl is-enabled foo

Mask
the Service
systemctl mask foo
Reload the updated unit file systemctl daemon-reload
Show Failed Services systemctl -failed
Reset any failed service systemctl reset-failed
Show properties of the Unit systemctl show < service >
Edit the Service Unit systemctl edit < service >
Edit the Full Service Unit systemctl edit --full < service >
Run on remote host systemctl -H < host_name > status network
Changing System State
Reboot host systemctl reboot
Poweroff host systemctl poweroff
Switch to Emergency mode systemctl emergency
Log back to default mode (Multi-User) systemctl default
Viewing Log Messages
Show all log messages journalctl
Show only kernel log messages journalctl -k
Show log for specific service journalctl -u network.service
Follow messages as they appear journalctl -f

Besides services, most systemd commands can work with these unit types: paths,
slices, snapshots, sockets, swaps, targets, and timers

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