Learn Modern Service Management System (Systemd) on Linux

systemd is a modern system and service manager for Linux operating systems. Running as the first process on boot (PID 1), it acts as an init system that initializes and maintains userspace services, replacing traditional SysV init scripts with a more efficient service management framework.

systemd provides comprehensive system initialization, service management, and system state control through its suite of tools, primarily systemctl for service control and systemd-analyze for performance analysis.

Basic systemd Information

To get help information about systemd, use the following command −

$ systemd -h

Starts up and maintains the system or user services.

-h --help                         Show this help
--test                           Determine startup sequence, dump it and exit
--no-pager                       Do not pipe output into a pager
--dump-configuration-items        Dump understood unit configuration items
--unit=UNIT                      Set default unit
--system                         Run a system instance, even if PID != 1
--user                          Run a user instance
--dump-core[=BOOL]              Dump core on crash
--crash-vt=NR                   Change to specified VT on crash
--crash-reboot[=BOOL]           Reboot on crash
--crash-shell[=BOOL]            Run shell on crash

Version Information

To check the systemd version and compile-time features −

$ systemd --version

systemd 229
+PAM +AUDIT +SELINUX +IMA +APPARMOR +SMACK +SYSVINIT +UTMP +LIBCRYPTSETUP
+GCRYPT +GNUTLS +ACL +XZ -LZ4 +SECCOMP +BLKID +ELFUTILS +KMOD -IDN

Boot Process Analysis

Analyze Boot Performance

To analyze systemd boot process timing −

$ systemd-analyze

Startup finished in 8.740s (firmware) + 4.483s (loader) + 3.616s (kernel) + 4min 21.244s (userspace) = 4min 38.084s

Detailed Service Boot Times

To see which services took the longest to start during boot −

$ systemd-analyze blame

3min 59.399s apt-daily.service
     16.585s NetworkManager-wait-online.service
     11.182s grub-common.service
      9.532s apport.service
      9.224s irqbalance.service
      8.986s networking.service
      8.916s snapd.refresh.service
      8.859s speech-dispatcher.service
      7.709s dev-sda2.device
      7.641s gpu-manager.service

Service Management with systemctl

systemctl is the primary command for controlling systemd services. It allows you to start, stop, restart, enable, disable, and monitor services.

Basic Service Operations

Example: Managing Bluetooth Service

Start the bluetooth service −

$ sudo systemctl start bluetooth.service

Check service status −

$ systemctl status bluetooth

bluetooth.service - Bluetooth service
Loaded: loaded (/lib/systemd/system/bluetooth.service; enabled; vendor preset: enabled)
Active: active (running) since Fri 2017-01-27 09:32:45 IST; 3 days ago
Docs: man:bluetoothd(8)
Main PID: 954 (bluetoothd)
Status: "Running"
CGroup: /system.slice/bluetooth.service
??954 /usr/lib/bluetooth/bluetoothd

Advanced Service Control

Masking and Unmasking Services

Masking prevents a service from being started manually or automatically −

$ sudo systemctl mask bluetooth.service
$ sudo systemctl unmask bluetooth.service

Service Dependencies

View service dependencies −

$ systemctl list-dependencies bluetooth.service

Critical Chain Analysis

Analyze the critical chain for a specific service −

$ systemd-analyze critical-chain bluetooth.service

bluetooth.service +886ms

System Control Groups (cgroups)

Control Group Hierarchy

View the control group hierarchy −

$ systemd-cgls

Control group /:
-.slice
??init.scope
? ??1 /lib/systemd/systemd --system --deserialize 19
??system.slice
? ??bluetooth.service
? ? ??954 /usr/lib/bluetooth/bluetoothd
? ??dbus.service
? ? ??759 /usr/bin/dbus-daemon --system
? ??cron.service
?   ??850 /usr/sbin/cron -f

Resource Usage Monitoring

Monitor resource usage by control groups −

$ sudo systemd-cgtop

Control Group                    Tasks   %CPU   Memory  Input/s Output/s
/                                    -   20.0     6.3G        -        -
/system.slice                      125      -        -        -        -
/user.slice                        695      -        -        -        -

Unit File Management

Listing Units and Unit Files

List all active units −

$ systemctl list-units

List all unit files and their states −

$ systemctl list-unit-files

List specific unit types −

$ systemctl list-unit-files --type=service
$ systemctl list-unit-files --type=socket

System State Control

systemd provides commands for system power management −

Conclusion

systemd revolutionizes Linux system management with its unified approach to service control, boot process optimization, and resource management. Its powerful tools like systemctl and systemd-analyze provide administrators with comprehensive control over system services and performance monitoring, making it an essential skill for modern Linux administration.