systemd

Official Site

Systemd is a suite of basic building blocks for a Linux system. It provides a system and service manager that runs as PID 1 and starts the rest of the system.

Units

Services (Daemons) [Uses systemctl]
Timers (similar to Cron)
Mounts (File system mounts, from /etc/fstab)
Devices
Sockets
List all units for a type:
```
systemctl list-units --type <name>
```

Serices

Start a service:
```
systemctl start <name>
```
Stop a service:
```
systemctl stop <name>
```
Enable a service to start on boot:
```
systemctl enable <name>
```
Disable a service from starting on boot:
```
systemctl disable <name>
```
Disable a service from being started by other services:
```
systemctl mask <name>
```

Status of a service:

systemctl status <name>

Enable [or disable or mask] and start immediatly:

systemctl enable [disable | mask] --now <name>

Reload service units

systemctl daemon-reload

Journal

Arch Wiki

systemd has its own logging system called the journal; running a separate logging daemon is not required. To read the log, use man journalctl(1)

Show all messages since boot:

journalctl -b [-n for nth boot, 0 = current, 1 = previous etc]

Show all messages with prority higher than n [2 = critical, 3 = error, 4 = warn]:
```
journalctl -p <n>
```
Show all messages for a unit:
```
journalctl -u <unit-name>
```
- If unit is in ~/.config/ then instead of -u, use --user-unit
Show all messages for an executable:
```
journalctl /path/to/executable
```

Clear up old logs

# Check size
journalctl --disk-usage

# Rotate logs
sudo journalctl --rotate

# Remove all logs beyond 7d
sudo journalctl --vacuum-time=7d

Timers

Arch Wiki

Timers are systemd unit files with a suffix of .timer. Timers are like other unit configuration files and are loaded from the same paths but include a [Timer] section which defines when and how the timer activates.

For each .timer file, a matching .service file exists (e.g. foo.timer and foo.service). The .timer file activates and controls the .service file.

Example of a timer service:

Website link #Demo
Executable to run (can be any language, using a bash script here). Inside $HOME/bin/schedule-test.sh:
```
#!/usr/bin/env bash
echo "This is only a test: $(date)" >> "$HOME/schedule-test-output.txt"
```
- Make sure the executable is... executable.
```
chmod u+x $HOME/bin/schedule-test.sh
```

Systemd Service Unit inside ~/.config/systemd/user/schedule-test.service:

[Unit]
Description=A job to test the systemd scheduler

[Service]
Type=simple
ExecStart=/home/<user>/bin/schedule-test.sh

[Install]
WantedBy=default.target

Systemd Timer Unit inside ~/.config/systemd/user/schedule-test.timer:

[Unit]
Description=Schedule a message every 1 minute

[Timer]

#Execute job if it missed a run due to machine being off
Persistent=true

#Run 120 seconds after boot for the first time
OnBootSec=120

#Run every 1 minute thereafter
OnUnitActiveSec=60

#File describing job to execute
Unit=schedule-test.service

[Install]

# So systemd can manage automatically
WantedBy=timers.target

Can use OnCalendar instead of OnUnitActiveSec

Reload daemons and start service to test:

systemctl --user daemon-reload
systemctl --user start schedule-test.service
tail $HOME/schedule-test-output.txt

Enable timer if test pass and check status:

systemctl --user enable --now schedule-test.timer
systemctl --user status schedule-test.timer
tail -f $HOME/schedule-test-output.txt

If want to run sudo, put files in /etc/systemd/system and use sudo systemctl
If want to run as user but need sudo as well, add User=username in [Service]

Systemd service order

local-fs-pre.target
         |
         v
(various mounts and   (various swap   (various cryptsetup
 fsck services...)     devices...)        devices...)       (various low-level   (various low-level
         |                  |                  |             services: udevd,     API VFS mounts:
         v                  v                  v             tmpfiles, random     mqueue, configfs,
  local-fs.target      swap.target     cryptsetup.target    seed, sysctl, ...)      debugfs, ...)
         |                  |                  |                    |                    |
         \__________________|_________________ | ___________________|____________________/
                                              \|/
                                               v
                                        sysinit.target
                                               |
          ____________________________________/|\________________________________________
         /                  |                  |                    |                    \
         |                  |                  |                    |                    |
         v                  v                  |                    v                    v
     (various           (various               |                (various          rescue.service
    timers...)          paths...)              |               sockets...)               |
         |                  |                  |                    |                    v
         v                  v                  |                    v              rescue.target
   timers.target      paths.target             |             sockets.target
         |                  |                  |                    |
         v                  \_________________ | ___________________/
                                              \|/
                                               v
                                         basic.target
                                               |
          ____________________________________/|                                 emergency.service
         /                  |                  |                                         |
         |                  |                  |                  To do this, we first need to add an [Install]
         v                  v                  v                                 emergency.target
     display-        (various system    (various system
 manager.service         services           services)
         |             required for            |
         |            graphical UIs)           v
         |                  |           multi-user.target
         |                  |                  |
         \_________________ | _________________/
                           \|/
                            v
                  graphical.target