Systemd

• 1: Enabling `systemd` Debug Shell During Boot
• 2: Troubleshooting Services With 'systemctl`
• 3: Analyzing System Logs with `journalctl`
• 4: Inspecting Services with `systemd-analyze`
• 5: Inspecting Services with `systemd-analyze`systemd

systemctl stop sshd
systemctl start sshd

1 - Enabling `systemd` Debug Shell During Boot

To diagnose systemd related boot issues, you can enable early shell access during boot.

Perform the following steps to enable early shell access:

1. Restart the Photon OS machine or the virtual machine running Photon OS.

   When the Photon OS splash screen appears, as it restarts, type the letter e quickly.

2. Append systemd.debug-shell=1 to the kernel command line.

   Optionally, to change logging level to debug, you can append systemd.log_level=debug.

3. Press F10 to proceed with the boot.

4. Press Alt+Ctrl+F9 to switch to tty9 to access the debug shell.

2 - Troubleshooting Services With 'systemctl`

To view a description of all the active, loaded units, execute the systemctl command without any options or arguments:

systemctl

To see all the loaded, active, and inactive units and their description, run this command:

systemctl --all

To see all the unit files and their current status but no description, run this command:

systemctl list-unit-files

The grep command filters the services by a search term, a helpful tactic to recall the exact name of a unit file without looking through a long list of names. Example:

systemctl list-unit-files | grep network
org.freedesktop.network1.busname           static
dbus-org.freedesktop.network1.service      enabled
systemd-networkd-wait-online.service       enabled
systemd-networkd.service                   enabled
systemd-networkd.socket                    enabled
network-online.target                      static
network-pre.target                         static
network.target  

For example, to list all the services that you can manage on Photon OS, you run the following command instead of ls /etc/rc.d/init.d/:

systemctl list-unit-files --type=service

Similarly, to check whether the sshd service is enabled, on Photon OS you run the following command instead of chkconfig sshd:

systemctl is-enabled sshd

The chkconfig --list command that shows which services are enabled for which runlevel on a SysVinit computer becomes substantially different on Photon OS because there are no runlevels, only targets:

ls /etc/systemd/system/*.wants

You can also display similar information with the following command:

systemctl list-unit-files --type=service

The following is list of some of the systemd commands that take the place of SysVinit commands on Photon OS:

USE THIS SYSTEMD COMMAND 	INSTEAD OF THIS SYSVINIT COMMAND
systemctl start sshd 		service sshd start
systemctl stop sshd 		service sshd stop
systemctl restart sshd 		service sshd restart
systemctl reload sshd 		service sshd reload
systemctl condrestart sshd 	service sshd condrestart
systemctl status sshd 		service sshd status
systemctl enable sshd 		chkconfig sshd on
systemctl disable sshd 		chkconfig sshd off
systemctl daemon-reload		chkconfig sshd --add

3 - Analyzing System Logs with `journalctl`

The journalctl tool queries the contents of the systemd journal. On Photon OS, all the system logs except the installation log and the cloud-init log are written into the systemd journal.

When you run the journalctl command without any parameters, it displays all the contents of the journal, beginning with the oldest entry.

To display the output in reverse order with new entries first, include the -r option in the command:

journalctl -r

The journalctl command includes many options to filter its output. For help troubleshooting systemd, two journalctl queries are particularly useful:

• Showing the log entries for the last boot.

  The following command displays the messages that systemd generated during the last time the machine started:

  journalctl -b

• Showing the log entries for a systemd service unit.Item

  The following command reveals the messages for only the systemd service unit specified by the -u option, which in the following example is the auditing service:

  journalctl -u auditd

You can look at the messages for systemd itself or for the network service:

journalctl -u systemd
journalctl -u systemd-networkd

Example:

root@photon-1a0375a0392e [ ~ ]# journalctl -u systemd-networkd
-- Logs begin at Tue 2016-08-23 14:35:50 UTC, end at Tue 2016-08-23 23:45:44 UTC. --
Aug 23 14:35:52 photon-1a0375a0392e systemd[1]: Starting Network Service...
Aug 23 14:35:52 photon-1a0375a0392e systemd-networkd[458]: Enumeration completed
Aug 23 14:35:52 photon-1a0375a0392e systemd[1]: Started Network Service.
Aug 23 14:35:52 photon-1a0375a0392e systemd-networkd[458]: eth0: Gained carrier
Aug 23 14:35:53 photon-1a0375a0392e systemd-networkd[458]: eth0: DHCPv4 address 198.51.100.1
Aug 23 14:35:54 photon-1a0375a0392e systemd-networkd[458]: eth0: Gained IPv6LL
Aug 23 14:35:54 photon-1a0375a0392e systemd-networkd[458]: eth0: Configured

For more information, see journalctl or the journalctl man page by running this command: man journalctl

4 - Inspecting Services with `systemd-analyze`

The systemd-analyze command reveals performance statistics for boot times, traces system services, and verifies unit files. It can help troubleshoot slow system boots and incorrect unit files. See the man page for a list of options.

Examples:

systemd-analyze blame

systemd-analyze dump

5 - Inspecting Services with `systemd-analyze`systemd

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

To manage the services run the following commands:

• systemctl or systemctl list-units : This command lists the running units.
• systemctl --failed : This command lists failed units.
• systemctl list-unit-files : This command lists all the installed unit files. The unit files are usually present in /usr/lib/systemd/system/ and /etc/systemd/system/.
• systemctl status pid : This command displays the cgroup slice, memory and parent for a PID.
• systemctl start unit : This command starts a unit immediately.
• systemctl stop unit : This command stops a unit.
• systemctl restart unit : This command restarts a unit.
• systemctl reload unit : This command asks a unit to reload its configuration.
• systemctl status unit : This command displays the status of a unit.
• systemctl enable unit : This command enables a unit to run on startup.
• systemctl enable --now unit : This command enables a unit to run on startup and start immediately.
• systemctl disable unit : This command disables a unit and removes it from the startup program.
• systemctl mask unit : This command masks a unit to make it impossible to start.
• systemctl unmask unit : This command unmasks a unit.

To get an overview of the system boot-up time, run the following command:

systemd-analyze

To view a list of all running units, sorted by the time they took to initialize (highest time on top), run the following command:

systemd-analyze blame