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

Last modified March 24, 2021: big docs v4 cleanup (80869a1a7)