Journalctl

journalctl command allows to query the contents of the systemd journal/logs.

Configuration

• /etc/systemd/journald.conf

Basic Commands

• journalctl or journalctl --no-pager
• journalctl -r (reverse order)
• journalctl -k: Show only kernel messages
• journalctl -f (follow)
• journalctl -u service-name.service

journalctl -u ssh (See also sshd logs)

-u unit_name

journalctl -u service-name.service -f

journalctl -u libvirtd

• journalctl --verify
• Disk Usage/Logging: journalctl --disk-usage
• journalctl --vacuum-size=200M
  • --vacuum-size=BYTES Reduce disk usage below specified size
  • --vacuum-files=INT Leave only the specified number of journal files
  • --vacuum-time=TIME Remove journal files older than specified time
• journalctl --list-boots. See also: last (command)
• journalctl -xb

-x --catalog Includes explanations in log lines from message catalgog

-b show message from current boot. if [ID] is provided will show messages from that specific boot.

See also

• Systemd: systemctl, systemd-journald, journalctl systemd-cat, systemd-logind, systemd-networkd:networkctl, systemd-timesyncd, systemd-resolved, systemd-udevd , Systemd-logind: loginctl, hostnamectl, udevadm, systemd-run, portablectl, systemd-nspawn, systemd resource management, Timer (systemd), systemd logs, /etc/systemd/, systemd services, systemd-mount, /run/systemd/, .service, /usr/lib/systemd/, /usr/lib/systemd/system/, /usr/lib/systemd/system/systemd-resolved.service, /lib/systemd/, machinectl, systemd-oomd, systemd-analyze
• /etc/inittab and respawn option
• launchd init and operating system service management daemon for MacOS
• Linux logging, Cisco IOS logging

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Source: https://en.wikiversity.org/wiki/Linux_Administration/System_Services/System_Logging/Journalctl