systemd/Journal

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

In Arch Linux, the directory /var/log/journal/ is a part of the systemd package, and the journal (when Storage= is set to auto in /etc/systemd/journald.conf) will write to /var/log/journal/. If that directory is deleted, systemd will not recreate it automatically and instead will write its logs to /run/systemd/journal in a nonpersistent way. However, the directory will be recreated if Storage=persistent is added to journald.conf and systemd-journald.service is restarted (or the system is rebooted).

Systemd journal classifies messages by Priority level and Facility. Logging classification corresponds to classic Syslog protocol (RFC 5424).

Priority level

A syslog severity code (in systemd called priority) is used to mark the importance of a message RFC 5424 6.2.1.

Value	Severity	Keyword	Description	Examples
0	Emergency	emerg	System is unusable	Severe Kernel BUG, systemd dumped core. This level should not be used by applications.
1	Alert	alert	Should be corrected immediately	Vital subsystem goes out of work. Data loss. kernel: BUG: unable to handle kernel paging request at ffffc90403238ffc.
2	Critical	crit	Critical conditions	Crashes, coredumps. Like familiar flash: systemd-coredump[25319]: Process 25310 (plugin-containe) of user 1000 dumped core Failure in the system primary application, like X11.
3	Error	err	Error conditions	Non-fatal error reported: kernel: usb 1-3: 3:1: cannot get freq at ep 0x84, systemd[1]: Failed unmounting /var., libvirtd[1720]: internal error: Failed to initialize a valid firewall backend
4	Warning	warning	May indicate that an error will occur if action is not taken	A non-root file system has only 1GB free. org.freedesktop. Notifications[1860]: (process:5999): Gtk-WARNING **: Locale not supported by C library. Using the fallback 'C' locale
5	Notice	notice	Events that are unusual, but not error conditions	systemd[1]: var.mount: Directory /var to mount over is not empty, mounting anyway, gcr-prompter[4997]: Gtk: GtkDialog mapped without a transient parent. This is discouraged
6	Informational	info	Normal operational messages that require no action	lvm[585]: 7 logical volume(s) in volume group "archvg" now active
7	Debug	debug	Messages which may need to be enabled first, only useful for debugging	kdeinit5[1900]: powerdevil: Scheduling inhibition from ":1.14" "firefox" with cookie 13 and reason "screen"

These rules are recommendations, and the priority level of a given error is at the application developer's discretion. It is always possible that the error will be at a higher or lower level than expected.

Facility

A syslog facility code is used to specify the type of program that is logging the message RFC 5424 6.2.1.

Facility code	Keyword	Description	Info
0	kern	Kernel messages
1	user	User-level messages
2	mail	Mail system	Archaic POSIX still supported and sometimes used (for more mail(1))
3	daemon	System daemons	All daemons, including systemd and its subsystems
4	auth	Security/authorization messages	Also watch for different facility 10
5	syslog	Messages generated internally by syslogd	For syslogd implementations (not used by systemd, see facility 3)
6	lpr	Line printer subsystem (archaic subsystem)
7	news	Network news subsystem (archaic subsystem)
8	uucp	UUCP subsystem (archaic subsystem)
9		Clock daemon	systemd-timesyncd
10	authpriv	Security/authorization messages	Also watch for different facility 4
11	ftp	FTP daemon
12	-	NTP subsystem
13	-	Log audit
14	-	Log alert
15	cron	Scheduling daemon
16	local0	Local use 0 (local0)
17	local1	Local use 1 (local1)
18	local2	Local use 2 (local2)
19	local3	Local use 3 (local3)
20	local4	Local use 4 (local4)
21	local5	Local use 5 (local5)
22	local6	Local use 6 (local6)
23	local7	Local use 7 (local7)

Useful facilities to watch: 0, 1, 3, 4, 9, 10, 15.

Filtering output

journalctl allows for the filtering of output by specific fields. If there are many messages to display, or if the filtering of large time spans has to be done, the output of this command can be extensively delayed.

Examples:

• Show all messages matching PATTERN:

  # journalctl --grep=PATTERN

• Show all messages from this boot:

  # journalctl -b

  However, often one is interested in messages not from the current, but from the previous boot (e.g. if an unrecoverable system crash happened). This is possible through optional offset parameter of the -b flag: journalctl -b -0 shows messages from the current boot, journalctl -b -1 from the previous boot, journalctl -b -2 from the second previous and so on – you can see the list of boots with their numbers by using journalctl --list-boots. See journalctl(1) for a full description; the semantics are more powerful than indicated here.
• Include explanations of log messages from the message catalog where available:

  # journalctl -x

  Note that this feature should not be used when attaching logs to bug reports and support threads, as to limit extraneous output. You can list all known catalog entries by running journalctl --list-catalog.
• Show all messages from date (and optional time):

  # journalctl --since="2012-10-30 18:17:16"

• Show all messages since 20 minutes ago:

  # journalctl --since "20 min ago"

• Follow new messages:

  # journalctl -f

• Show all messages by a specific executable:

  # journalctl /usr/lib/systemd/systemd

• Show all messages by a specific identifier:

  # journalctl -t sudo

• Show all messages by a specific process:

  # journalctl _PID=1

• Show all messages by a specific unit:

  # journalctl -u man-db.service

• Show all messages from user services by a specific unit:

  $ journalctl --user -u dbus

• Show kernel ring buffer:

  # journalctl -k

• Show only error, critical and alert priority messages:

  # journalctl -p err..alert

  You can use numeric log level too, like journalctl -p 3..1. If single number/log level is used, journalctl -p 3, then all higher priority log levels are also included (i.e. 0 to 3 in this case).
• Show auth.log equivalent by filtering on syslog facility:

  # journalctl SYSLOG_FACILITY=10

• If the journal directory (by default located under /var/log/journal) contains a large amount of log data then journalctl can take several minutes to filter output. It can be sped up significantly by using --file option to force journalctl to look only into most recent journal:

  # journalctl --file /var/log/journal/*/system.journal -f

See journalctl(1), systemd.journal-fields(7), or Lennart Poettering's blog post for details.

Tip:

• By default, journalctl truncates lines longer than screen width, but in some cases, it may be better to enable wrapping instead of truncating. This can be controlled by the SYSTEMD_LESS environment variable, which contains options passed to less (the default pager) and defaults to FRSXMK (see less(1) and journalctl(1) for details).

By omitting the S option, the output will be wrapped instead of truncated. For example, start journalctl as follows:

$ SYSTEMD_LESS=FRXMK journalctl

To set this behaviour as default, export the variable from ~/.bashrc or ~/.zshrc.

• While the journal is stored in a binary format, the content of stored messages is not modified. This means it is viewable with strings, for example for recovery in an environment which does not have systemd installed, e.g.:

  $ strings /mnt/arch/var/log/journal/af4967d77fba44c6b093d0e9862f6ddd/system.journal | grep -i message

Journal size limit

If the journal is persistent (non-volatile), its size limit is set to a default value of 10% of the size of the underlying file system but capped at 4 GiB. For example, with /var/log/journal/ located on a 20 GiB partition, journal data may take up to 2 GiB. On a 50 GiB partition, it would max at 4 GiB. To confirm current limits on your system review systemd-journald unit logs:

# journalctl -b -u systemd-journald

The maximum size of the persistent journal can be controlled by uncommenting and changing the following:

/etc/systemd/journald.conf

SystemMaxUse=50M

It is also possible to use the drop-in snippets configuration override mechanism rather than editing the global configuration file. In this case, place the overrides under the [Journal] header:

/etc/systemd/journald.conf.d/00-journal-size.conf

[Journal]
SystemMaxUse=50M

Restart the systemd-journald.service after changing this setting to apply the new limit.

See journald.conf(5) for more info.

Per unit size limit by a journal namespace

Edit the unit file for the service you wish to configure (for example sshd) and add LogNamespace=ssh in the [Service] section.

Then create /etc/systemd/journald@ssh.conf by copying /etc/systemd/journald.conf. After that, edit journald@ssh.conf and adjust SystemMaxUse to your liking.

Restarting the service should automatically start the new journal service systemd-journald@ssh.service. The logs from the namespaced service can be viewed with journalctl --namespace ssh.

See systemd-journald.service(8) § JOURNAL NAMESPACES for details about journal namespaces.

Clean journal files manually

Journal files can be globally removed from /var/log/journal/ using e.g. rm, or can be trimmed according to various criteria using journalctl. For example:

• Remove archived journal files until the disk space they use falls below 100M:

  # journalctl --vacuum-size=100M

• Make all journal files contain no data older than 2 weeks.

  # journalctl --vacuum-time=2weeks

Journal files must have been rotated out and made inactive before they can be trimmed by vacuum commands. Rotation of journal files can be done by running journalctl --rotate. The --rotate argument can also be provided alongside one or more vacuum criteria arguments to perform rotation and then trim files in a single command.

See journalctl(1) for more info.

Journald in conjunction with syslog

Compatibility with a classic, non-journald aware syslog implementation can be provided by letting systemd forward all messages via the socket /run/systemd/journal/syslog. To make the syslog daemon work with the journal, it has to bind to this socket instead of /dev/log (official announcement).

The default journald.conf for forwarding to the socket is ForwardToSyslog=no to avoid system overhead, because rsyslog or syslog-ng pull the messages from the journal by itself.

See Syslog-ng#Overview and Syslog-ng#syslog-ng and systemd journal, or rsyslog respectively, for details on configuration.

Forward journald to /dev/tty12

Create a drop-in directory /etc/systemd/journald.conf.d and create a fw-tty12.conf file in it:

/etc/systemd/journald.conf.d/fw-tty12.conf

[Journal]
ForwardToConsole=yes
TTYPath=/dev/tty12

Then restart systemd-journald.service.

Note: By design, this does not forward kernel ring log messages.

Specify a different journal to view

There may be a need to check the logs of another system that is dead in the water, like booting from a live system to recover a production system. In such case, one can mount the disk in e.g. /mnt, and specify the journal path via -D/--directory, like so:

# journalctl -D /mnt/var/log/journal -e

Journal access as user

By default, a regular user only has access to their own per-user journal. To grant read access for the system journal as a regular user, you can add that user to the systemd-journal user group. Members of the adm and wheel groups are also given read access.

See journalctl(1) § DESCRIPTION and Users and groups#User groups for more information.

Tips and tricks

Desktop notifications

Desktop notifications can help you quickly notice error messages, improving awareness compared to manually checking logs or not noticing them at all.

The journalctl-desktop-notification^AUR package provides an automatic desktop notification for each error message logged by any process. For more details and configuration options, visit the GitLab project page.