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Applies to SUSE Linux Enterprise Server 15 SP5

21 journalctl: query the systemd journal

 

systemd features its own logging system called journal. There is no need to run a syslog-based service, as all system events are written to the journal.

The journal itself is a system service managed by systemd. Its full name is systemd-journald.service. It collects and stores logging data by maintaining structured indexed journals based on logging information received from the kernel, user processes, standard input, and system service errors. The systemd-journald service is on by default: 

> sudo systemctl status systemd-journald
systemd-journald.service - Journal Service
   Loaded: loaded (/usr/lib/systemd/system/systemd-journald.service; static)
   Active: active (running) since Mon 2014-05-26 08:36:59 EDT; 3 days ago
     Docs: man:systemd-journald.service(8)
           man:journald.conf(5)
 Main PID: 413 (systemd-journal)
   Status: "Processing requests..."
   CGroup: /system.slice/systemd-journald.service
           └─413 /usr/lib/systemd/systemd-journald
[...]

21.1 Making the journal persistent

 

The journal stores log data in /run/log/journal/ by default. Because the /run/ directory is volatile by nature, log data is lost at reboot. To make the log data persistent, create the directory /var/log/journal/ and make sure it has the correct access modes and ownership, so the systemd-journald service can store its data. To switch to persistent logging, execute the following commands: 

> sudo  mkdir /var/log/journal
> sudo  systemd-tmpfiles --create --prefix=/var/log/journal
> sudo  journalctl --flush

Any log data stored in /run/log/journal/ will be flushed into /var/log/journal/.

21.2 journalctl: useful switches

 

This section introduces several common useful options to enhance the default journalctl behavior. All switches are described in the journalctl manual page, man 1 journalctl.

Tip
Tip: Messages related to a specific executable

To show all journal messages related to a specific executable, specify the full path to the executable: 

> sudo journalctl /usr/lib/systemd/systemd
-f

Shows only the most recent journal messages, and prints new log entries as they are added to the journal.

Prints the messages and jumps to the end of the journal, so that the latest entries are visible within the pager.

-r

Prints the messages of the journal in reverse order, so that the latest entries are listed first.

-k

Shows only kernel messages. This is equivalent to the field match _TRANSPORT=kernel (see Section 21.3.3, “Filtering based on fields”).

-u

Shows only messages for the specified systemd unit. This is equivalent to the field match _SYSTEMD_UNIT=UNIT (see Section 21.3.3, “Filtering based on fields”). 

> sudo journalctl -u apache2
[...]
Jun 03 10:07:11 pinkiepie systemd[1]: Starting The Apache Webserver...
Jun 03 10:07:12 pinkiepie systemd[1]: Started The Apache Webserver.

21.3 Filtering the journal output

 

When called without switches, journalctl shows the full content of the journal, the oldest entries listed first. The output can be filtered by specific switches and fields.

21.3.1 Filtering based on a boot number

 

journalctl can filter messages based on a specific system boot. To list all available boots, run 

> sudo journalctl --list-boots
-1 097ed2cd99124a2391d2cffab1b566f0 Mon 2014-05-26 08:36:56 EDT—Fri 2014-05-30 05:33:44 EDT
 0 156019a44a774a0bb0148a92df4af81b Fri 2014-05-30 05:34:09 EDT—Fri 2014-05-30 06:15:01 EDT

The first column lists the boot offset: 0 for the current boot, -1 for the previous one, -2 for the one before that, etc. The second column contains the boot ID followed by the limiting time stamps of the specific boot.

Show all messages from the current boot: 

> sudo journalctl -b

If you need to see journal messages from the previous boot, add an offset parameter. The following example outputs the previous boot messages: 

> sudo journalctl -b -1

Another way is to list boot messages based on the boot ID. For this purpose, use the _BOOT_ID field: 

> sudo journalctl _BOOT_ID=156019a44a774a0bb0148a92df4af81b

21.3.2 Filtering based on time interval

 

You can filter the output of journalctl by specifying the starting and/or ending date. The date specification should be of the format "2014-06-30 9:17:16". If the time part is omitted, midnight is assumed. If seconds are omitted, ":00" is assumed. If the date part is omitted, the current day is assumed. Instead of numeric expression, you can specify the keywords "yesterday", "today", or "tomorrow". They refer to midnight of the day before the current day, of the current day, or of the day after the current day. If you specify "now", it refers to the current time. You can also specify relative times prefixed with - or +, referring to times before or after the current time.

Show only new messages since now, and update the output continuously: 

> sudo journalctl --since "now" -f

Show all messages since last midnight till 3:20am: 

> sudo journalctl --since "today" --until "3:20"

21.3.3 Filtering based on fields

 

You can filter the output of the journal by specific fields. The syntax of a field to be matched is FIELD_NAME=MATCHED_VALUE, such as _SYSTEMD_UNIT=httpd.service. You can specify multiple matches in a single query to filter the output messages even more. See man 7 systemd.journal-fields for a list of default fields.

Show messages produced by a specific process ID: 

> sudo journalctl _PID=1039

Show messages belonging to a specific user ID: 

# journalctl _UID=1000

Show messages from the kernel ring buffer (the same as dmesg produces): 

> sudo journalctl _TRANSPORT=kernel

Show messages from the service's standard or error output: 

> sudo journalctl _TRANSPORT=stdout

Show messages produced by a specified service only: 

> sudo journalctl _SYSTEMD_UNIT=avahi-daemon.service

If two different fields are specified, only entries that match both expressions at the same time are shown: 

> sudo journalctl _SYSTEMD_UNIT=avahi-daemon.service _PID=1488

If two matches refer to the same field, all entries matching either expression are shown: 

> sudo journalctl _SYSTEMD_UNIT=avahi-daemon.service _SYSTEMD_UNIT=dbus.service

You can use the '+' separator to combine two expressions in a logical 'OR'. The following example shows all messages from the Avahi service process with the process ID 1480 together with all messages from the D-Bus service: 

> sudo journalctl _SYSTEMD_UNIT=avahi-daemon.service _PID=1480 + _SYSTEMD_UNIT=dbus.service

21.4 Investigating systemd errors

 

This section introduces a simple example to illustrate how to find and fix the error reported by systemd during apache2 start-up.

  1. Try to start the apache2 service: 

    # systemctl start apache2
Job for apache2.service failed. See 'systemctl status apache2' and 'journalctl -xn' for details.

  2. Let us see what the service's status says: 

    > sudo systemctl status apache2
apache2.service - The Apache Webserver
   Loaded: loaded (/usr/lib/systemd/system/apache2.service; disabled)
   Active: failed (Result: exit-code) since Tue 2014-06-03 11:08:13 CEST; 7min ago
  Process: 11026 ExecStop=/usr/sbin/start_apache2 -D SYSTEMD -DFOREGROUND \
           -k graceful-stop (code=exited, status=1/FAILURE)

    The ID of the process causing the failure is 11026.

  3. Show the verbose version of messages related to process ID 11026: 

    > sudo journalctl -o verbose _PID=11026
[...]
MESSAGE=AH00526: Syntax error on line 6 of /etc/apache2/default-server.conf:
[...]
MESSAGE=Invalid command 'DocumenttRoot', perhaps misspelled or defined by a module
[...]

  4. Fix the typo inside /etc/apache2/default-server.conf, start the apache2 service, and print its status: 

    > sudo systemctl start apache2 && systemctl status apache2
apache2.service - The Apache Webserver
   Loaded: loaded (/usr/lib/systemd/system/apache2.service; disabled)
   Active: active (running) since Tue 2014-06-03 11:26:24 CEST; 4ms ago
  Process: 11026 ExecStop=/usr/sbin/start_apache2 -D SYSTEMD -DFOREGROUND
           -k graceful-stop (code=exited, status=1/FAILURE)
 Main PID: 11263 (httpd2-prefork)
   Status: "Processing requests..."
   CGroup: /system.slice/apache2.service
           ├─11263 /usr/sbin/httpd2-prefork -f /etc/apache2/httpd.conf -D [...]
           ├─11280 /usr/sbin/httpd2-prefork -f /etc/apache2/httpd.conf -D [...]
           ├─11281 /usr/sbin/httpd2-prefork -f /etc/apache2/httpd.conf -D [...]
           ├─11282 /usr/sbin/httpd2-prefork -f /etc/apache2/httpd.conf -D [...]
           ├─11283 /usr/sbin/httpd2-prefork -f /etc/apache2/httpd.conf -D [...]
           └─11285 /usr/sbin/httpd2-prefork -f /etc/apache2/httpd.conf -D [...]

21.5 Journald configuration

 

The behavior of the systemd-journald service can be adjusted by modifying /etc/systemd/journald.conf. This section introduces only basic option settings. For a complete file description, see man 5 journald.conf. You need to restart the journal for the changes to take effect with 

> sudo systemctl restart systemd-journald

21.5.1 Changing the journal size limit

 

If the journal log data is saved to a persistent location (see Section 21.1, “Making the journal persistent”), it uses up to 10% of the file system the /var/log/journal resides on. For example, if /var/log/journal is located on a 30 GB /var partition, the journal may use up to 3 GB of the disk space. To change this limit, change (and uncomment) the SystemMaxUse option: 

SystemMaxUse=50M

21.5.2 Forwarding the journal to /dev/ttyX

 

You can forward the journal to a terminal device to inform you about system messages on a preferred terminal screen, for example /dev/tty12. Change the following journald options to 

ForwardToConsole=yes
TTYPath=/dev/tty12

21.5.3 Forwarding the journal to syslog facility

 

Journald is backward compatible with traditional syslog implementations such as rsyslog. Make sure the following is valid:

  • rsyslog is installed. 

    > sudo rpm -q rsyslog
rsyslog-7.4.8-2.16.x86_64

  • rsyslog service is enabled. 

    > sudo systemctl is-enabled rsyslog
enabled

  • Forwarding to syslog is enabled in /etc/systemd/journald.conf. 

    ForwardToSyslog=yes

21.6 Using YaST to filter the systemd journal

 

For an easy way of filtering the systemd journal (without dealing with the journalctl syntax), you can use the YaST journal module. After installing it with sudo zypper in yast2-journal, start it from YaST by selecting System › Systemd Journal. Alternatively, start it from command line by entering sudo yast2 journal.

YaST systemd journal
Figure 21.1: YaST systemd journal
 

The module displays the log entries in a table. The search box on top allows you to search for entries that contain certain characters, similar to using grep. To filter the entries by date and time, unit, file or priority, click Change filters and set the respective options.

21.7 Viewing logs in GNOME

 

You can view the journal with GNOME Logs. Start it from the application menu. To view system log messages, it needs to be run as root, for example with xdg-su gnome-logs. This command can be executed when pressing Alt–F2.