Mastering System Logs: From systemd-journald to journalctl
- WHAT?
View and analyze logs from
systemd-journaldusing thejournalctlcommand-line tool.- WHY?
This article provides a complete overview of tasks that can be performed using the
journalctlcommand-line tool.- EFFORT
The average reading time of this article is approximately 30 minutes.
- GOAL
You will be able to view your system logs using
journalctl.- REQUIREMENTS
You must have
sudoprivileges.
1 The systemd-journald service #
journald is a systemd service responsible for collecting, indexing, and storing log data. The collected events include kernel messages, initrd (initial RAM disk) messages,
service startup/shutdown, application events, and authentication and session data.
On SUSE Linux Enterprise Server, the systemd-journald service is enabled and started by
default. The service logs the data into the journal described in Section 2, “About the journal”.
1.1 Configuring systemd-journald #
You can configure the basic behavior of systemd-journald service by modifying /etc/systemd/journald.conf.
To configure systemd-journald, proceed as follows:
systemd-journald #Open the
systemd-journaldconfiguration file:>sudovi /etc/systemd/journald.conf[Journal] # Store journals on disk so logs survive reboot Storage=persistent # Disk space limits SystemMaxUse=2G SystemKeepFree=10% # Per-file max size SystemMaxFileSize=200M # RAM (runtime) journal limits RuntimeMaxUse=500M RuntimeKeepFree=10% # Log retention MaxRetentionSec=4weeks # Compression for old logs Compress=yes # Optional: also forward to rsyslog/syslog ForwardToSyslog=yes
Modify the configurations:
To modify the storage type, modify
Storage=auto.The available options are:
: RAM only (clears on reboot).
: stored in
/var/log/journal.: persistent if directory exists, otherwise volatile.
: no logs written to disk or memory.
If the journal log data is saved to a persistent location, it uses up to 10% of the file system the
/var/log/journalresides on. For example, if/var/log/journalis 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) theSystemMaxUseoption:SystemMaxUse=50M
>sudosystemctl restart systemd-journaldTo limit the log rate to prevent flooding logs:
RateLimitIntervalSec=30s RateLimitBurst=1000
To send the journal to a terminal device to inform you about system messages on a preferred terminal screen, for example
/dev/tty12:ForwardToConsole=yes TTYPath=/dev/tty12
To forward logs to syslog, modify
ForwardToSyslog=yes.journaldis backward compatible with traditional syslog implementations such as rsyslog.Install rsyslog:
rpm -q rsyslog
Enable rsyslog:
systemctl is-enabled rsyslog
Enable forwarding to rsyslog in
/etc/systemd/journald.conf:ForwardToSyslog=yes
For more information on file descriptions, see
man 5 journald.conf.
Save and restart
systemd-journald.>sudosystemctl restart systemd-journald
2 About the journal #
The journal is a centralized, unified storage system for log data from all sources. It stores events in a structured manner, which helps with error identification and crash recovery.
2.1 Where can I find the journal? #
By default, journald stores the collected data in the volatile memory in
/run/log/journal, so after reboot the data is lost.
If systemd-journald is configured to store the logs in a persistent
manner, you can find the journal in /var/log/journal. See if the
directory is there:
Check if the directory exist:
>sudols /var/log... journal ...If the
/var/log/journaldirectory does not exist, create it:>sudomkdir /var/log/journalSet the correct ownership and access permissions:
>sudosystemd-tmpfiles --create --prefix=/var/log/journal(Optional) To flush the data from RAM to the directory:
>sudojournalctl --flush
For details about
the service configuration, refer to Section 1.1, “Configuring systemd-journald”.
2.2 The journal log structure #
The logs are stored in binary format across several files. To view the structure of a log entry, proceed as follows:
>sudojournalctl -n 1 -o verboseFri 2025-12-05 10:52:53.284321 CET [s=5b7ae2e926794210bf832d014f5f560a;i=d49752> _UID=1000 _AUDIT_SESSION=3 _AUDIT_LOGINUID=1000 _SYSTEMD_OWNER_UID=1000 _SYSTEMD_UNIT=user@1000.service _SYSTEMD_SLICE=user-1000.slice _MACHINE_ID=d3489468c8534c5d81a2860cf9a2a20e _RUNTIME_SCOPE=system _SELINUX_CONTEXT=unconfined PRIORITY=6 _TRANSPORT=syslog _BOOT_ID=d095069bd59b4bc4bf67c8c71999243b SYSLOG_IDENTIFIER=sudo SYSLOG_TIMESTAMP=Dec 5 10:52:53 _PID=25806 _COMM=sudo _EXE=/usr/bin/sudo _CMDLINE=sudo journalctl -n 1 -o verbose ...
The log entry is a data structure containing the log message and metadata fields:
- Timestamp
The exact time the event occurred
- Source fields
_PID: process ID of the sender_UID/_GID: User/Group ID of the sender_EXE: path to the executable_COMM: name of the executable
- System fields
_SYSTEMD_UNIT: The systemd unit (service) that generated the log (for example,sshd.service)_BOOT_ID: A unique identifier for the specific system boot session
- Kernel fields
_TRANSPORT: How the message was logged (e.g., kernel, syslog, stdout)- Priority level
A numeric value indicating the severity of the message (0=emerg, 7=debug)
3 The journalctl command #
journalctl is a command-line utility used to query and display logs collected by
the systemd-journald.
Because the logging system stores data in a binary format (for performance and security) rather
than plain text, you cannot use standard tools like cat or
less to open the files directly. So, the journalctl
command decodes the data and displays it according to the provided parameters.
3.1 Using journalctl #
This section describes the generic usage of the journalctl command.
Running journalctl without any options displays all logged messages, usually starting from the oldest, and pipes the output through a pager (like less) for easy navigation.
The general syntax of the journalctl command is as follows:
>sudojournalctl [OPTIONS...] [MATCHES...]
To show all journal messages related to a specific executable, specify the full path to the executable:
>sudojournalctl /usr/lib/systemd/systemd
Listed below are the common useful options to enhance the default journalctl behavior:
- -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.- -u
Shows only messages for the specified
systemdunit. This is equivalent to the field match_SYSTEMD_UNIT=UNIT.>sudojournalctl -u apache2
For a complete list of options refer to man page, man 1 journalctl.
4 Filtering the journal output #
This section describes how to refine searches in logs, for example, by boot number, by a specific time interval, or to view specific data fields. By default, the journal lists the oldest entries first. The output can be filtered using specific switches and fields.
You can filter journals based on boot number, time interval, and fields. For details, refer to the following sections.
4.1 Filter logs based on a specific system boot #
To list logs for all the available boots, run the command as follows:
>sudojournalctl --list-boots
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.
To show all messages from the current boot:
>sudojournalctl -b
To view journal messages from the previous boot, add an offset parameter. The following example command shows the previous boot messages:
>sudojournalctl -b -1
To view boot messages based on the boot ID, 156019a44a774a0bb0148a92df4af81b:
>sudojournalctl _BOOT_ID=156019a44a774a0bb0148a92df4af81b
4.2 Filtering logs 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 YYYY-MM-DD H:MM:SS. 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 a 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.
To view only new messages since now, and update the output continuously:
>sudojournalctl --since "now" -f
To view all messages since last midnight until 3:20am:
>sudojournalctl --since "today" --until "3:20"
4.3 Filtering logs 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.
To view messages produced by a specific process ID: PID_1039:
>sudojournalctl _PID=1039
To view messages belonging to a specific user ID: UID_1000:
>sudojournalctl _UID=1000
To view messages from the kernel ring buffer (the same as dmesg displays):
>sudojournalctl _TRANSPORT=kernel
To view messages from the service's standard or error output:
>sudojournalctl _TRANSPORT=stdout
To view messages produced by a specified service only:
>sudojournalctl _SYSTEMD_UNIT=avahi-daemon.service
If two different fields are specified, only entries that match both expressions at the same time are shown:
>sudojournalctl _SYSTEMD_UNIT=avahi-daemon.service _PID=1488
If two matches refer to the same field, all entries matching either expression are shown:
>sudojournalctl _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:
>sudojournalctl _SYSTEMD_UNIT=avahi-daemon.service _PID=1480 + _SYSTEMD_UNIT=dbus.service
5 Troubleshooting systemd errors #
This section introduces ways to interpret and fix the log messages retrieved through journalctl.
5.1 View systemd errors #
View the list of failed
systemdunits:systemctl --failed
The list of all failed services appears.
Identify the severity and content of the error.
journalctl -p 0..7 -b
The list of errors with priority levels
0 to 7appears. The errors are marked with priority level::
emerg, alert, crit: Critical issues, system collapse imminent.:
err (Error): A service or application failed to complete a requested operation.:
warning: Something undesirable happened but is not an immediate failure.:
notice, info, debug: Normal operational information and developer diagnostics.
View the error log for the failing service using the following command:
journalctl -u <failing_service_name>
The lines preceding the termination message include information about the error.
: If a service stops, look for a message like
Process xxx exitedwith status 1/FAILURE. A status of 0 indicates success. Any nonzero status indicates an error.: Messages containing phrases like
No such file or directory,Permission deniedorAddress already in useusually point to a problem in the service's configuration file.: The service was terminated because it exceeded memory limits.
Use detailed views of logs to get detailed information about the error.
journalctl -xe
For example, to view all messages from the Apache service during the current boot with detailed explanations, run the following command:
journalctl -u httpd.service -b -xe
5.2 Troubleshoot systemd error #
This section introduces an example to illustrate how to find and fix
the error reported by systemd during apache2 start-up.
Start the apache2 service:
systemctl start apache2 Job for apache2.service failed. See 'systemctl status apache2' and 'journalctl -xn' for details.
View the service status:
>sudosystemctl status apache2 apache2.service - The Apache Webserver Loaded: loaded (/usr/lib/systemd/system/apache2.service; disabled) Active: failed (Result: exit-code) since Tue 2025-09-03 11:08:13 EDT; 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.
View the verbose version of messages related to process ID 11026:
>sudojournalctl -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 [...]Fix the typo inside
/etc/apache2/default-server.conf, start the apache2 service, and print its status:>sudosystemctl start apache2 && systemctl status apache2 apache2.service - The Apache Webserver Loaded: loaded (/usr/lib/systemd/system/apache2.service; disabled) Active: active (running) since Tue 2025-09-03 11:26:24 EDT; 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 [...]
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ADDENDUM: How to use this License for your documents #
Copyright (c) YEAR YOUR NAME. Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.2 or any later version published by the Free Software Foundation; with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts. A copy of the license is included in the section entitled “GNU Free Documentation License”.
If you have Invariant Sections, Front-Cover Texts and Back-Cover Texts, replace the “with...Texts.” line with this:
with the Invariant Sections being LIST THEIR TITLES, with the Front-Cover Texts being LIST, and with the Back-Cover Texts being LIST.
If you have Invariant Sections without Cover Texts, or some other combination of the three, merge those two alternatives to suit the situation.
If your document contains nontrivial examples of program code, we recommend releasing these examples in parallel under your choice of free software license, such as the GNU General Public License, to permit their use in free software.