Journald : Basic Usage2026/06/19

	This is Basic Usage of Journald that is the Log Management Service Daemon.
[1]	By default, Journald is running and many logging data on the System are collected by Journald. Therefore, if [Journald (systemd-journald.service systemd-journald.socket systemd-journald-dev-log.socket)] would be down, collecting of many logging data will also stop.

root@dlp:~#

systemctl status systemd-journald.service

● systemd-journald.service - Journal Service
     Loaded: loaded (/usr/lib/systemd/system/systemd-journald.service; static)
    Drop-In: /usr/lib/systemd/system/systemd-journald.service.d
             +-- nice.conf
     Active: active (running) since Thu 2026-06-18 23:54:28 UTC; 19s ago
 Invocation: 3f99e5609596430e8d9603b9a59034ed
TriggeredBy: ● systemd-journald.socket
             ● systemd-journald-dev-log.socket
             ○ systemd-journald-audit.socket
       Docs: man:systemd-journald.service(8)
             man:journald.conf(5)
   Main PID: 818 (systemd-journal)
     Status: "Processing requests..."
      Tasks: 1 (limit: 3991)
   FD Store: 13 (limit: 4224)
     Memory: 10.1M (peak: 10.7M)
        CPU: 66ms
     CGroup: /system.slice/systemd-journald.service
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[2]	It's possible to change settings of Journald on [/etc/systemd/journald.conf]. All options are commented out by default, however they are the default parameters of Journald on Ubuntu.

root@dlp:~#

cat /etc/systemd/journald.conf

[Journal]
#Storage=persistent
#Compress=yes
#Seal=yes
#SplitMode=uid
#SyncIntervalSec=5m
#RateLimitIntervalSec=30s
#RateLimitBurst=10000
#SystemMaxUse=
#SystemKeepFree=
#SystemMaxFileSize=
#SystemMaxFiles=100
#RuntimeMaxUse=
#RuntimeKeepFree=
#RuntimeMaxFileSize=
#RuntimeMaxFiles=100
#MaxRetentionSec=0
#MaxFileSec=1month
#ForwardToSyslog=no
#ForwardToKMsg=no
#ForwardToConsole=no
#ForwardToWall=yes
#TTYPath=/dev/console
#MaxLevelStore=debug
#MaxLevelSyslog=debug
#MaxLevelKMsg=notice
#MaxLevelConsole=info
#MaxLevelWall=emerg
#MaxLevelSocket=debug
#LineMax=48K
#ReadKMsg=yes
#Audit=yes

[3]

The place of stored logging data is set on [Storage=***] of [/etc/systemd/journald.conf].
For the place of stored logging data, if syslog service like Rsyslog are installed and running, they are also stored in conventional files like [/var/log/syslog] and so on by syslog service with the setting [ForwardToSyslog=yes] on Journald.

# parameters of [Storage=***]
#
# volatile   : stored only in memory : under the [/run/log/journal]
# persistent : stored on disk : under the [/var/log/journal]
#              but if impossible to write on disk like  early boot, fallback to memory
# auto       : stored on disk if [/var/log/journal] exists
#              if not exists, stored in memory
# none       : not stored all data
#              but forwarding to other targets like Syslog daemon if they are configured
#
# * storing in memory is not persistent, when system restarted, logging data are cleared

# on default settings of Ubuntu, it's set [persistent] and 
# logging data are stored in [/var/log/journal]

root@dlp:~#

grep Storage /etc/systemd/journald.conf

#Storage=persistent

root@dlp:~#

ll -d /var/log/journal

drwxr-sr-x+ 3 root systemd-journal 4096 May  6 02:46 /var/log/journal/

root@dlp:~#

ll -Rh /var/log/journal

/var/log/journal:
total 12K
drwxr-sr-x+ 3 root systemd-journal 4.0K May  6 02:46 ./
drwxrwxr-x 11 root syslog          4.0K Jun 18 23:54 ../
drwxr-sr-x+ 2 root systemd-journal 4.0K Jun 18 23:53 07f56090dad245fc9a9afd8b1df7d909/

/var/log/journal/07f56090dad245fc9a9afd8b1df7d909:
total 23M
drwxr-sr-x+2 root systemd-journal 4.0K Jun 18 23:53 ./
drwxr-sr-x+3 root systemd-journal 4.0K May  6 02:46 ../
-rw-r-----+1 root systemd-journal 8.0M Jun 18 23:54 system.journal
-rw-r-----+1 root systemd-journal 8.0M Jun 18 23:53 system@2bc3cd8eac864a5bbd41f7dfc31ccfe7-0000000000000001-0006511d2bfc19c7.journal
-rw-r-----+1 root systemd-journal 8.0M May  6 02:51 user-1000.journal

[4]	To show stored logging data by Journald, it's possible with [journalctl] command.

# show all data without any option : results are send to [less] command
# if not send to [less], add [--no-pager] option

root@dlp:~#

journalctl

May 06 02:46:34 ubuntu kernel: Linux version 7.0.0-15-generic (buildd@lcy02-amd64-048) (x86_64-linux-gnu-gcc (Ubuntu 15.2.0-16ubuntu1) 15.2.0, GNU ld (GNU Binutils for Ubuntu) 2.46) #15-Ubuntu SMP PREEMPT_DYNAMIC Wed Apr 22 16:06:43 UTC 2026 (Ubuntu 7.0.0-15.15-generic 7.0.0)
May 06 02:46:34 ubuntu kernel: Command line: BOOT_IMAGE=/vmlinuz-7.0.0-15-generic root=/dev/mapper/ubuntu--vg-ubuntu--lv ro console=ttyS0,115200n8 crashkernel=2G-4G:320M,4G-32G:512M,32G-64G:1024M,64G-128G:2048M,128G-:4096M
May 06 02:46:34 ubuntu kernel: KERNEL supported cpus:
May 06 02:46:34 ubuntu kernel:   Intel GenuineIntel
May 06 02:46:34 ubuntu kernel:   AMD AuthenticAMD
May 06 02:46:34 ubuntu kernel:   Hygon HygonGenuine
May 06 02:46:34 ubuntu kernel:   Centaur CentaurHauls
May 06 02:46:34 ubuntu kernel:   zhaoxin   Shanghai
May 06 02:46:34 ubuntu kernel: BIOS-provided physical RAM map:
May 06 02:46:34 ubuntu kernel: BIOS-e820: [mem 0x0000000000000000-0x000000000009fbff]  System RAM
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# [-u UNIT] : show logs of a specific UNIT

root@dlp:~#

journalctl -u cron.service

May 06 02:47:12 ubuntu systemd[1]: Started cron.service - Regular background program processing daemon.
May 06 02:47:12 ubuntu (cron)[1275]: cron.service: Referenced but unset environment variable evaluates to an empty string: EXTRA_OPTS
May 06 02:47:12 ubuntu cron[1275]: (CRON) INFO (pidfile fd = 3)
May 06 02:47:12 ubuntu cron[1275]: (CRON) INFO (Running @reboot jobs)
May 06 02:51:19 ubuntu systemd[1]: Stopping cron.service - Regular background program processing daemon...
May 06 02:51:19 ubuntu systemd[1]: cron.service: Deactivated successfully.
May 06 02:51:19 ubuntu systemd[1]: Stopped cron.service - Regular background program processing daemon.
-- Boot 0af5b92fb9fa4df3941e548d09fd471f --
May 06 02:51:27 ubuntu systemd[1]: Started cron.service - Regular background program processing daemon.
May 06 02:51:27 ubuntu (cron)[1322]: cron.service: Referenced but unset environment variable evaluates to an empty string: EXTRA_OPTS
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root@dlp:~#

journalctl -u systemd-tmpfiles-clean.timer

May 06 02:47:12 ubuntu systemd[1]: Started systemd-tmpfiles-clean.timer - Daily Cleanup of Temporary Directories.
May 06 02:51:19 ubuntu systemd[1]: systemd-tmpfiles-clean.timer: Deactivated successfully.
May 06 02:51:19 ubuntu systemd[1]: Stopped systemd-tmpfiles-clean.timer - Daily Cleanup of Temporary Directories.
-- Boot 0af5b92fb9fa4df3941e548d09fd471f --
May 06 02:51:26 ubuntu systemd[1]: Started systemd-tmpfiles-clean.timer - Daily Cleanup of Temporary Directories.
May 06 02:52:33 ubuntu systemd[1]: systemd-tmpfiles-clean.timer: Deactivated successfully.
May 06 02:52:33 ubuntu systemd[1]: Stopped systemd-tmpfiles-clean.timer - Daily Cleanup of Temporary Directories.
-- Boot 7e40068c373d480ca0188a89d7a1861d --
May 06 02:52:40 ubuntu systemd[1]: Started systemd-tmpfiles-clean.timer - Daily Cleanup of Temporary Directories.
May 06 02:52:55 ubuntu systemd[1]: systemd-tmpfiles-clean.timer: Deactivated successfully.
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# [-k] : show logs of kernel message

root@dlp:~#

journalctl -k

Jun 18 23:54:26 ubuntu kernel: Linux version 7.0.0-15-generic (buildd@lcy02-amd64-048) (x86_64-linux-gnu-gcc (Ubuntu 15.2.0-16ubuntu1) 15.2.0, GNU ld (GNU Binutils for Ubuntu) 2.46) #15-Ubuntu SMP PREEMPT_DYNAMIC Wed Apr 22 16:06:43 UTC 2026 (Ubuntu 7.0.0-15.15-generic 7.0.0)
Jun 18 23:54:26 ubuntu kernel: Command line: BOOT_IMAGE=/vmlinuz-7.0.0-15-generic root=/dev/mapper/ubuntu--vg-ubuntu--lv ro console=ttyS0,115200n8 crashkernel=2G-4G:320M,4G-32G:512M,32G-64G:1024M,64G-128G:2048M,128G-:4096M
Jun 18 23:54:26 ubuntu kernel: KERNEL supported cpus:
Jun 18 23:54:26 ubuntu kernel:   Intel GenuineIntel
Jun 18 23:54:26 ubuntu kernel:   AMD AuthenticAMD
Jun 18 23:54:26 ubuntu kernel:   Hygon HygonGenuine
Jun 18 23:54:26 ubuntu kernel:   Centaur CentaurHauls
Jun 18 23:54:26 ubuntu kernel:   zhaoxin   Shanghai
Jun 18 23:54:26 ubuntu kernel: BIOS-provided physical RAM map:
Jun 18 23:54:26 ubuntu kernel: BIOS-e820: [mem 0x0000000000000000-0x000000000009fbff]  System RAM
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# [-p Priority] : show logs of a specific priority

root@dlp:~#

journalctl -p err

May 06 02:46:34 ubuntu kernel: KHO: Failed to reserve lowmem scratch buffer
May 06 02:51:20 ubuntu systemd-tmpfiles[2395]: Failed to create symlink '/run/initramfs/lib64/ld-linux-x86-64.so.2': Not a directory
-- Boot 0af5b92fb9fa4df3941e548d09fd471f --
May 06 02:51:23 ubuntu kernel: KHO: Failed to reserve lowmem scratch buffer
May 06 02:52:34 ubuntu systemd-tmpfiles[2439]: Failed to create symlink '/run/initramfs/lib64/ld-linux-x86-64.so.2': Not a directory
-- Boot 7e40068c373d480ca0188a89d7a1861d --
May 06 02:52:37 ubuntu kernel: KHO: Failed to reserve lowmem scratch buffer
May 06 02:52:55 ubuntu systemd-tmpfiles[1700]: /run/initramfs/etc/initrd-release exists and is not a regular file.
May 06 02:52:55 ubuntu (sd-pam)[1526]: pam_systemd(systemd-user:session): Varlink call io.systemd.Login.ReleaseSession failed: io.systemd.Login.NoSuchSession
-- Boot 0c4275c4b26e47968f876f0586c47158 --
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# [-g PATTERN] : show logs that include specific word [PATTERN] in [MESSAGE] field

root@dlp:~#

journalctl -g "apparmor"

May 06 02:46:34 ubuntu kernel: AppArmor: AppArmor initialized
May 06 02:46:34 ubuntu kernel: AppArmor: AppArmor secmark mediation reserved: ready to be enabled
May 06 02:46:34 ubuntu kernel: AppArmor: AppArmor Filesystem Enabled
May 06 02:46:34 ubuntu kernel: AppArmor: AppArmor sha256 policy hashing enabled
May 06 02:46:34 ubuntu kernel: evm: security.apparmor
May 06 02:46:34 ubuntu systemd[1]: systemd 259.5-0ubuntu3 running in system mode (+PAM +AUDIT +SELINUX +APPARMOR +IMA +IPE +SMACK +SECCOMP +GCRYPT -GNUTLS +OPENSSL +ACL +BLKID +CURL +ELFUTILS +FIDO2 +IDN2 -IDN +KMOD +LIBCRYPTSETUP +LIBCRYPTSETUP_PLUGINS +LIBFDISK +PCRE2 +PWQUALITY +P11KIT +QRENCODE +TPM2 +BZIP2 +LZ4 +XZ +ZLIB +ZSTD +BPF_FRAMEWORK +BTF -XKBCOMMON -UTMP +SYSVINIT +LIBARCHIVE)
May 06 02:46:36 ubuntu systemd[1]: systemd 259.5-0ubuntu3 running in system mode (+PAM +AUDIT +SELINUX +APPARMOR +IMA +IPE +SMACK +SECCOMP +GCRYPT -GNUTLS +OPENSSL +ACL +BLKID +CURL +ELFUTILS +FIDO2 +IDN2 -IDN +KMOD +LIBCRYPTSETUP +LIBCRYPTSETUP_PLUGINS +LIBFDISK +PCRE2 +PWQUALITY +P11KIT +QRENCODE +TPM2 +BZIP2 +LZ4 +XZ +ZLIB +ZSTD +BPF_FRAMEWORK +BTF -XKBCOMMON -UTMP +SYSVINIT +LIBARCHIVE)
May 06 02:46:37 ubuntu systemd[1]: Starting apparmor.service - Load AppArmor profiles...
May 06 02:46:37 ubuntu apparmor.systemd[1008]: Restarting AppArmor
May 06 02:46:37 ubuntu apparmor.systemd[1008]: Reloading AppArmor profiles
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# [-S DATE] : show logs Since DATE
# [-U DATE] : show logs Until DATE

root@dlp:~#

journalctl -S "2026-06-10 00:00:00" -U "2026-06-20 23:59:59"

Jun 18 23:53:07 ubuntu kernel: Linux version 7.0.0-15-generic (buildd@lcy02-amd64-048) (x86_64-linux-gnu-gcc (Ubuntu 15.2.0-16ubuntu1) 15.2.0, GNU ld (GNU Binutils for Ubuntu) 2.46) #15-Ubuntu SMP PREEMPT_DYNAMIC Wed Apr 22 16:06:43 UTC 2026 (Ubuntu 7.0.0-15.15-generic 7.0.0)
Jun 18 23:53:07 ubuntu kernel: Command line: BOOT_IMAGE=/vmlinuz-7.0.0-15-generic root=/dev/mapper/ubuntu--vg-ubuntu--lv ro console=ttyS0,115200n8 crashkernel=2G-4G:320M,4G-32G:512M,32G-64G:1024M,64G-128G:2048M,128G-:4096M
Jun 18 23:53:07 ubuntu kernel: KERNEL supported cpus:
Jun 18 23:53:07 ubuntu kernel:   Intel GenuineIntel
Jun 18 23:53:07 ubuntu kernel:   AMD AuthenticAMD
Jun 18 23:53:07 ubuntu kernel:   Hygon HygonGenuine
Jun 18 23:53:07 ubuntu kernel:   Centaur CentaurHauls
Jun 18 23:53:07 ubuntu kernel:   zhaoxin   Shanghai
Jun 18 23:53:07 ubuntu kernel: BIOS-provided physical RAM map:
Jun 18 23:53:07 ubuntu kernel: BIOS-e820: [mem 0x0000000000000000-0x000000000009fbff]  System RAM
.....
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# show help

root@dlp:~#

journalctl --help

journalctl [OPTIONS...] [MATCHES...]

Query the journal.

Options:
     --system                Show the system journal
     --user                  Show the user journal for the current user
  -M --machine=CONTAINER     Operate on local container
  -S --since=DATE            Show entries not older than the specified date
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