Journald : Basic Usage2021/06/30 |
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This is Basic Usage of Journald that is the Log Management Service Daemon.
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| [1] | By default, Journald is running and almost all 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 almost all logging data will also stop. |
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[root@dlp ~]# systemctl status systemd-journald.service
* systemd-journald.socket - Journal Socket
Loaded: loaded (/usr/lib/systemd/system/systemd-journald.socket; static; ven>
Active: active (running) since Wed 2021-06-29 21:38:57 JST; 4min 57s ago
Docs: man:systemd-journald.service(8)
man:journald.conf(5)
Listen: /run/systemd/journal/stdout (Stream)
/run/systemd/journal/socket (Datagram)
Tasks: 0 (limit: 23673)
Memory: 0B
CGroup: /system.slice/systemd-journald.socket
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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 CentOS Stream 8. |
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[root@dlp ~]# cat /etc/systemd/journald.conf #Storage=auto #Compress=yes #Seal=yes #SplitMode=uid #SyncIntervalSec=5m #RateLimitIntervalSec=30s #RateLimitBurst=10000 #SystemMaxUse= #SystemKeepFree= #SystemMaxFileSize= #SystemMaxFiles=100 #RuntimeMaxUse= #RuntimeKeepFree= #RuntimeMaxFileSize= #RuntimeMaxFiles=100 #MaxRetentionSec= #MaxFileSec=1month #ForwardToSyslog=no #ForwardToKMsg=no #ForwardToConsole=no #ForwardToWall=yes #TTYPath=/dev/console #MaxLevelStore=debug #MaxLevelSyslog=debug #MaxLevelKMsg=notice #MaxLevelConsole=info #MaxLevelWall=emerg #LineMax=48K |
| [3] | The place of stored logging data is set on [Storage=***] of [/etc/systemd/journald.conf]. |
# 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 CentOS Stream 8, it's set [auto] and also # [/var/log/journal] does not exist, so logging data are stored in [/run/log/journal]
[root@dlp ~]#
grep Storage /etc/systemd/journald.conf #Storage=auto
[root@dlp ~]#
[root@dlp ~]# ll -d /var/log/journal ls: cannot access '/var/log/journal': No such file or directory ll /run/log/journal total 0 drwxr-s---+ 2 root systemd-journal 60 Jul 1 20:41 54f4bd7df8464337a3009215fdef55fb # [/run/log] on CentOS Stream 8 is [tmpfs] filesystem # [tmpfs] is on memory # for [tmpfs] partition size, it's set as half of physical memory size if not set manually # the size is not kept always on memory but it's used dynamically as needed df -h /run/log Filesystem Size Used Avail Use% Mounted on tmpfs 1.9G 8.5M 1.9G 1% /run # if you'd like to change stored place to disk, create the [/var/log/journal] directory [root@dlp ~]# mkdir /var/log/journal [root@dlp ~]# systemctl restart systemd-journald.service \
systemd-journald.socket \ systemd-journald-dev-log.socket
[root@dlp ~]#
[root@dlp ~]# ll /run/log/journal ls: cannot access '/run/log/journal': No such file or directory ll /var/log/journal total 0 drwxr-xr-x. 2 root root 28 Jul 1 23:42 54f4bd7df8464337a3009215fdef55fb # * Note # on default settings of CentOS Stream 8, Rsyslog which is the syslog daemon is also running and # it stores logging data received from Journald in [/var/log] directory # so logging data are stored on disk even if not change storage setting of Journald # Rsyslog imports logging data from Journald with Rsyslog [imjournal] module, so # [ForwardToSyslog=***] parameters on Journald does not influence to sending data to Rsyslog |
| [4] | To show stored logging data by Journald, it's possible with [journalctl] command. |
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# show all data without any option : results are send to [less] command # if not send to [less], add [--no-pager] option [root@dlp ~]# journalctl -- Logs begin at Thu 2021-07-01 23:57:55 JST, end at Thu 2021-07-02 14:30:31 JS> Jul 01 23:57:55 localhost.localdomain kernel: Linux version 4.18.0-310.el8.x86_> Jul 01 23:57:55 localhost.localdomain kernel: Command line: BOOT_IMAGE=(hd0,msd> Jul 01 23:57:55 localhost.localdomain kernel: x86/fpu: Supporting XSAVE feature> ..... ..... # [-u UNIT] : show logs of a specific UNIT [root@dlp ~]# journalctl -u sshd.service -- Logs begin at Thu 2021-07-01 23:57:55 JST, end at Thu 2021-07-02 15:01:01 JS> Jul 01 23:58:01 dlp.srv.world systemd[1]: Starting OpenSSH server daemon... Jul 01 23:58:01 dlp.srv.world sshd[897]: Server listening on 0.0.0.0 port 22. Jul 01 23:58:01 dlp.srv.world sshd[897]: Server listening on :: port 22. ..... .....[root@dlp ~]# journalctl -u systemd-tmpfiles-clean.timer -- Logs begin at Thu 2021-07-01 23:57:55 JST, end at Thu 2021-07-02 15:01:01 JS> Jul 01 23:58:00 dlp.srv.world systemd[1]: Started Daily Cleanup of Temporary Di> ..... ..... # [-k] : show logs of kernel message [root@dlp ~]# journalctl -k -- Logs begin at Thu 2021-07-01 23:57:55 JST, end at Thu 2021-07-02 14:30:31 JS> Jul 01 23:57:55 localhost.localdomain kernel: Linux version 4.18.0-310.el8.x86_> Jul 01 23:57:55 localhost.localdomain kernel: Command line: BOOT_IMAGE=(hd0,msd> Jul 01 23:57:55 localhost.localdomain kernel: x86/fpu: Supporting XSAVE feature> ..... ..... # [-p Priority] : show logs of a specific priority [root@dlp ~]# journalctl -p err -- Logs begin at Thu 2021-07-01 23:57:55 JST, end at Thu 2021-07-02 16:02:09 JS> Jul 01 23:57:55 localhost.localdomain systemd-vconsole-setup[288]: /usr/bin/set> Jul 01 23:57:56 localhost.localdomain kernel: pcieport 0000:00:01.6: Failed to > Jul 01 23:57:58 localhost.localdomain systemd-vconsole-setup[656]: /usr/bin/set> ..... ..... # [-g PATTERN] : show logs that include specific word [PATTERN] in [MESSAGE] field [root@dlp ~]# journalctl -g "sealert" -- Logs begin at Thu 2021-07-01 23:57:55 JST, end at Thu 2021-07-02 17:01:01 JST. -- Jul 01 23:58:33 dlp.srv.world setroubleshoot[1473]: SELinux is preventing /usr/bin/login from getattr access on the filesystem /sys/fs/cgroup. For complete SELinux messages run: sealert -l 99d9d41e-aa3f-4856-9d81-6ca26aa6405c ..... ..... # [-S DATE] : show logs Since DATE # [-U DATE] : show logs Until DATE [root@dlp ~]# journalctl -S "2021-06-30 00:00:00" -U "2021-07-01 23:59:59" -- Logs begin at Fri 2021-06-25 23:37:03 JST, end at Thu 2021-07-01 17:12:47 JST. -- Jun 30 00:00:16 dlp.srv.world systemd[1]: Starting update of the root trust anchor for DNSSEC validation in unbound... Jun 30 00:00:17 dlp.srv.world systemd[1]: unbound-anchor.service: Succeeded. Jun 30 00:00:17 dlp.srv.world systemd[1]: Started update of the root trust anchor for DNSSEC validation in unbound. ..... ..... # show help [root@dlp ~]# journalctl --help --no-pager
journalctl [OPTIONS...] [MATCHES...]
Query the journal.
Options:
--system Show the system journal
--user Show the user journal for the current user
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