Detailed usage of journalctl

1. Brief description of journalctl command

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

Query the journal.

Flags:
     --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
  -U --until=DATE          Show entries not newer than the specified date
  -c --cursor=CURSOR       Show entries starting at the specified cursor
     --after-cursor=CURSOR Show entries after the specified cursor
     --show-cursor         Print the cursor after all the entries
  -b --boot[=ID]           Show current boot or the specified boot
     --list-boots          Show terse information about recorded boots
  -k --dmesg               Show kernel message log from the current boot
  -u --unit=UNIT           Show logs from the specified unit
  -t --identifier=STRING   Show entries with the specified syslog identifier
  -p --priority=RANGE      Show entries with the specified priority
  -e --pager-end           Immediately jump to the end in the pager
  -f --follow              Follow the journal
  -n --lines[=INTEGER]     Number of journal entries to show
     --no-tail             Show all lines, even in follow mode
  -r --reverse             Show the newest entries first
  -o --output=STRING       Change journal output mode (short, short-iso,
                                   short-precise, short-monotonic, verbose,
                                   export, json, json-pretty, json-sse, cat)
     --utc                 Express time in Coordinated Universal Time (UTC)
  -x --catalog             Add message explanations where available
     --no-full             Ellipsize fields
  -a --all                 Show all fields, including long and unprintable
  -q --quiet               Do not show privilege warning
     --no-pager            Do not pipe output into a pager
  -m --merge               Show entries from all available journals
  -D --directory=PATH      Show journal files from directory
     --file=PATH           Show journal file
     --root=ROOT           Operate on catalog files underneath the root ROOT
     --interval=TIME       Time interval for changing the FSS sealing key
     --verify-key=KEY      Specify FSS verification key
     --force               Override of the FSS key pair with --setup-keys

Commands:
  -h --help                Show this help text
     --version             Show package version
  -F --field=FIELD         List all values that a specified field takes
     --new-id128           Generate a new 128-bit ID
     --disk-usage          Show total disk usage of all journal files
     --vacuum-size=BYTES   Reduce disk usage below specified size
     --vacuum-time=TIME    Remove journal files older than specified date
     --flush               Flush all journal data from /run into /var
     --header              Show journal header information
     --list-catalog        Show all message IDs in the catalog
     --dump-catalog        Show entries in the message catalog
     --update-catalog      Update the message catalog database
     --setup-keys          Generate a new FSS key pair
     --verify              Verify journal file consistency

Two, journalctl common options

1. journalctl -e // Immediately jump to the end in the pager (immediately jump to the end of the journal page)

journalctl -e

1月 04 18:30:32 server1 systemd[1]: Started Session 22 of user root.
1月 04 18:30:32 server1 systemd[1]: Starting Session 22 of user root.
1月 04 18:30:32 server1 CROND[55734]: (root) CMD (/usr/lib64/sa/sa1 1 1)
1月 04 18:40:01 server1 systemd[1]: Started Session 23 of user root.
1月 04 18:40:01 server1 systemd[1]: Starting Session 23 of user root.
1月 04 18:40:01 server1 CROND[55818]: (root) CMD (/usr/lib64/sa/sa1 1 1)
1月 04 18:50:01 server1 systemd[1]: Started Session 24 of user root.
1月 04 18:50:01 server1 systemd[1]: Starting Session 24 of user root.
1月 04 18:50:01 server1 CROND[55907]: (root) CMD (/usr/lib64/sa/sa1 1 1)
1月 04 19:00:01 server1 systemd[1]: Started Session 25 of user root.
1月 04 19:00:01 server1 systemd[1]: Starting Session 25 of user root.
1月 04 19:00:01 server1 CROND[55997]: (root) CMD (/usr/lib64/sa/sa1 1 1)
1月 04 19:01:01 server1 systemd[1]: Started Session 26 of user root.
1月 04 19:01:01 server1 systemd[1]: Starting Session 26 of user root.
1月 04 19:01:01 server1 CROND[56011]: (root) CMD (run-parts /etc/cron.hourly)
1月 04 19:01:01 server1 run-parts(/etc/cron.hourly)[56014]: starting 0anacron
1月 04 19:01:01 server1 run-parts(/etc/cron.hourly)[56020]: finished 0anacron
1月 04 19:10:01 server1 systemd[1]: Started Session 27 of user root.
1月 04 19:10:01 server1 systemd[1]: Starting Session 27 of user root.
1月 04 19:10:01 server1 CROND[56105]: (root) CMD (/usr/lib64/sa/sa1 1 1)

2. journalctl -u //View the log of the specified unit (unit)

journalctl -u httpd

[root@server1 ~]# journalctl -u httpd
-- Logs begin at 日 2020-11-22 04:33:29 CST, end at 一 2021-01-04 19:30:01 CST. --
1月 04 19:24:46 server1 systemd[1]: Starting The Apache HTTP Server...
1月 04 19:25:06 server1 httpd[56288]: AH00558: httpd: Could not reliably determine the server's fully qualified domain name, using fe80::4f00:5a8e:3923:429d. Set the 'ServerName' directive 
1月 04 19:25:26 server1 systemd[1]: Started The Apache HTTP Server.
1月 04 19:27:57 server1 systemd[1]: Stopping The Apache HTTP Server...
1月 04 19:27:58 server1 systemd[1]: Stopped The Apache HTTP Server.