nixpkgs/nixos/doc/manual/administration/logging.xml

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 <title>Logging</title>
 <para>
  System-wide logging is provided by systemd’s <emphasis>journal</emphasis>, which subsumes traditional logging daemons such as syslogd and klogd. Log entries are kept in binary files in <filename>/var/log/journal/</filename>. The command <literal>journalctl</literal> allows you to see the contents of the journal. For example,
<screen>
<prompt>$ </prompt>journalctl -b
</screen>
  shows all journal entries since the last reboot. (The output of <command>journalctl</command> is piped into <command>less</command> by default.) You can use various options and match operators to restrict output to messages of interest. For instance, to get all messages from PostgreSQL:
<screen>
<prompt>$ </prompt>journalctl -u postgresql.service
-- Logs begin at Mon, 2013-01-07 13:28:01 CET, end at Tue, 2013-01-08 01:09:57 CET. --
...
Jan 07 15:44:14 hagbard postgres[2681]: [2-1] LOG:  database system is shut down
-- Reboot --
Jan 07 15:45:10 hagbard postgres[2532]: [1-1] LOG:  database system was shut down at 2013-01-07 15:44:14 CET
Jan 07 15:45:13 hagbard postgres[2500]: [1-1] LOG:  database system is ready to accept connections
</screen>
  Or to get all messages since the last reboot that have at least a “critical” severity level:
<screen>
<prompt>$ </prompt>journalctl -b -p crit
Dec 17 21:08:06 mandark sudo[3673]: pam_unix(sudo:auth): auth could not identify password for [alice]
Dec 29 01:30:22 mandark kernel[6131]: [1053513.909444] CPU6: Core temperature above threshold, cpu clock throttled (total events = 1)
</screen>
 </para>
 <para>
  The system journal is readable by root and by users in the <literal>wheel</literal> and <literal>systemd-journal</literal> groups. All users have a private journal that can be read using <command>journalctl</command>.
 </para>
</chapter>