journalctl Command Builder
Build a journalctl command that filters by unit, time range, priority, boot and text, with the output format you need and disk cleanup commands.
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Input
Build a journalctl command
Output
Command
72 characters
Disk usage and cleanup
journalctl --disk-usage sudo journalctl --vacuum-time=14d sudo journalctl --vacuum-size=1G
Make it permanent with SystemMaxUse=1G in /etc/systemd/journald.conf.
Reading the journal of other users' services needs root or membership of the systemd-journal group. The journal of a previous boot only exists when it is persistent: create /var/log/journal or set Storage=persistent in journald.conf.
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About this tool
Method, assumptions, practical next steps, and limits.
Filters worth knowing
-b -1 shows the previous boot, which is where the reason for a crash or an unexpected reboot is. -p err shows errors and worse. -o json writes one JSON object per line with every field for scripts, and -o cat prints just the messages. The journalctl manual lists every option.
Persistent logs
On some distributions the journal lives in memory and is lost on reboot, so -b -1 shows nothing. Creating /var/log/journal, or setting Storage=persistent in journald.conf, keeps logs across boots, and SystemMaxUse caps their size.