Show Journal Messages

# journalctl -b
show all messages of the current boot.

# journalctl --since=2013-01-01 --until="2013-01-02 23:59:59" -p err
show errors that happened in selected time interval (--since=today or --since=yesterday can be also used).

# journalctl -f
show most recent journal entries and print new entries "live".

# journalctl _PID=number
show all messages by a specific process.

# journalctl -u unit
show all messages by a specific unit.

Clean Journal Files Manually

# journalctl --vacuum-time=2d
remove journal data older than 2 days.

# journalctl --vacuum-size=100M
remove archived journal files until the disk space they use falls below 100 MB.