journalctl: New option --cursor-file

journalctl: New option --cursor-file

The option cursor-file takes a filename as argument. If the file exists and
contains a valid cursor, this is used to start the output after this position.
At the end, the last cursor gets written to the file.

This allows for an easy implementation of a timer that regularly looks in the
journal for some messages.

    journalctl --cursor-file err-cursor -b -p err
    journalctl --cursor-file audit-cursor -t audit --grep DENIED

Or you might want to walk the journal in steps of 10 messages:

    journalctl --cursor-file ./curs -n10 --since=today -t systemd