Is there a way to make journalctl show logs from "the last time foo.service ran"?

I'm particularly interested in this for looking at the output of oneshot services that run on a timer. The --unit flag is close, but it concatenates all the runs of the service together. The most obvious way I can think of would be to filter on PID, but that makes me worry about PID reuse / services that fork, and getting the last PID is pretty inconvenient. Is there some other identifier that corresponds to a single run of a service, that I could use to filter the logs?

EDIT: I would happily accept an authoritative "no" if that's the real answer.

Solution 1:

Since systemd version 232, we have the concept of invocation ID. Each time a unit is ran, it has a unique 128 bit invocation ID. Unlike MainPID which can be recycled, or ActiveEnterTimestamp which can have resolution troubles, it is a failsafe way to get all the log of a particular systemd unit invocation.

To obtain the latest invocation ID of a unit

$ systemctl show --value -p InvocationID openipmi
bd3eb84c3aa74169a3dcad2af183885b

To obtain the journal of the latest invocation of, say, openipmi, whether it failed or not, you can use the one liner

$ journalctl _SYSTEMD_INVOCATION_ID=`systemctl show -p InvocationID --value openipmi.service`
-- Logs begin at Thu 2018-07-26 12:09:57 IDT, end at Mon 2019-07-08 01:32:50 IDT. --
Jun 21 13:03:13 build03.lbits openipmi[1552]:  * Starting ipmi drivers
Jun 21 13:03:13 build03.lbits openipmi[1552]:    ...fail!
Jun 21 13:03:13 build03.lbits openipmi[1552]:    ...done.

(Note that the --value is available since systemd 230, older than InvocationID)

Solution 2:

I'm not sure which timestamp makes the most sense but this works for me. Hopefully there is a better way of working with the timestamps from systemctl show than awk - could not figure out how to control the format of timestamps.

unit=foo.service

ts=$(systemctl show -p ActiveEnterTimestamp $unit)

echo $ts
ActiveEnterTimestamp=Fri 2016-11-11 12:30:01 MST

journalctl -u $unit --since "$(echo $ts | awk '{print $2 $3}')"