Question

Is it possible to only purge/delete a job log by invocation id in multi-instance jobs from the DataStage command line?

Answer

No, this is not possible because log records for job invocations are stored together in the RT_LOG file for the job. Currently the purging mechanism only allows deletion by date/time and by number of previous runs, but it does not allow the identification of individual invocations within the log file,
Why is this different from DataStage Director Client?

In the DataStage Director, you can clean up the individual instance log. For example, take an ETL job named etl_a, which has 2 instances, etl_a.1 and etl_a.2. From DataStage Director, highlight etl_a.1, right-click, then select delete. This will delete etl_a.1 from job list. etl_a, while etl_a.2 will still remain in the job list.

In DataStage Director, the delete action described above only removes the invocation entry from the list of jobs and does not actually delete any log records, which remain in the log file until the purge mechanism discards them.

So the difference here is that the Director delete action only deletes records from RT_STATUS whereas the purging mechanism deletes records from RT_LOG.