How to run a command after an already running, existing one finishes?

You can separate multiple commands by ;, so they are executed sequentially, for example:

really_long_script.sh ; echo Finished

If you wish to execute next program only if the script finished with return-code 0 (which usually means it has executed correctly), then:

really_long_script.sh && echo OK

If you want the opposite (i.e. continue only if current command has failed), than:

really_long_script.sh || echo FAILED

You could run your script in a background (but beware, scripts output (stdout and stderr) would continue to go to your terminal unless you redirect it somewhere), and then wait for it:

really_long_script.sh &
dosomethingelse
wait; echo Finished

If you have already run script, you could suspend it with Ctrl-Z, and then execute something like:

fg ; echo Finished

Where fg brings the suspended process to foreground (bg would make it run in background, pretty much like started with &)

You can also use bash's job control. If you started

$ really_long_script.sh

then press ctrl+z to suspend it:

^Z
[1]+  Stopped                 really_long_script.sh
$ bg

to restart the job in the background (just as if started it with really_long_script.sh &). Then you can wait for this background job with

$ wait N && echo "Successfully completed"

where N is the job ID (probably 1 if you didn't run any other background jobs) which is also displayed as [1] above.

If the process does not run on current tty, then try this:

watch -g ps -opid -p <pid>; mycommand