Linux – kill – no process found

On Linux, when you kill a parent the child gets sent a SIGHUP which will generally kill it unless it was meant to stay alive as a daemon in which case it will trap sighup. (I think this is why one usually uses SIGHUP to tell a daemon to refresh itself, since it's conveniently always trapped).

On Mac OS X I can't find documentation, but it appears that no SIGHUP gets sent. As a result, the child process is orphaned, and its new parent is the grandparent.

The way you deal with this is you send the kill signal to the process group of the parent, not the parent process itself. This will nuke all the children and grand children as well with one caveat. If any child process does a setpgrp() or setsid() then it escapes the process group membership. It will not get the kill sent to its old process group. Usually one need not worry about the latter since it's usually intentional when used to achieve that purpose.

I'm sorry, but this simply is not possible (that's by design). However, if members of a common group, user1 could write to a file that user2's process checks, indicating to the process that it should terminate.

Or, user2 could run something in the background that checks a file, then sends the appropriate signals. User1 then simply has to write to that file. This may be easier, as it would not require any modification of user2's programs.

Conventionally, no, user1 can not send POSIX signals to user2's process.