Say we have a parent process P, child of a grandparent process G, spawning a child process C and waiting for it. Either C dies normally with an exit code from 0 to 255, or it is killed by a signal. How can we make sure that P reports to G what happened to C, with as much precision as possible ?
The problem is, there's more information in a wstat (the structure filled in by waitpid()) than a process can report by simply exiting. P could exit with the same exit code as C, but then what should it do if C has been killed by a signal ?
An idea is to have P kill itself with the same signal that killed C. But that's actually not right, because P itself could be killed by a signal from another source, and G needs that information. "P has been killed by a signal" and "C has been killed by a signal" are two different informations, so they should not be reported in the same way.
So, any way you look at it, there is always more information than we can report.
Shells have their own convention for reporting crashes, but since any exit code greater than 127 is reported as is, the information given by the shell is unreliable: "child exited 129" and "child was killed by SIGHUP" are indistinguishable. When shells get nested, all bets are off - the information conveyed by exit codes becomes devoid of meaning pretty fast. We need something better.
execline commands such as forx, that can report a child's exit code, proceed that way when they're in the position of P: