Chris Lindee
3de092fad7
The exit status should be compared against 128 — not 127 — as the Bash
manual, under the EXIT STATUS section, states [0]:
When a command terminates on a fatal signal N, bash uses the value
of 128+N as the exit status.
If a command is not found, the child process created to execute it
returns a status of 127. If a command is found but is not
executable, the return status is 126.
Furthermore, UAPI.15 specifies the `signal` value must be the symbolic
signal name, not a numerical value [1]. Luckily, POSIX.1-2008 ensures
the shell must supply `kill -l`, which must be able to convert `$?` to
the signal that could cause such an exit in that shell [2]. It should
be noted that only a select handful of signals have a standard numeric
value; all others are implementation-defined [3]. This is why UAPI.15
requires the symbolic name, as the value may differ across systems.
Notably, not every value above 128 needs to correspond to a signal; on
my Linux system, only exit codes 129–192 indicate a signal. Again, we
can use `kill -l`, which will exit non-zero when an exit code does not
come from a signal (plus 128) [4].
Signal 0 (which would correspond to exit code 128) is reserved for the
null signal, per POSIX.1-2008 [5], and therefore will never be an exit
signal.
This change brings the implementation into compliance.
References:
[0] https://www.man7.org/linux/man-pages//man1/bash.1.html#EXIT_STATUS
[1] https://uapi-group.org/specifications/specs/osc_context/
[2] https://pubs.opengroup.org/onlinepubs/9699919799.2008edition/utilities/kill.html#:~:text=The%20letter%20ell
[3] https://pubs.opengroup.org/onlinepubs/9699919799.2008edition/utilities/kill.html#:~:text=The%20effects%20of%20specifying%20any%20signal_number
[4] https://www.man7.org/linux/man-pages//man1/bash.1.html#:~:text=false%20if%20an%20error%20occurs%20or%20an%20invalid%20option%20is%20encountered
[5] https://pubs.opengroup.org/onlinepubs/9699919799.2008edition/basedefs/signal.h.html#:~:text=The%20value%200%20is%20reserved%20for%20use%20as%20the%20null%20signal
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