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$? The status returned by the last pipe close, backtick (``) com-
mand, successful call to wait() or waitpid(), or from the sys-
tem() operator. This is just the 16-bit status word returned
by the wait() system call (or else is made up to look like it).
Thus, the exit value of the subprocess is really ("$? >> 8"),
and "$? & 127" gives which signal, if any, the process died
from, and "$? & 128" reports whether there was a core dump.
(Mnemonic: similar to sh and ksh.)