There's really no point in having bash do arithmetic expansion on the
systemd_exitstatus variable, i.e. have it convert the string to a number, do no
actual arithmetic, then convert it back to a string to be used as the argument to
the printf, which will again convert it to a number for the %d specifier, and
finally emit it as a decimal. Also, it deviates for no obvious reason from how
it is passed to printf in the "probably died by signal" case just above.