sendmail, 4th Edition

Correct exit(2) Values

The sendmail program expects its A= programs to exit with reasonable exit(2) values. The values that it expects are listed in <sysexits.h>. Exiting with unexpected values causes sendmail to bounce mail and gives an unclear message:

554 5.0.0 Unknown status val

Here, val is the unexpected error value. To illustrate, consider the following rewrite of the previous script:

#!/bin/sh
EX_OK=0                   # From <sysexits.h>
EX_USAGE=64               # From <sysexits.h>
if [ ${#} -ne 2 ]; then
        echo $0 needs a username.
        exit $EX_USAGE
fi
/usr/ucb/mail -s "$1 gone" postmaster
exit $EX_OK

Here, if the argument count is wrong, we exit with the value EX_USAGE, thus producing a clearer (two-line) error message:

/usr/local/bin/gone needs a username.
/usr/local/bin/gone... Bad usage.

If all goes well, we then exit with EX_OK so that sendmail knows the mail was successfully delivered.