Comparison to sysexits.h Exit Codes

Semantic Exit Code	sysexits.h Exit Code	Name	Meaning
80	64	Usage Error	A required argument was omitted or an invalid value was supplied for a flag.
81		Unknown Command	An unrecognized subcommand was invoked.
82		Requirement Not Met	A prerequisite wasn't met.
83	77	Forbidden	The user isn't authorized to perform the requested action.
84		Moved Permanently	The program has been migrated to a new location.
	65	Data Error	The input data was incorrect in some way. This should only be used for user's data and not system file.
	66	No Input	An input file (not a system file) did not exist or was not readable. This could also include errors like "No message" to a mailer (if it cared to catch it).
	67	No User	The user specified did not exist. This might be used for mail addresses or remote logins.
	68	No Host	The host specified did not exist. This is used in mail addresses of network requests.
	73	Cannot Create	A (user specified) output file cannot be created.
	78	Config Error	Something was found in an unconfigured or misconfigured state.
100	70	Internal Error	An error occurred in the program's own code or in one of its dependencies.
101	69	Unavailable	A local daemon or remote service did not respond, a connection was closed unexpectedly, an HTTP service responded with 503.
	71	Os Error	An operating system error has been detected. This is intended to be used for such things as "cannot fork", "cannot create pipe", or the like. It includes things like getuid returning a user that does not exist in the passwd file.
	72	Os File Error	Some system file (e.g., /etc/passwd, /var/run/utmp, etc.) does not exist, cannot be opened, or has some sort of error (e.g., syntax error).
	74	Io Error	An error occurred while doing I/O on some file.
	75	Temporary Failure	Temporary failure, indicating something that is not really an error. In sendmail, this means that a mailer (e.g.) could not create a connection, and the request should be reattempted later.
	76	Protocol Error	The remote system returned something that was "not possible" during a protocol exchanged.

Why did we depart from the sysexits.h codes?

This library's goals are to define exit codes that are:

1. Broadly applicable to heterogenous Command Line tools
2. Easy to partition into user errors and system errors

sendmail's exit codes weren't ideally suited to either goal. While several of its codes are broadly applicable (like Usage Error, Internal Error, and Forbidden), others are over-fit to sendmail (like No User and No Host). And its numbering scheme interleaves user errors with software errors.