Unix Exit Status Code Reference

gistfile1.txt

0 # successful termination 64 # base value for error messages
64 # command line usage error
65 # data format error
66 # cannot open input
67 # addressee unknown
68 # host name unknown
69 # service unavailable
70 # internal software error
71 # system error (e.g., can't fork)
72 # critical OS file missing
73 # can't create (user) output file
74 # input/output error
75 # temp failure; user is invited to retry
76 # remote error in protocol
77 # permission denied
78 # configuration error

Source: <http://kennethreitz.com/unix-exit-status-code-reference.html>