Unix mystery; error status = 1 and 2;

Question

In a terminal:
$ cat nonexistantfile ; echo $?
cat: nonexistantfile: No such file or directory
1
$ cp /usr/include/sys/errno.h /usr/include/sys/abc.h; echo $?
cp: /usr/include/sys/abc.h: Permission denied
1
The error messages are correct; the numeric values are the same, even though they are different errors. One of them should be a 2, no?
Why?

Answer 1

Most (but not all) C tools and many C routines will use zero as a success code and non-zero as failure, though that can vary. Outside of the ERRNO stuff, there's no particular agreement on individual errors, and there's no particular uniformity on whether you'll get ERRNO or some other value.
Check the man pages for the return values for the various commands.
"The cp utility exits 0 on success, and >0 if an error occurs."
"The cat utility exits 0 on success, and >0 if an error occurs."
Beyond that, there's not a particularly dependable consistency. Sometimes from one version of the tool to the next, unfortunately.