That's because it's only seemingly simple. The return value is whatever exit status the command you executed returned. The only general convention is that 0 indicates success and not-zero means something went some degree of wrong. You have to look at the documentation for whatever it is you ran to know exactly what it means when /usr/bin/fnord exited with status 23.

The cake is a lie.
The cake is a lie.
The cake is a lie.

In reply to Re: Return codes from a system() call by Fletch
in thread Return codes from a system() call by oltranzista

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