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(bashref.info)Exit Status

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Exit Status
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   For the shell's purposes, a command which exits with a zero exit
status has succeeded.  A non-zero exit status indicates failure.  This
seemingly counter-intuitive scheme is used so there is one well-defined
way to indicate success and a variety of ways to indicate various
failure modes.  When a command terminates on a fatal signal whose
number is N, Bash uses the value 128+N as the exit status.

   If a command is not found, the child process created to execute it
returns a status of 127.  If a command is found but is not executable,
the return status is 126.

   If a command fails because of an error during expansion or
redirection, the exit status is greater than zero.

   The exit status is used by the Bash conditional commands (Note:
Conditional Constructs) and some of the list constructs (Note:
Lists).

   All of the Bash builtins return an exit status of zero if they
succeed and a non-zero status on failure, so they may be used by the
conditional and list constructs.  All builtins return an exit status of
2 to indicate incorrect usage.