3.7.5 Exit Status
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
(see section 3.2.5 Conditional Constructs) and some of the list
constructs (see section 3.2.3 Lists of Commands).
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.