The exit status of an executed command is the value returned by
the waitpid system call or equivalent function. Exit statuses
fall between 0 and 255, though, as explained below, the shell may
use values above 125 specially. Exit statuses from shell
builtins and compound commands are also limited to this range.
Under certain circumstances, the shell will use special values to
indicate specific failure modes.
For the shell's purposes, a command which exits with a zero exit
status has succeeded. An exit status of zero indicates success.
A non-zero exit status indicates failure. When a command
terminates on a fatal signal N, bash uses the value of 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.
Shell builtin commands return a status of 0 (true) if successful,
and non-zero (false) if an error occurs while they execute. All
builtins return an exit status of 2 to indicate incorrect usage,
generally invalid options or missing arguments.
Bash itself returns the exit status of the last command executed,
unless a syntax error occurs, in which case it exits with a non-
zero value. See also the exit builtin command below.
