Shell Commands Pipelines
A simple command is the kind of command encountered most often.
It's just a sequence of words separated by
by one of the shell's control operators (see Definitions). The
first word generally specifies a command to be executed.
The return status (see Exit Status) of a simple command is
its exit status as provided
by the POSIX.1
waitpid function, or 128+n if the command
was terminated by signal n.