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If a command name contains no slashes, the shell attempts to locate it. If there exists a shell function by that name, the function is invoked as described in Functions. If there exists a shell builtin by that name, the builtin is invoked.
Otherwise, the shell searches each element of $path for a
directory containing an executable file by that name.
If execution fails: an error message is printed, and one of the following values is returned.
The search was unsuccessful. The error message is
‘command not found: cmd’.
The executable file has insufficient permissions, is a directory or special file, or is not a script and is in a format unrecognized by the operating system. The exact conditions and error message are operating system-dependent; see execve(2).
If execution fails because the file is not in executable format,
and the file is not a directory, it is assumed to be a shell
script. /bin/sh is spawned to execute it. If the program
is a file beginning with ‘#!’, the remainder of the first line
specifies an interpreter for the program. The shell will
execute the specified interpreter on operating systems that do
not handle this executable format in the kernel.
If no external command is found but a function command_not_found_handler
exists the shell executes this function with all
command line arguments. The return status of the function becomes the
status of the command. Note that the handler is executed in a
subshell forked to execute an external command, hence changes to
directories, shell parameters, etc. have no effect on the main shell.
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