texinode(Command Execution)(Functions)(Redirection)(Top)
chapter(Command Execution)
ifzman(\
sect(Command Execution)
)\
cindex(command execution)
cindex(execution, of commands)
cindex(command not found, handling of)
findex(command_not_found_handler)
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 noderef(Functions).  If there exists
a shell builtin by that name, the builtin is invoked.

vindex(path, use of)
Otherwise, the shell searches each element of tt($path) for a
directory containing an executable file by that name.  If the
search is unsuccessful, the shell prints an error message and returns
a nonzero exit status.

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.  tt(/bin/sh) is spawned to execute it.  If the program
is a file beginning with `tt(#!)', 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 tt(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.  If the function wishes to mimic the
behaviour of the shell when the command is not found, it should
print the message `tt(command not found:) var(cmd)' to standard error
and return status 127.  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.