What does “--” (double-dash) mean? (also known as “bare double dash”)

Question

I have seen -- used in the compgen command.

For example:

compgen -W "foo bar baz" -- b

What is the meaning of the -- in there?

Answer 1

More precisely, a double dash (--) is used in bash built-in commands and many other commands to signify the end of command options, after which only positional parameters are accepted.

Example use: lets say you want to grep a file for the string -v - normally -v will be considered the option to reverse the matching meaning (only show lines that do not match), but with -- you can grep for string -v like this:

grep -- -v file

Answer 2

This marks end of parameter (option) list.

http://linux.about.com/library/cmd/blcmdl1_compgen.htm

Answer 3

In man bash we can read in Shell Builtin Commands section:

Unless otherwise noted, each builtin command documented in this section as accepting options preceded by - accepts -- to signify the end of the options.

The :, true, false, and test builtins do not accept options and do not treat -- specially. The exit, logout, break, continue, let, and shift builtins accept and process arguments beginning with - without requiring --. Other builtins that accept arguments but are not specified as accepting options interpret arguments beginning with - as invalid options and require -- to prevent this interpretation.

Note that echo does not interpret -- to mean the end of options.