427

I have seen -- used in the compgen command.

For example:

compgen -W "foo bar baz" -- b

What is the meaning of the -- in there?

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525

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
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  • 2
    Most notably in the Bash set built-in, where it's absolutely necessary. – l0b0 Oct 18 '12 at 9:03
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    -- works to separate options from regular expressions in grep, but the canonical way is to use -e/--regexp. – l0b0 Oct 18 '12 at 9:05
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    @l0b0: the pattern to search is normally one of the positional parameters, so it can fit after the --, though you are correct in noting that my example above could also be written as grep -e -v file (although that is very confusing). – Guss Jan 21 '15 at 16:12
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    Not all bash builtin commands accept -- as the end of option marker. [ and echo don't for instance (one of the reasons echo can't be used reliably). – Stéphane Chazelas Mar 21 '16 at 10:06
42

This marks end of parameter (option) list.

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

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  • Link redirects to something about xargs. This is the closest article I could find that references the double dash – KymikoLoco Mar 28 at 15:58
  • Why does this have 40 upvotes when it's a link-only answer and -- doesn't even appear in the article? – Dan Dascalescu May 2 at 0:20
35

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.

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  • 2
    +1 for referencing man bash (note-to-self for next time : ). – user3773048 Oct 18 '18 at 21:24
6

POSIX.1-2017

12.2 Utility Syntax Guidelines

Guideline 10:

The first -- argument that is not an option-argument should be accepted as a delimiter indicating the end of options. Any following arguments should be treated as operands, even if they begin with the '-' character.

http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap12.html#tag_12_02

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