What is the double dash in this bash script excerp

Question

I'm looking at an excerpt of bash and I'm trying to understand what exactly is happening here, particularly in the COMPREPLY assignment:

 case "$cur" in
    -*)
    COMPREPLY=( $( compgen -W '-a -d -f -l -t -h --aoption --debug \
                               --file --log --test --help --' -- $cur ) );;
#   Generate the completion matches and load them into $COMPREPLY array.
#   xx) May add more cases here.
#   yy)
#   zz)
  esac

I understand we're assigning a value to COMPREPLY here based on the output of compgen, but what exactly is -- doing in this assignment? Also, why the double semi-colon?

Answer 1

The meaning of -- really depends on the program you are executing. In this example compgen. Check the documentation of this program, it should be explained there.

A common convention that is not necessarily always followed is to treat everything after -- as arguments, and do not try to parse as options or flags. As a concrete example, consider the case of running the GNU ls command to get a detailed listing of a file named -t. Running ls -l -t will treat -t as an option (order output by time), not as a filename argument. The solution is ls -l -- -t, this way ls will not try to parse the arguments after --. This is just a convention and not all programs may follow it. Note also that I wrote GNU ls not just ls, because other implementations may behave differently.

Answer 2

compgen is a bash builtin command, and the man pages say:

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

Since $cur is a variable that contains options (starting with -), the -- is required (as mentioned in the man pages) to make a distinction between compgen options and input to be processed.

The following command is turning the option --d in --debug:

compgen -W '-a -d -f -l -t -h --aoption --debug --file --log --test --help --' -- --d
--debug

If you remove the separator --, the command throws an error, because compgen doesn't have any option starting with --d:

compgen -W '-a -d -f -l -t -h --aoption --debug --file --log --test --help --' --d
-bash: compgen: --: invalid option
compgen: usage: compgen [-abcdefgjksuv] [-o option]  [-A action] [-G globpat] [-W wordlist]  [-F function] [-C command] [-X filterpat] [-P prefix] [-S suffix] [word]

---

The ;; is a separator used in the case, esac statement to terminate in your example the section started with -*). Look at bash man pages for Compound Commands to get more info.

Answer 3

The double dashes end the argument portion in getopt_long. From the manual.

The special argument "--" forces an end of option-scanning regardless of the scanning mode.

https://linux.die.net/man/3/getopt_long

The double semi colons end a case branch.