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Some tutorials (such as the Getting Started on linuxcontainers.org) recommend the use of a double-hyphen (--) when using lxc exec to run a program inside a LXD container, as in

$ lxc exec my-container -- apt update

I suspect -- tells lxc exec to pass provided flags to the program, because e.g. lxc exec my-container apt list --upgradable fails with

error: flag provided but not defined: --upgradable

However, I can't find any documentation on the issue. Can someone confirm, or clarify if that's not exactly what it does?

Also, I saw instances where people use -- in different places, e.g.:

$ lxc exec my-container apt -- list --upgradable
$ lxc exec my-container apt list -- --upgradable

What's the difference?

1 Answer 1

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Partly this is a duplicate of what does double dash mean.

You ask an interesting question about lxc so let me answer that. In your two examples, both instances of -- tell lxc to not interpret options after that point. The different positioning in this case is irrelevant. However, because you're running a command line in a container, you might want to use -- as part of that command line. Consider the following: I created a file /tmp/bar containing the line --foo. The following commands were run in the container:

root@myContainer:~# echo "here is --foo" >/tmp/bar
root@myContainer:~# echo "this line has no double dash" >> /tmp/bar
root@myContainer:~# cat /tmp/bar
here is --foo
this line has no double dash
root@myContainer:~# exit

Now I'm going to search for that using grep. I need to let grep know that the -- is part of the search pattern, not an option to grep:

$ lxc exec myContainer -- grep -- --foo /tmp/bar
here is --foo

If I use a single -- it gets consumed by the lxc command line and I get the error:

$ lxc exec myContainer grep -- --foo /tmp/bar
grep: unrecognized option '--foo'
Usage: grep [OPTION]... PATTERN [FILE]...
Try 'grep --help' for more information.

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