How to source `. filename` reliably?

Question

The current POSIX spec does not specify any options for dot . builtin.

If I do something like:

$ echo 'echo .' > /tmp/-foo
$ PATH=/tmp "$shell" -c '. -foo'

then the result is varied between shells:

• dash, ash, ksh88, Bourne shell, schily sh, schily osh, heirloom sh work well.
• bash, yash, ksh93, pdksh, mksh, posh don't. Changing the command to . -- -foo works in these shells.

And also, using -- is a non compliant way, because POSIX spec says that builtin which are not conformed to Utility Syntax Guidelines will ignore --.

zsh is the only shell that works with both cases.

So how can I make . filename work reliably in Bourne-like or POSIX compliant shells?

Answer 1

To avoid shell-dependent effects, pass a full path to .. . /absolute/path/to/script and . relative/path/to/script work in all shells.

PATH lookup is rarely useful for sourced scripts anyway. If you do want PATH lookup, then you can do the lookup manually in case the filename starts with -. Or you can require that the filename does not start with -, to keep things simple.

Answer 2

I assume this is partly a theoretical question.

In practical terms I'd make it work reliably either by avoiding - as the first character of the included script or by providing a script path (relative or absolute doesn't matter).

Again from a practical perspective, I prefer to code for a known shell, but if that wasn't possible I'd probably end up with an ugly case statement wrapped inside a function. Unless the generalisation had to extend to *csh shells too, in which case I'd probably just run away.