I haven't read your whole message yet, but to unshift a new value in front
of the positional params, all you have to do is this:


    set -- "$new" "$@"

And now $1 is "$new", $2 is whatever $1 used to be, and so on.

On Mon, Apr 1, 2024 at 9:52 AM Ray Andrews <rayandrews@eastlink.ca> wrote:

> Can we unshift the positional params?  I have a complex function that has
> shifts throughout and at one point there's a while loop:
>
> shift
> while [ "$1" ]; do
>     if [ a ]; then b; c  fi
>     if [ d ]; then e; f  fi
>     if [ g ]; then h; i  fi
> ...
> shift
> done
>
> ... works fine, but I'm thinking to restructure it:
>
> shift
> while [ "$1" ]; do
>     if [ a ]; then b; c; continue fi
>     if [ d ]; then e; f; continue fi
>     if [ g ]; then h; i; continue fi
> ...
> shift
> done
>
> ... because only one 'if' test will be relevant per loop and the remaining
> 'if' tests might even do some bad stuff.  The problem of course is that if
> I 'continue', the final 'shift' is missed.  I could do this:
>
> # shift
> while [ "$2" ]; do
>     shift
>     if [ a ]; then b; c; continue fi
>     if [ d ]; then e; f; continue fi
>     if [ g ]; then h; i; continue fi
> ...
> # shift
> done
>
> ... except that the leading (commented) 'shift' might happen in any number
> of places and changing them all would be troublesome.  It can be done of
> course, but I'm wondering if we have some way of  faking this:
>
> shift
> unshift
> while [ "$1" ]; do
>     shift
>     if [ a ]; then b; c; continue fi
>     if [ d ]; then e; f; continue fi
>     if [ g ]; then h; i; continue fi
> ...
> # shift
> done
>
> ... it looks pointless above, however note that the leading 'shift' might
> in fact have happened in any one of several different places all of which
> would need to be reworked and it's fragile, so the 'unshift' would take
> care of it in one place, at least temporarily.  I know we don't have an
> official unshift, but can it be faked?  Of course there's also this:
>
>     ...
>     if [ a ]; then b; c; shift; continue fi
>     ...
>
> ... but I'm wondering about a more succinct logic.  The quick and dirty
> 'unshift' idea would solve a temporary problem very nicely.
>
>
>
>

-- 
Mark J. Reed <markjreed@gmail.com>