How can I use a variable line number with a command?

Question

I want to move a line to another specified in a variable.

So where I might do this to move the current line up seven lines:

:m-7

I want to do it with a variable line destination. Something like this:

:let lineOffset=-7
:m$lineOffset

Ultimately, I'm trying to append a collections of lines. So given this:

1
2
3
one
two
three

I want a simple way of getting this:

1 one
2 two
3 three

It seemed to me that the quickest way would be a macro. Something like this:

:m$(lineOffset)
kJ
$(lineOffset)j
:let lineOffset+=1

I know you can't use shell variable syntax as shown above. But used here for purpose of comprehension.

Where lineOffset, using the example above, would be -3, so would move the current line 3 lines back when used as :m-3.

And where those statements

1. Move current line to line number specified by lineOffset (move sends the cursor to that line as well);
2. Move to line above moved line and join it with the moved line;
3. Go to next line to be moved;
4. Set the lineOffset to the next line down from the last.

Answer 1

:g/^/m+2|j

to move the lines down by 2 and join with the next.

That applies the command to the whole document/buffer. As a side-effect, it leaves a search highlight on every line-start. :nohlsearch will clear the current search highlight. Additionally, a range can limit global to certain lines. So to apply it to only, say, lines 1 to 3, and remove the residual highlight:

:1,3g/^/m+2|j|noh

And if you really want a variable to be involved:

:execute("g/^/m+".lineOffset."|j")