I put my hands up to being a complete beginner, so apologies if I'm doing this wrong.

Say I have the following data frame that continues indefinitely:

 -1 2 3 4 -5 9
 2 3 -4 5 -6 11

And I want to only keep the minus signs and every other column, so I get this:

 - 2 4 - 9
 3 - 5 - 11

And eventually end up with this:

 -2 4 -9
 3 -5 -11

Is there a way to do this with awk/sed?

This is about a far as I get:

 awk '{ for (i=2;i<=NF;i+=2) $i="" }1' FILE.txt | sed 's/[0-9,.]*//g'