Question

Why this awk script:

awk '{FS = "\t" ; print $1 " - " $2}' A.txt

with this input file A.txt

B A A1
C B A2
D A A3

outputs these results

B - A
C B - A2
D A - A3

Note that between first B and A there is a space and not a tab character. I double checked this

Answer 1

I believe it’s because FS is being set in the first action. Before the first action is invoked, the splitting of the first line is done already, and it uses the default FS (whitespace).

So to get it consistent, you should invoke awk with -F option.