UNIX for Advanced & Expert Users

You are doing something like this:
1) Borrow 100$
2) Buy gold for 100$
3) Buy more gold since you have now 100$ and gold worth of 100$
The (3) has logical error.
You are writing to a file that is already printed (the same way as you would be going to buy gold for money that you have already spent).

In other words:
1) cat "a.txt"
2) remember output from (1)
3) execute "while read something" using (2)
4) printo into "1.txt" that is the same file on the disk but different than (1)

You should avoid updating the object you rely on. That could lead to a deadlock. Just like giving a note to a blonde.... but the note has "turn around" on both sides.

You should probably change your approach and then it should be simple.