In addition to what rdrtx1 already explained:

If you want to change something only on certain lines and not on others you should do that in sed, not outside, with grep::

Code:

sed '/^special_line/ s/Y/X/g'

will change character "Y" to "X", but only on lines starting with "special_line". Likewise with your problem:

Code:

sed '/connections_per/ s/[<>]!*--//g' a

If you want only the changed lines as output:

Code:

sed -n '/connections_per/ s/[<>]!*--//g' a

As a rule of thumb: whenever you use sed, awk or a similar programmable text processing tool, do everything with it or use another tool. Any line with "...grep | sed | grep..." or "awk | sed | ..." is (very very rare cases aside) nonsense.

I hope this helps.

bakunin