ABC.txt has 2400 lines in total and out of that 200 line of some pattern that needs to be commented. rest of the lines should be untouched.

For the lines to be commented pattern, i have got those in a temp file to use it as an identified key.

---------- Post updated at 08:40 PM ---------- Previous update was at 08:36 PM ----------

I am facing hard luck in the previous solution as well. The command runs but not commenting enteries.

sed -e 's/\(^FIM$\)/#\1/g' /tmp/abc


san% grep FIM /tmp/abc
FIM1
FIM2
FIMR45

---------- Post updated at 08:54 PM ---------- Previous update was at 08:40 PM ----------

Got the quick workaround to enjoy my weekend


perl -p -i -e "s/PM/#PM/g" ABC.TXT