# add a string in a field to line number (15)
perl -pi -e 'if($.==14){s/\n/\n\/\/ ABC\n/g}if(eof){$.=0}' Makefile.txt

# replace regex with environment variable
perl -pi -e 's/(([0-9]+.){3}([0-9]+))/'$dIPmysql'/g' t.sh