VIM search and replace - replace string that is found on particular lines only

Question

So I have a bunch of lines like this -

$ns duplex-link n1 n2 10mb 10ms DropTail
$ns duplex-link-op n1 n2 10mb 10ms queuePos 0.5
$ns duplex-link n2 n3 10mb 10ms DropTail
$ns duplex-link-op n2 n3 10mb 10ms queuePos 0.5
$ns duplex-link n3 n4 10mb 10ms DropTail
$ns duplex-link-op n3 n4 10mb 10ms queuePos 0.5

Now here's the problem. I want the "10mb 10ms" string removed only where the second word happens to be "duplex-link-op". Therefore I can't do a general replace "10mb 10ms" with a " " command.

On a similar note, how do I do a search and replace of a particular string that happens to occur in a line that has another string? I am guessing it has something to do with backreferences ... but I am unable to find enough tutorials on the web on how to do so :(

Answer 1

:g/duplex-link-op/s/10mb 10ms//g should replace them.

And try :help sub-replace-expression, and :help sub-replace-special for nearest thing to backreferences in Vim.

Answer 2

If you don't mind using other tools,

awk '$2=="duplex-link-op"{ sub("10mb 10ms","")}1' file > t && mv t file

sed -i.bak '/duplex-link-op/s/10mb 10ms//' file