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Answering to this question:

I found the following situation with sed version 4.2.2.

From this input:

send host-name = gethostname();
 domain-name, domain-name-servers, domain-search, host-name,
# option host-name "andare.swiftmedia.com";

I want to remove the string "host-name" only from the second line, so the expected output is:

send host-name = gethostname();
 domain-name, domain-name-servers, domain-search,
# option host-name "andare.swiftmedia.com";

Then I used the following command:

sed -e 's/" host-name,"//g' input_file

However, it won't remove the " host-name," string on input_file leaving it as it was.

If I don't use double quotation:

sed -e 's/ host-name,//g' input_file

It gives properly the expected output.

I thought that the correct way would be by using double quotation, but can't figure out why it's not working here.

The character " is not special for the shell inside '...'. In fact, inside '...' no character is special for the shell; as a consequence, the " characters are passed to sed; and they are not special to sed.

The command sed -e 's/" host-name,"//g' is looking literally for "   h o s t n a m e , ", which it won't find because it's not there.