How do I get rid of “--” line separator when using grep with context lines?

Question

I have a text file named compare.txt where I want to extract the single line that follows every line that contains the pattern nmse_gain_constant.

Answer 1

There is no need to pipe to so many greps or use other tools (for example, sed) if you use AWK:

awk '/nmse_gain_constant/{getline;print }' compare.txt

Answer 2

There is an undocumented parameter of grep: "--group-separator", which overrides the default "--". You can set it to "" to get rid of the double dash. Though, you still get an empty line. I had the same trouble, and found this param by reading the source code of grep.

Answer 3

One solution will be:

grep -A 1 nmse_gain_constant compare.txt | grep -v nmse_gain_constant  | grep -v "\-\-"

Answer 4

I do this:

 grep ... | grep -v -- "^--$"

But this works too (on many, not all OS'es)!

grep --no-group-separator ...

And it doesn't spit out that "--" or even a blank line.

Answer 5

Well, the A switch by default will add those characters, so it's no mystery.

man grep states:

-A NUM

    Places  a  line  containing  a  group  separator  (--)   between
    contiguous  groups  of  matches.  With the -o or --only-matching
    option, this has no effect and a warning is given.

But you can use a simple sed to clean up the result:

yourgrep | sed '/^--$/d'