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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. The following command gets me close:

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

But this includes a separator -- line between every line of desired text. Any easy ideas how to get rid of the -- lines?

Example: for an input file that looks like

line line nmse_gain_constant matching line line after first match line line nmse_gain_constant another matching line line after second match line nmse_gain_constant a third matching line line after third match

the output is

line after first match -- line after second match -- line after third match

but I'd like to have just

line after first match line after second match line after third match
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5 Answers 5

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120

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.

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4 Comments

grep: unrecognized option '--no-group-separator' on Mac OS X
instead of adding additional grep just remove the A flag
--no-group-separator is not supported by all versions of grep
For Mac OS X, you can install the Gnu versions of various command line tools using brew; this includes grep. I believe by default it is installed as ggrep (Gnu Grep). This supports the --no-group-separator option. I have done this and it works. See this link as a reference How to install and use GNU Grep in macOS
25

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.

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6 Comments

You can use --no-group-separator (see below)
even if I set --group-separator="", the group separator is not actually empty, but still gets colored. So instead of an empty line, the separator line contains the color code, i.e [[36m[[K[[m[[K. Is there any way to disable coloring of the separator, while keeping grep --color=always?
there is no such option on mac for grep (BSD grep) 2.5.1-FreeBSD
I can confirm Tim's statement - I've got the same version of grep and it is not available.
@eddie the A flag was needed for the search to find what the person was looking for. very often you're looking for "the line after something else"
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20

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'
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3 Comments

Regexp with sed is overkill for this task. Why not use grep -v -- --?
The -A doesn't just add "--" it also prints NUM extra lines After the match. Similarly -B prints NUM lines Before the match, and -C prints NUM lines of Context from around the match. The "--" gets added to separate the groups.
This answer presumes the GNU version of grep. That excerpt of the man page, for instance, does not match at least BSD grep, GNU compatible 2.6.0-FreeBSD. Nor, in this version, is the -A flag exclusively responsible for the inclusion of the -- separators.
8

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
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2 Comments

for grep -B use: awk '/regex/{ print x; print }; { x=$0 }'
an explanation of the getline;print part of the awk command would suit this answer as most people who try to grep when they should awk are not familiar with the awk expressions used to obtain grep equivalent results
6

One solution will be:

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

Unecessary quoting and escaping: it's simpler to use ...| grep -v -- --

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