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search – grep a file, but show several surrounding lines?

Posted by: admin February 21, 2020 Leave a comment

Questions:

I would like to grep for a string, but also show the preceding five lines and the following five lines as well as the matched line. How would I be able to do this?

How to&Answers:

For BSD or GNU grep you can use -B num to set how many lines before the match and -A num for the number of lines after the match.

grep -B 3 -A 2 foo README.txt

If you want the same number of lines before and after you can use -C num.

grep -C 3 foo README.txt

This will show 3 lines before and 3 lines after.

Answer:

-A and -B will work, as will -C n (for n lines of context), or just -n (for n lines of context… as long as n is 1 to 9).

Answer:

ack works with similar arguments as grep, and accepts -C. But it’s usually better for searching through code.

Answer:

grep astring myfile -A 5 -B 5

That will grep “myfile” for “astring”, and show 5 lines before and after each match

Answer:

I normally use

grep searchstring file -C n # n for number of lines of context up and down

Many of the tools like grep also have really great man files too. I find myself referring to grep’s man page a lot because there is so much you can do with it.

man grep

Many GNU tools also have an info page that may have more useful information in addition to the man page.

info grep

Answer:

Use grep

$ grep --help | grep -i context
Context control:
  -B, --before-context=NUM  print NUM lines of leading context
  -A, --after-context=NUM   print NUM lines of trailing context
  -C, --context=NUM         print NUM lines of output context
  -NUM                      same as --context=NUM

Answer:

Search for “17655” in “/some/file.txt” showing 10 lines context before and after (using Awk), output preceded with line number followed by a colon. Use this on Solaris when ‘grep’ does not support the “-[ACB]” options.

awk '

/17655/ {
        for (i = (b + 1) % 10; i != b; i = (i + 1) % 10) {
                print before[i]
        }
        print (NR ":" ($0))
        a = 10
}

a-- > 0 {
        print (NR ":" ($0))
}

{
        before[b] = (NR ":" ($0))
        b = (b + 1) % 10
}' /some/file.txt;

Answer:

ripgrep

If you care about the performance, use ripgrep which has similar syntax to grep, e.g.

rg -C5 "pattern" .

-C, --context NUM – Show NUM lines before and after each match.

There are also parameters such as -A/--after-context and -B/--before-context.

The tool is built on top of Rust’s regex engine which makes it very efficient on the large data.

Answer:

Here is the @Ygor solution in awk

awk 'c-->0;$0~s{if(b)for(c=b+1;c>1;c--)print r[(NR-c+1)%b];print;c=a}b{r[NR%b]=$0}' b=3 a=3 s="pattern" myfile

Note: Replace a and b variables with number of lines before and after.

It’s especially useful for system which doesn’t support grep’s -A, -B and -C parameters.

Answer:

$ grep thestring thefile -5

-5 gets you 5 lines above and below the match ‘thestring’ is equivalent to -C 5 or -A 5 -B 5.

Answer:

Grep has an option called Context Line Control, you can use the --context in that, simply,

| grep -C 5

or

| grep -5

Should do the trick

Answer:

I use to do the compact way

grep -5 string file

That is the equivalent of

grep -A 5 -B 5 string file