grep: show lines surrounding each match

Asked 2023-09-20 20:12:41 View 839,604

How do I grep and show the preceding and following 5 lines surrounding each matched line?

  • I keep a copy of Brendan Gregg's perl script around for this purpose. Works well. - anyone
  • For a solution that works on Solaris, check out this link. - anyone
  • man grep | grep -C 1 context :) - anyone
  • man grep | grep -C 1 "\-C" ;) - anyone
  • @StvnW ... I don't know whether to call that meta (in a more general, rather than SO context), or what to call it. You answered the question by showing how to use the answer to find the answer. - anyone

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.

Answered   2023-09-20 20:12:41

  • It is good but unfortunately the Solaris grep does not support that. See that link for solaris: unix.com/solaris/33533-grep-display-few-lines-before-after.html - anyone
  • Ok, but what if want to show all lines of output after the match? grep -A0 and grep -A-1 don't cut it... - anyone
  • does not work for me for some reason, although mentioned in my man pages. - anyone
  • If you are HP-UX env, none of the grep versions will work like in Solaris. Was able to use the Solaris link but replace nawk with awk in that link. - anyone
  • -n is for line numbers, but for some versions of grep -n# will show # surrounding lines (like -c) with line numbers. That's a useful shortcut that's my go-to when I need context. - anyone

-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).

Answered   2023-09-20 20:12:41

  • I tried the -n format and found out it only works till 9. For 15 it returns 5 lines - anyone
  • @DeepakMahakale This is probably related to how command-line arguments / options are typically parsed by POSIX programs. The option specifier is a single character (such as -A, -B or -C). Usually, the option specifier is followed by a value (-o a.out to specify output file in GCC), but it can also function as a simple switch / flag (-g to enable debugging info in GCC). However spaces between options are optional, so for options without a value, it is possible to merge them (-ABC), which means that -15 is interpreted -1 -5 (two separate options) and the -5 overrides the -1. - anyone
  • -5 is quicker than both -A 5 -B 5. Those are not meant to be used together. It is cleaner to other readers of the script if you choose -A or -B or -C over -9. - anyone
  • The -n option currently works for more than 9 lines on (GNU grep) 3.1. - anyone
grep astring myfile -A 5 -B 5

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

Answered   2023-09-20 20:12:41

  • "grep astring myfile -C 5 " will do the same - anyone
  • Wow that's great! A = after, B = before, C = context - anyone

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.

Answered   2023-09-20 20:12:41

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

Answered   2023-09-20 20:12:41

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

Answered   2023-09-20 20:12:41

  • Did you not read the accepted answer? You are just repeating what has already been said on a question almost 10 years old... - anyone
  • Oh I'm sorry Yokai. But I don't read anything about grepping the help section of grep to retrieve the answer. - anyone
  • @Yokai Besides what Chiel ten Brinke said, the accepted answer does not mention the long options - anyone

Let's understand using an example.
We can use grep with options:

-A 5  # this will give you 5 lines after searched string.
-B 5  # this will give you 5 lines before searched string.
-C 5  # this will give you 5 lines before & after searched string

Example. File.txt contains 6 lines and following are the operations.

[abc@xyz]~/% cat file.txt # print all file data
this is first line
this is 2nd line
this is 3rd line
this is 4th line
this is 5th line
this is 6th line

[abc@xyz]~% grep "3rd" file.txt # we are searching for keyword '3rd' in the file
this is 3rd line

[abc@xyz]~% grep -A 2 "3rd" file.txt # print 2 lines after finding the searched string
this is 3rd line
this is 4th line
this is 5th line

[abc@xyz]~% grep -B 2 "3rd" file.txt # Print 2 lines before the search string.
this is first line
this is 2nd line
this is 3rd line

[abc@xyz]~% grep -C 2 "3rd" file.txt # print 2 line before and 2 line after the searched string 
this is first line
this is 2nd line
this is 3rd line
this is 4th line
this is 5th line

Trick to remember options:

  1. -A  → A means "After"
  2. -B  → B means "Before"
  3. -C  → C means "in between"

Answered   2023-09-20 20:12:41

If you search code often, AG the silver searcher is much more efficient (ie faster) than grep.

You show context lines by using the -C option.

Eg:

ag -C 3 "foo" myFile

line 1
line 2
line 3
line that has "foo"
line 5
line 6
line 7

Answered   2023-09-20 20:12:41

  • On what system (incl. distribution name and version) did you try it? Was it installed by default? If not, what are some installation instructions? It is not installed by default on Ubuntu MATE 20.04 (Focal Fossa). - anyone

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;

Answered   2023-09-20 20:12:41

I do it the compact way:

grep -5 string file

That is the equivalent of:

grep -A 5 -B 5 string file

Answered   2023-09-20 20:12:41

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.

Answered   2023-09-20 20:12:41

  • Very nice. Yes, it is more complex-looking, but it is actually cross-platform. Tested on AIX and HP-UX. - anyone

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

Answered   2023-09-20 20:12:41

$ 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.

Answered   2023-09-20 20:12:41