Grep characters before and after match?

Question

Using this:

grep -A1 -B1 "test_pattern" file

will produce one line before and after the matched pattern in the file. Is there a way to display not lines but a specified number of characters?

The lines in my file are pretty big so I am not interested in printing the entire line but rather only observe the match in context. Any suggestions on how to do this?

Solution 1

3 characters before and 4 characters after

$> echo "some123_string_and_another" | grep -o -P '.{0,3}string.{0,4}'
23_string_and

Solution 2

grep -E -o ".{0,5}test_pattern.{0,5}" test.txt 

This will match up to 5 characters before and after your pattern. The -o switch tells grep to only show the match and -E to use an extended regular expression. Make sure to put the quotes around your expression, else it might be interpreted by the shell.

Solution 3

You could use

awk '/test_pattern/ {
    match($0, /test_pattern/); print substr($0, RSTART - 10, RLENGTH + 20);
}' file

Solution 4

You mean, like this:

grep -o '.\{0,20\}test_pattern.\{0,20\}' file

?

That will print up to twenty characters on either side of test_pattern. The \{0,20\} notation is like *, but specifies zero to twenty repetitions instead of zero or more.The -o says to show only the match itself, rather than the entire line.

Solution 5

I'll never easily remember these cryptic command modifiers so I took the top answer and turned it into a function in my ~/.bashrc file:

cgrep() {
    # For files that are arrays 10's of thousands of characters print.
    # Use cpgrep to print 30 characters before and after search pattern.
    if [ $# -eq 2 ] ; then
        # Format was 'cgrep "search string" /path/to/filename'
        grep -o -P ".{0,30}$1.{0,30}" "$2"
    else
        # Format was 'cat /path/to/filename | cgrep "search string"
        grep -o -P ".{0,30}$1.{0,30}"
    fi
} # cgrep()

Here's what it looks like in action:

$ ll /tmp/rick/scp.Mf7UdS/Mf7UdS.Source
-rw-r--r-- 1 rick rick 25780 Jul  3 19:05 /tmp/rick/scp.Mf7UdS/Mf7UdS.Source
$ cat /tmp/rick/scp.Mf7UdS/Mf7UdS.Source | cgrep "Link to iconic"
1:43:30.3540244000 /mnt/e/bin/Link to iconic S -rwxrwxrwx 777 rick 1000 ri
$ cgrep "Link to iconic" /tmp/rick/scp.Mf7UdS/Mf7UdS.Source
1:43:30.3540244000 /mnt/e/bin/Link to iconic S -rwxrwxrwx 777 rick 1000 ri

The file in question is one continuous 25K line and it is hopeless to find what you are looking for using regular grep.

Notice the two different ways you can call cgrep that parallels grep method.

There is a "niftier" way of creating the function where "$2" is only passed when set which would save 4 lines of code. I don't have it handy though. Something like ${parm2} $parm2. If I find it I'll revise the function and this answer.

View more solutions

Comments

[sondra.kinsey]

Duplicate of unix.stackexchange.com/q/163726 Near duplicate of stackoverflow.com/q/2034799

[Benubird]

A good answer for small amounts of data, but it starts getting slow when you are matching >100 characters - e.g. in my giant xml file, I want {1,200} before and after, and it is too slow to use.

[ssobczak]

The awk version by @amit_g is much faster.

[Xofo]

Not available on Mac OSX, so really this is not a widely available solution. The -E version (listed below) is a better solution. What is -P? Read on ... -P, --perl-regexp Interpret PATTERN as a Perl regular expression (PCRE, see below). This is highly experimental and grep -P may warn of unimplemented features.

[Apollon]

Works nicely even with somewhat bigger files also

[Kev]

Inexplicably, for me, this prints a certain number of lines of beautiful output, then says "Aborted", every time the same number of lines, which depends on what I'm searching for, but is never the full number of matches, by far. bash 4.1.2(1) and grep 2.6.3, CentOS 6.5.

[Kev]

The -E version below does not have this trouble, for some reason. Also, if I search for something that doesn't exist, I get only the Aborted line.

[glerYbo]

On OSX install via: brew install homebrew/dupes/grep and run it as ggrep.

[koox00]

how can you use this to find multiple matches per line?

[Alexander Pravdin]

This command is not working for me: grep: Invalid content of \{\}

[Lew Rockwell Fan]

What's the significance of the first number in the curly-bracketed pairs? Like the 0s in "grep -E -o ".{0,5}test_pattern.{0,5}" test.txt "?

[matanster]

As implied by @Benubird this will be performance-wise impossible to use for huge files with moderately wide surroundings desired for the match target.

[CodeMonkey]

Good answer, interesting that it's capped at 2^8-1 for length in the {} so {0,255} works {0,256} gives grep: invalid repetition count(s)

[Abdollah]

It's really faster but not as accurate as @ekse's answer.

[Adam Hughes]

This seems to get considerably less performant as I increase the number of matching chars (5 -> 25 ->50), any idea why?

[GKP]

not working for me bash-5.1$ echo "some123_string_and_another" | grep -o -P '.{0,3}string.{0,4}' grep: unrecognized option: P