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Seeing Context with `grep`

Ryan Palo on June 20, 2019

Have you ever searched for some text in a file on the command line only to realize that grep's output isn't quite enough for you to understand what...

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Valentin Baca • Jun 21 '19

Meta advice: "tldr" (tldr.sh/) is a wonderful, pragmatic way to find common flags for commands. It's much more approachable than man.

$ tldr grep

grep

Matches patterns in input text.
Supports simple patterns and regular expressions.

- Search for an exact string:
    grep search_string path/to/file

- Search in case-insensitive mode:
    grep -i search_string path/to/file

- Search recursively (ignoring non-text files) in current directory for an exact string:
    grep -RI search_string .

- Use extended regular expressions (supporting `?`, `+`, `{}`, `()` and `|`):
    grep -E ^regex$ path/to/file

- Print 3 lines of [C]ontext around, [B]efore, or [A]fter each match:
    grep -C|B|A 3 search_string path/to/file

- Print file name with the corresponding line number for each match:
    grep -Hn search_string path/to/file

- Use the standard input instead of a file:
    cat path/to/file | grep search_string

- Invert match for excluding specific strings:
    grep -v search_string

Diretnan Domnan • Jun 21 '19

Wow... a lot nicer than man

Valentin Baca • Jun 21 '19

It's best to also use them in conjunction:

Use tldr for the quick easy things, then with that knowledge, you know what to look for in the man page.

Ryan Palo • Jun 21 '19

Nice! That’s a neat service. Thanks for sharing!

anpos231 • Jun 21 '19

If I remember correctly, you can install tldr locally on your PC.

Valentin Baca • Jun 21 '19

You absolutely can: tldr.sh/#installation

anpos231 • Jun 21 '19 • Edited

WOW so many clients.
We've got to make even more :P

Sundeep • Jun 21 '19 • Edited

Nicely presented!

Suggestion: as a good practice, always use single quotes unless you need double quotes (you've mixed it up in the article)

With GNU grep, you have two options to control how to separate different matching groups: --no-group-separator and --group-separator (see my tutorial for details)

As pointed by others, ripgrep has --passthru option to show entire input file instead of using regex trick. You can also use -E '^|pattern' or -E 'pattern|$' instead of -e option. always color option is useful to highlight multiple terms in different color, see stackoverflow.com/questions/172360... for example

For those interested in detailed learning of GNU grep and ripgrep, I have a book on that: github.com/learnbyexample/learn_gn... (I had a free offer running, but that ended yesterday, bad timing)

Ryan Palo • Jun 21 '19

Very helpful, thanks! I appreciate the input from someone who literally wrote the book.

The —always tip is especially good, because I was not sure that there was ever a good reason for that option. The -e choice of syntax was deliberate, as I think it gives an ever so slight readability boost over the more opaque regex syntax for showing separate patterns, but showing folks that the extended regex flag is available is a good tip. Thanks again!

Alfredo • Jun 21 '19 • Edited

2nd Meta advice: grep is awfully slow, there are many replacement such as The Platinum Searcher or - my personal favourite - RipGrep. Both support almost the same options as grep, so switching from grep to one of those program will be easy.

James McPherson • Jun 21 '19

An even better way of "seeing the whole file" is to use cat... though if you only want a page or screenful at a time then less is definitely more.

Ryan Palo • Jun 21 '19

Does cat do search highlighting?

James McPherson • Jun 21 '19

No, because that's not its job. If you want search highlighting use a colourised grep. If you just want to see the whole file, use cat. Do one thing, do it well.

Ryan Palo • Jun 21 '19

Yeah cool, just checking. Thanks for clarifying. This post was mostly about different ways to see more of the file as you searched for specific things, but you are right that if all you want to do is see the contents of the file, cat is the best option for short files. I appreciate the addition!