CODEWITHSUNDEEP
linuxbashgrep
Alex
Alex

Reputation: 657

How to make grep to stop searching in each file after N lines?

It's best to describe the use by a hypothetical example:

Searching for some useful header info in a big collection of email storage (each email in a separate file). e.g. doing stats of top mail client apps used.

Normally if you do grep you can specify -m to stop at first match but let's say an email does not contact X-Mailer or whatever it is we are looking for in a header? It will scan through the whole email. Since most headers are <50 lines performance could be increased by telling grep to search only 50 lines on any file. I could not find a way to do that.

Upvotes: 2

Views: 1514

Answers (4)

Vijay
Vijay

Reputation: 67231

ls *.txt | xargs head -<N lines>| grep 'your_string'

Upvotes: 0

kev
kev

Reputation: 161684

Try this command:

for i in *
do
    head -n 50 $i | grep -H --label=$i pattern
done

output:

1.txt: aaaaaaaa pattern aaaaaaaa
2.txt: bbbb pattern bbbbb

Upvotes: 1

rici
rici

Reputation: 241741

I don't know if it would be faster but you could do this with awk:

 awk '/match me/{print;exit}FNR>50{exit}' *.mail

will print the first line matching match me if it appears in the first 50 lines. (If you wanted to print the filename as well, grep style, change print; to print FILENAME ":" $0;)

awk doesn't have any equivalent to grep's -r flag, but if you need to recursively scan directories, you can use find with -exec:

find /base/dir -iname '*.mail' \
     -exec awk '/match me/{print FILENAME ":" $0;exit}FNR>50{exit}' {} +

You could solve this problem by piping head -n50 through grep but that would undoubtedly be slower since you'd have to start two new processes (one head and one grep) for each file. You could do it with just one head and one grep but then you'd lose the ability to stop matching a file as soon as you find the magic line, and it would be awkward to label the lines with the filename.

Upvotes: 2

Avichal Badaya
Avichal Badaya

Reputation: 3629

you can do something like this 

head -50 <mailfile>| grep <your keyword>

Upvotes: 1

Related Questions