Searching for text using grep

grep – A utility for searching for specified text in files, file names, etc. from the command line.

You can search using grep, even for example in the output of some command:

COMMAND | grep TEXT

You can calculate the number of lines where the specified text occurs:

grep -c TEXT FILE
grep TEXT FILE | wc -l

To search and display the lines with the specified text, execute:

grep TEXT FILE

Search all the files in the specified directory:

grep TEXT /DIRECTORY/*
grep -r TEXT /DIRECTORY/*

Search only for the whole specified word:

grep -w WORD FILE

Display the lines where the specified words occur:

grep -E "(WORD|WORD|WORD)" FILE

Display lines where the specified words occur several times:

grep -E "(WORD|WORD).*\1"

Display also the lines after the search, for example 5 lines:

grep -A 5 WORD FILE

Or the lines before the search:

grep -B 5 WORD FILE

Show lines where the WORD1, WORD2, WORD3 occurs:

grep WORD[1-3] FILE

Show all lines where the IP address occurs:

grep -E "[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}" FILE

Show lines where capital letters occur:

grep "[[:upper:]]" FILE

Show lines where the specified word appears with a tab on both sides:

grep "[[:blank:]]WORD[[:blank:]]" FILE
grep " WORD " FILE

Find and replace the text with another command:

grep 'the_text_we_are_looking_for' -P -R -I -l  * | xargs sed -i 's/the_text_we_are_looking_for/the text to which we replace/g'

I’ll describe some startup options:
-b (displaying the block number in which the fragment was found, the blocks are numbered from 0)
-c (display only the number of rows in which the fragment was found)
-r (search also in subdirectories)
-n (displays line number)
-h (hides the filenames before the found fragments, when the search is performed on several files)
-l (display only file names where the fragment occurs)
-i (ignoring the case of characters, that is, the big and small letters are equal when searching)
-s (hides messages about nonexistent or inaccessible files)
-v (displays all lines except the specified sample)

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