Grep-command

Grep is an acronym that stands for Global Regular Expression Print.

Grep is a Linux / Unix command-line tool used to search for a string of characters in a specified file/files. 
The text search pattern is called a regular expression. When it finds a match, it prints the line with the result. The grep command is handy when searching through large log files.

NOTE : It is case-sensitive by default
Syntax

grep [pattern] [file]

Example 1 : Search a string in a file, error.log

grep ERROR error.log

Example 2 : Search a complete word

grep -w ERROR error.log

Example 3 : It is case-sensitive by default, to search a string as case-insenstive 

grep -i error error.log

Example 4 : Search a string in complete folder

grep ERROR path-to-folder/*

Example 5 : Search a string in all txt files

grep ERROR path-to-folder/*.txy

Example 6 : Search a string in all files starting with specific characters

grep ERROR path-to-folder/2020-10-10-*.logs

Example 7 : Limit the output with max-count

grep -m10 ERROR path-to-folder/*

Example 8 : Sometimes you need to look some lines before your search

grep -B 10 ERROR path-to-folder/*
result will include 10 lines previous to search

Example 9 : Sometimes you need to look some lines after your search

grep -A 10 ERROR path-to-folder/*
result will include 10 lines lines to search

Example 10 : Nested grep

grep ERROR path-to-folder/* | grep TimeOut

Example 11 : Include subdirectories 

grep -r ERROR path-to-folder/*

Example 12 : Inverse grep, you can search what do not match with search 

grep -v ERROR path-to-folder/*

Example 13 : Search a complete line

grep -x "Exception at line 100" path-to-folder/*

Example 14 : List the names of matching files

grep -l ERROR path-to-folder/*

Example 15 : Count the number of matches

grep -c ERROR path-to-folder/*