Basics

Match regular expression in files

grep "lines.*empty" demo_file

Character	Function
?	The preceding item is optional and matched at most once.
*	The preceding item will be matched zero or more times.
+	The preceding item will be matched one or more times.
{n}	The preceding item is matched exactly n times.
{n,}	The preceding item is matched n or more times.
{,m}	The preceding item is matched at most m times.
{n,m}	The preceding item is matched at least n times, but not more than m times.

One-Liners

Task	Command
Search for the given string in a single file	grep "this" demo_file
Checking for the given string in multiple files	grep "this" demo_*
Case insensitive search	grep -i "the" demo_file
Checking for full words, not for sub-strings	grep -iw "is" demo_file
Display N lines after match	grep -A 3 -i "example" demo_text
Display N lines before match	grep -B 2 "single WORD" demo_text
Display N lines around match	grep -C 2 "Example" demo_text
Highlighting the search using GREP_OPTIONS	export GREP_OPTIONS='--color=auto' GREP_COLOR='100;8'
Searching in all files recursively	grep -r "ramesh" *
Invert match	grep -v "go" demo_text
display the lines which does not matches all the given pattern	grep -v -e "a" -e "b" -e "c" test-file.txt
How many lines matches the given pattern	grep -c "go" demo_text grep -c this demo_file
How many lines that does not match the pattern	grep -v -c this demo_file
Display only the file names which matches the given pattern	grep -l this demo_*
Show only the matched string	grep -o "is.*line" demo_file
Show line number while displaying the output	grep -n "go" demo_text
Filter comments from a config file	grep -vE '^#\|^;\|^$' server.conf
Search for "virus" in all files in a dir	grep virus /etc/snort/rules/*
Search this or that using Extended Regex	grep -E '(then\|there)' demo_text
Search this or that without Extended Regex	grep '$then\\|there$' demo_text
Search this or that grouping not necessary	grep 'then\\|there' demo_text
Search this or that grouping required	grep 'the$n\\|re$' demo_text

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