Grep

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

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

Color Filtered Output

grep "Gecko" /data --color=always

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

Search Email addresses using regex

grep -E -o "\b[A-Za-z0-9._%+-]+@[A-Za-z0-9.-]+\.[A-Za-z]{2,6}\b" filename.txt

Grep from compressed files

zgrep -I "free space" ./messages*

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