Linux comand: grep

Command-line utility to search lines matching regular expresions in plain-text data sets.

Option	Description	Example
-c	Count	grep -c 'root' /etc/passwd
-e	Specify multiple search patterns	grep -e 'root' -e 'system' /etc/passwd
-r	Recursive
-v	Show lines not matching the pattern
-i	Case insensitive
-n	Output line numbering
-E	Enable regex use (similar to egrep)
-o	Show only the match, not the full line
-f FILE	Specify file to search in.
-H	Print file name
-P	Perl Regex	grep -P '\d{16}'

Examples:

curl myip.is | grep -oE "\b([0-9]{1,3}.){3}[0-9]{1,3}\b" # grep IP Address
grep '\<a.*\>' archivo
cat archivo | grep "\<a.*\>"
grep "#" /boot/grub/menu.lst
grep -v "#" /boot/grub/menu.lst
grep -c "iface" /etc/network/interfaces
grep -e "root" -e "password" archivo
grep -n -e "root" -e "password" archivo
grep -r "password" *
ifconfig eth0 | grep -oiE '([0-9A-F]{2}:){5}[0-9A-F]{2}' # Show eth0 MAC address
grep -Eio '[a-z0-9._-]+@[a-z0-9.-]+[a-z]{2,4}' file.txt  # Extract e-main addresses from file.txt

Find files containing text pattern

grep -rnw '/path/' -e "pattern"

• -r or -R is recursive,
• -n is line number, and
• -w stands match the whole word.
• -l (lower-case L) can be added to just give the file name of matching files.

--exclude or --include parameter could be used for efficient searching. Something like below:
grep --include=\*.{c,h} -rnw '/path/to/somewhere/' -e "pattern"
grep --exclude=*.o -rnw '/path/to/somewhere/' -e "pattern"
grep --exclude-dir={dir1,dir2,*.dst} -rnw '/path/to/somewhere/' -e "pattern"

Remove comments and blank lines

cat /etc/apache2/sites-enabled/default-ssl.conf | grep -v '^$\|^\s*\#'

Grep with blanks

cat dig_output.txt | grep -E 'INblank:A'

Grep remove comments and blank lines

cat file.txt | grep -v ^\#|grep .