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

Examples:

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