Master Linux Regex: 11 Essential Commands to Extract System Information

Regular expressions (regex) are powerful patterns used to match and filter text streams, making it easy to extract the exact information you need from command‑line output.

1. Retrieve the IPv4 address from ifconfig output

ifconfig | head -n 2 | tail -1 | tr -s " " | cut -d" " -f3

2. Find the maximum partition usage percentage

df | tr -s " " | cut -d" " -f5

3. Get the username, UID and shell of the user with the highest UID

cat /etc/passwd | cut -d: -f1,3,7 | sort -nt: -k2 | tail -n 1

4. Display the permissions of /tmp

stat /tmp | head -n 4 | tail -n 1 | cut -c10-13

5. List all system users with their usernames and UIDs

cat /etc/passwd | cut -d: -f1,3 | egrep -v "[0-9]{4,}"

6. Show UID and default shell for users root , linuxmi and mi (placeholder A8 )

cat /etc/passwd | egrep "^(root|A8)" | cut -d: -f1,3

7. Use egrep to extract lines ending with a lowercase letter from /etc/rc.d/init.d/functions

echo /etc/rc.d/init.d/functions | egrep "[a-z]$"

8. Extract the directory name from a path using egrep

echo /etc/rc.d/init.d/functions | egrep "/.*/"

9. Count how many times each host IP logged in as root (output shown in image)

10. Express IP octet ranges with extended regular expressions

Match a single digit (0‑9): egrep "\<[0-9]\>" Match numbers 10‑99: egrep "\<1[0-9]\>" Match numbers 100‑199: egrep "\<1[0-9][0-9]\>" Match numbers 200‑249: egrep "\<2[0-4][0-9]\>" Match numbers 250‑255:

egrep "\<25[0-5]\>"

11. Show all IPv4 addresses from ifconfig output

ifconfig | egrep "[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}"

In short, regular expressions provide a concise way to describe patterns in text, enabling powerful search, extraction, and validation operations directly from the Linux shell.