Master Linux Regex: 11 Essential Commands to Extract System Info

Regular expressions are patterns used to match each line of input, offering powerful text search capabilities.

1. Find the IPv4 address from the ifconfig output

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

2. Get the maximum partition usage percentage

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

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

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

4. Show the permissions of the /tmp directory

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 (A8 as placeholder)

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

7. 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 the same path using egrep

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

9. Count how many times each host IP logged in as root via the last command

10. Use extended regular expressions to match numeric ranges (0‑9, 10‑99, 100‑199, 200‑249, 250‑255)

echo {1..255} | egrep "\<[0-9]\>"

egrep "\<1[0-9]\>"

egrep "\<1[0-9][0-9]\>"

egrep "\<2[0-4][0-9]\>"

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

11. Display all IPv4 addresses from the ifconfig command

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 and process text patterns on Linux systems.