Essential Linux Regex Commands for Quick System Queries

This guide presents a series of practical Linux one‑liners that leverage regular expressions to retrieve common system information.

1. Extract the IPv4 address from ifconfig

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 default shell of the user with the highest UID

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

4. Show permissions of /tmp

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

5. List all system users with their UID

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

6. Display UID and default shell for specific users (root, linuxmi, mi)

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 login occurrences per host IP for root in last output

(Command omitted in source; image only illustrates the result.)

10. Represent numeric ranges with extended regular expressions

0‑9: egrep "<[0-9]>" 10‑99: egrep "<1[0-9]>" 100‑199: egrep "<1[0-9][0-9]>" 200‑249: egrep "<2[0-4][0-9]>" 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 summary, regular expressions provide a powerful, concise way to filter and extract specific text patterns from Linux command outputs, enabling quick system diagnostics and data collection.