Master Essential Linux Commands and Regex Tricks for System Operations

Regular expressions are patterns used to match each line of input, providing powerful character search and convenient filtering of desired content.

Linux system: CentOS Linux release 8.1.1911 (Core)

1. Find the IPv4 address of the local machine from the ifconfig output

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

2. Get the maximum partition usage percentage

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

3. Show 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. Display 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 "[0-9]{4,}"

6. Show UID and default shell for users root, linuxmi, and mi (replace mi with 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. Use egrep to extract directory names from the same path

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

9. Count login attempts per host IP for root users from the last command

10. Use extended regular expressions to match numeric ranges

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. Show all IPv4 addresses from the 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 and process text patterns.

Source: https://www.linuxmi.com/linux-zhengzebiaodashi.html