20 Essential Linux Command-Line Tricks Every Sysadmin Should Know

Linux Knowledge Pocket

This article gathers a set of handy Linux command‑line tricks that can speed up everyday system administration work.

1. Quickly empty a file

cat /dev/null > haodao.txt

2. Count unique IP addresses in a Tomcat access log

awk '{print $1}' access.log | sort | uniq | wc -l

3. Generate a large test file

Use dd to create a file of a specific size for disk‑read/write testing. dd if=/dev/zero of=test.txt bs=1M count=1000 The command above creates test.txt with a size of 1 GB.

4. Create many files with similar names

touch haodao{1..100}.txt

5. Find and delete all .txt files in the current directory

find . -name "*.txt" -exec rm -rf {} \;

Alternative forms:

find . -name "*.txt" | xargs rm -rf

rm -rf $(find . -name "*.txt")

6. Delete .txt files older than 7 days

find . -mtime +7 -name "*.txt" | xargs rm -rf

7. Move files larger than 1000 M to /root/home

find . -size +1000M -exec mv {} /root/home \;

8. Delete .txt files older than 30 days and larger than 1000 M

find . -name "*.txt" -mtime +30 -type f -size +1000M | xargs rm -rf

9. Show the 10 most frequently used commands from history

cat /root/.bash_history | grep -v ^# | awk '{print $1}' | sort | uniq -c | sort -nr | head -10

10. Count IP accesses to Tomcat on a specific date and hour (e.g., 2021‑12‑11 09:00)

awk '{print $4,$1}' access.log | grep '11/Dec/2021:09' | awk '{print $2}' | sort | uniq | wc -l

Providing the log file allows the calculation; the same method works for any timestamp.

11. List the top 10 IP addresses by request count

cat access.log | awk '{print $1}' | sort | uniq -c | sort -nr | head -10

12. Count how many times a specific page was accessed

grep "/portal/index.html" access.log | wc -l

13. Show which pages a particular IP accessed

grep ^192.168.20.100 access.log | awk '{print $1,$7}'

14. Use tcpdump to inspect traffic on port 8080

tcpdump -i ens120 -tnn dst port 8080 -c 1000 | awk -F "." '{print $1"."$2"."$3"."$4}' | sort | uniq -c | sort -nr | head -10

15. History command shortcuts

!!

– repeat the previous command. !N – repeat command number N from history. !awk – repeat the most recent command starting with awk. !$ – use the last argument of the previous command.

Example: mv !$ !$ .bak renames test.txt to test.txt.bak.

16. Replace all occurrences of "haodao" with "HAODAO" in test.txt

sed -i "s/haodao/HAODAO/g" test.txt

17. Replace a directory path in haodao.txt

sed -i "s:/etc/dhcp:/home:g" haodao.txt

18. Common sed tricks on haodao.txt

Remove leading # characters: sed -i "s/^#//g" haodao.txt Add the word linux at the beginning of each line: sed -i "s/^/linux/g" haodao.txt Append study at the end of each line: sed -i "s/$/study/" haodao.txt After a line containing we love, insert you: sed -i "/we love/a you" haodao.txt Before a line containing haodao, insert where:

sed -i "/haodao/i where" haodao.txt

19. List network interfaces that are up

ip addr | awk -F ':' '/state UP/ {print $2}'

20. Show IP addresses and their corresponding interfaces

ip -o addr | awk '/inet/ {print $2,$4}'

Conclusion

The commands above are simple yet powerful tricks that many developers overlook; mastering them can greatly improve daily productivity.