Essential Linux Command-Line Tricks for System Administration

1. Move all *.tar files to a backup directory

Use find . -name "*.tar" -exec mv {} ./backup/ \; to locate every .tar archive in the current tree and move it to ./backup/. The -exec action processes each match immediately.

2. Batch unzip all *.zip files to a target folder

Iterate over zip files with a for loop:

for i in `find . -name "*.zip" -type f`
do
  unzip -d $i /data/www/img/
 done

3. Common sed one‑liners (example file: test.txt)

Remove a leading dot: sed -i '/^\./d' test.txt Insert an a at the start of each line: sed -i '/^/a a' test.txt Append an a at the end of each line: sed -i '$a a' test.txt After a line containing wuguangke, add c: sed -i '/wuguangke/a c' test.txt Replace wuguangke with c on the same line:

sed -i 's/wuguangke/c/' test.txt

4. Test whether a directory exists

if [ ! -d /data/backup/ ]; then
  mkdir -p /data/backup/
else
  echo "The Directory already exists, please exit"
fi

5. Monitor root partition usage and send an email when it exceeds 90 %

Extract the usage percentage: df -h | awk 'NR==2 {gsub(/%/, ""); print $5}' Continuously check every 5 minutes and mail when the threshold is crossed:

while sleep 5m
do
  for i in `df -h | awk 'NR==2 {gsub(/%/, ""); print $5}'`
  do
    if [ $i -ge 90 ]; then
      echo "More than 90% disk space used" | mail -s "Disk warning $i%" [email protected]
    fi
  done
done

6. List the top 20 IP addresses from an Nginx access log

cat access.log | awk '{print $1}' | sort | uniq -c | sort -nr | head -20

7. Disable SELinux enforcing mode with sed

sed -i '/SELINUX/s/enforcing/disabled/' /etc/selinux/config

8. Print the maximum and minimum values in a file

Using sort and awk:

cat a.txt | sort -nr | awk 'END{print}'   # maximum
cat a.txt | sort -n  | awk 'END{print}'   # minimum

9. Retrieve SNMP data with version 2c

snmpwalk -v2c -c public 192.168.0.241

10. Replace lines ending with jk by yz

sed -e 's/jk$/yz/g' b.txt

11. Capture network traffic with tcpdump

Capture HTTP traffic from a specific host: tcpdump -nn host 192.168.56.7 and port 80 Capture all traffic except a host:

tcpdump -nn host 192.168.56.7 or ! host 192.168.0.22 and port 80

12. Show the 20 most frequently used commands from Bash history

cat .bash_history | grep -v '^#' | awk '{print $1}' | sort | uniq -c | sort -nr | head -20

13. Delete *.log files older than three days

find . -mtime +3 -name "*.log" | xargs rm -rf

14. Move files larger than 100 KB to /tmp

find . -size +100k -exec mv {} /tmp \;

15. Simple firewall script allowing only remote access to port 80

iptables -F
iptables -X
iptables -A INPUT -p tcp --dport 80 -j ACCEPT
iptables -A INPUT -p tcp -j REJECT

16. Nginx log statistics – top 10 IPs

cd /home/logs/nginx/default
cat access.log | awk '{print $1}' | sort | uniq -c | sort -nr | head -10

17. Replace a directory path inside a file

sed -i 's:/usr/local:/tmp:g' test.txt