Essential Linux Command Cheat Sheet for File Management, Monitoring, and Automation

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

Use find . -name "*.tar" -exec mv {} ./backup/ \; to locate every file ending with .tar in the current directory tree and move it to ./backup/.

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

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 's/^\.//g' test.txt Insert an a at the beginning of each line: sed -i 's/^/a/' test.txt Append an a at the end of each line: sed -i 's/$/a/' test.txt Add a c after a specific line (example pattern wuguangke): sed -i '/wuguangke/a c' test.txt Prepend a c before a specific line:

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

4. Test for a directory and create it if missing

Script snippet:

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 %

Print the usage percentage:

df -h | sed -n '/\/$/p' | awk '{print $5}' | awk -F'%' '{print $1}'

Continuously check and alert:

while sleep 5m; do
  for i in $(df -h | sed -n '/\/$/p' | awk '{print $5}' | sed 's/%//g'); do
    echo $i
    if [ $i -ge 90 ]; then
      echo "More than 90% Linux disk space, please check!" |
        mail -s "Warn Linux / Parts is $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. Replace SELinux mode in /etc/selinux/config

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

8. Print the maximum and minimum values in a numeric file

Using sort and awk:

cat a.txt | sort -nr | awk 'NR==1{print}'   # maximum
cat a.txt | sort -n  | awk 'END{print}'   # minimum

9. Retrieve SNMP v2c data with snmpwalk

snmpwalk -v2c -c public 192.168.0.241

10. Change file extensions ending with jk to 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 particular host/port:

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

12. Show the 20 most frequently used shell commands

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 to allow only remote TCP traffic on port 80

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

Alternative rule using state tracking:

iptables -A INPUT -m state --state NEW -p tcp --dport 80 -j ACCEPT

16. Nginx log analysis to list the top 10 IP addresses

cd /home/logs/nginx/default
sort -m -k4 -o access.logok access.1 access.2 access.3 ...
cat access.logok | awk '{print $1}' | sort | uniq -c | sort -nr | head -10

17. Replace directory paths inside a file

sed 's:/user/local:/tmp:g' test.txt

These snippets provide a quick reference for everyday Linux system administration, file handling, text processing, monitoring, and security configuration.