Master Essential Linux Commands: Find, Sed, Disk Monitoring, and More

1. Move all .tar files to a backup directory

Find files ending with .tar in the current directory and move them to ./backup/:

find . -name "*.tar" -exec mv {} ./backup/ ;

2. Delete large log files older than 30 days

Locate .log files modified more than 30 days ago and larger than 100 MB, then remove them:

find . -name "*.log" -mtime +30 -type f -size +100M | xargs rm -rf {} ;

3. Batch unzip all .zip files in the current directory

Iterate over each .zip file and extract it to a target directory:

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

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

Remove the first character of each line: sed -i 's/^\.//g' test.txt Add an a at the beginning of each line: sed -i 's/^/a/' test.txt Add an a at the end of each line: sed -i 's/$/a/' test.txt Append a c after a specific line (e.g., line containing "wuguangke"): sed '/wuguangke/a c' test.txt Insert a c before a specific line:

sed '/wuguangke/i c' test.txt

5. Check if a directory exists, create it if not

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

6. Monitor root partition usage and alert when ≥90%

Print the usage percentage and send an email warning if it exceeds 90%:

while sleep 5m; do
    for i in $(df -h | awk 'NR>1 {print $5}' | sed 's/%//g'); do
        if [ $i -ge 90 ]; then
            echo "Root partition usage is $i% – please check!" | mail -s "Disk Space Warning $i%" [email protected]
        fi
    done
 done

7. List the top 20 IP addresses in an Nginx access log

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

8. Disable SELinux enforcing mode

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

9. Print the maximum and minimum values from a file

cat a.txt | sort -nr | awk 'NR==1{print "max="$0} END{print "min="$0}'

10. Retrieve SNMP data (v2c) from a host

snmpwalk -v2c -c public 192.168.0.241

11. Replace lines ending with jk by yz

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

12. 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 from a given host:

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

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

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

14. Delete .log files created more than 3 days ago

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

15. Move files larger than 100 KB to /tmp

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

16. Simple firewall script allowing only port 80

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

17. Nginx log aggregation and top‑10 IP statistics

Merge split logs, then count the most frequent IPs:

cd /home/logs/nginx/default
sort -m -k 4 -o access.log.ok access.1 access.2 access.3 ...
cat access.log.ok | awk '{print $1}' | sort -n | uniq -c | sort -nr | head -10

18. Replace a directory path in a file using sed

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