Master Essential Linux Shell Commands for File Management, Monitoring, and Automation

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

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

Note: find -name searches by filename; -exec or xargs can process the results. Common options include -mtime for modification time, -type (e.g., f for files, d for directories), and -size for size filtering.

2. Find and delete log files older than 30 days and larger than 100 MB:

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

3. Batch unzip all .zip files in the current directory to a specified folder:

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

Note: for i in (command); do … done is a common loop format where i is a user‑defined variable.

4. Common sed commands (example file test.txt):

Remove the first character of each line: sed -i 's/^.{1}//' test.txt Add an a at the beginning of each line: sed 's/^/a/' test.txt Add an a at the end of each line: sed 's/$/a/' test.txt Insert a c after a specific line: sed '/pattern/a c' test.txt Insert a c before a specific line: sed '/pattern/i c' test.txt 5. Check if a directory exists; create it if not, otherwise print a message:

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

6. Monitor root partition usage; if usage reaches 90% or more, send an email alert:

# Print root partition usage percentage
usage=$(df -h | awk 'NR==2 {print $5}' | tr -d '%')
while sleep 5m; do
  if [ $usage -ge 90 ]; then
    echo "More than 90% disk space used on Linux, please check!" | mail -s "Disk Warning $usage%" [email protected]
  fi
done

7. Show the top 20 IP addresses by request count in an Nginx access log:

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

Note: sort orders lines, uniq -c counts duplicate lines.

8. Replace SELinux enforcing mode with disabled in /etc/selinux/config:

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
cat a.txt | sort -n  | awk 'END{print}'   # min

10. Use snmpwalk to retrieve data from a device with SNMP v2c: snmpwalk -v2c -c public 192.168.0.241 11. Replace lines ending with jk to end with yz in a file: sed -e 's/jk$/yz/g' b.txt 12. Capture network packets with tcpdump for a specific host and port: tcpdump -nn host 192.168.56.7 and port 80 13. List the 20 most frequently used commands from Bash history:

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

14. Find .log files created more than three days ago and delete them:

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

15. Move files larger than 100 KB from a directory to /tmp: find . -size +100k -exec mv {} /tmp ; 16. Configure a firewall to allow 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

Alternative rule using state matching:

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

17. Generate Nginx log statistics to find the top 10 IP addresses:

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