Master Essential Linux Commands: Find, Sed, Iptables, and More

1. Find all files ending with .tar in the current directory and move them to a backup folder: 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 to specify file or directory, and -size for size filtering, e.g., find log files older than 30 days larger than 100 M and delete them.

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

2. Batch unzip all .zip files in the current directory to a target directory:

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

Note: The for i in $(command); do … done loop iterates over command output.

3. Common sed commands (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 's/^/a/g' test.txt
# Add an "a" at the end of each line
sed 's/$/a/' test.txt
# Append a "c" after a specific line
sed '/wuguangke/a c' test.txt
# Insert a "c" before a specific line
sed '/wuguangke/i c' test.txt

Refer to the sed documentation for more patterns.

4. 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

5. Monitor the root partition; if usage reaches ≥ 90 % send an email alert:

# Print root partition usage percentage
df -h | sed -n '/\/$/p' | awk '{print $5}' | awk -F '%' '{print $1}'
# Loop to check every 5 minutes and send mail when >90%
while sleep 5m; do
  for i in `df -h | sed -n '/\/$/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 Disk $i%" [email protected]
    fi
  done
done

6. List the top 20 IP addresses by request count from an Nginx access log:

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

7. Replace a line in /etc/selinux/config to disable SELinux enforcement:

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

8. Print the maximum and minimum values from a file:

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

9. Retrieve SNMP v2c data using snmpwalk: snmpwalk -v2c -c public 192.168.0.241 10. Replace lines ending with jk to yz in b.txt: sed -e 's/jk$/yz/g' b.txt 11. Capture network packets with tcpdump:

# Capture traffic to host 192.168.56.7 on port 80
tcpdump -nn host 192.168.56.7 and port 80
# Capture all traffic except host 192.168.0.22 on port 80
tcpdump -nn host 192.168.56.7 or ! host 192.168.0.22 and port 80

12. Configure SNMP community name on H3C devices (set version first, then community):

snmp-agent sys-info version v1 v2c
snmp-agent community read public

13. 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

14. Find *.log files created more than 3 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. 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

Alternative rule using state tracking:

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

17. Nginx log statistics to get the top 10 IPs:

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

18. Replace a directory path in a file using sed:

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