Essential Linux Shell Commands for Everyday System Administration

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

Find every file ending with .tar in the current directory and move it to /backup/:

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

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

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

Iterate over every .zip file and extract it to /data/www/img/:

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

Note: The for i in (command); do … done pattern is a common shell loop where i holds each filename.

3. 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 '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 containing the pattern wuguangke: sed '/wuguangke/ac' test.txt Insert a c before a specific line containing the pattern wuguangke: sed '/wuguangke/ic' test.txt Refer to the sed manual for additional commands.

4. 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

Explanation: ! -d tests for non‑existence; mkdir -p creates the directory hierarchy; echo prints a message.

5. Monitor root partition usage and email when ≥90%

Step 1 – Retrieve the usage percentage:

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

Step 2 – Periodically check and send an 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 Disk $i%" [email protected]
        fi
    done
 done

6. List top 20 IP addresses by Nginx access log

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

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

7. Disable SELinux enforcement

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

Another example replaces /tmp with /tmp/abc/ in test.txt:

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

8. Print maximum and minimum values from a file

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

Alternative using sed:

sed 's/ / /g' a.txt | sort -nr | sed -n '1p;$p'

9. Retrieve SNMP v2c data with snmpwalk

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 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. Show the 20 most frequently used commands

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

13. Delete *.log files older than 3 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

Alternative rule:

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

16. Nginx log statistics – 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

17. Replace directory paths in a file

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

Or in‑place editing:

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