17 Must-Know Linux Ops Tricks for Sysadmins: From Log Analysis to Disk Monitoring

This guide shares 17 essential Linux command-line techniques that every operations engineer should master. Each tip includes a concrete command example and a brief annotation explaining the key flags and logic.

1. Find and move .tar files to a backup directory

Use find with -name and -exec to locate all .tar files in the current directory and move them to ./backup/.

find . -name “*.tar” -exec mv {} ./backup/ \;

Annotation: find -name matches filenames; -exec or xargs passes results to a subsequent command. Extensions: -mtime for modification time, -type for file type (f=file, d=directory), -size for size. Example: delete .log files older than 30 days and larger than 100 MB:

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

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

A for loop iterates over find results and extracts each archive with unzip -d.

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

Annotation: for i in (command); do ... done is a common loop pattern; the variable name is arbitrary.

3. Common sed one-liners (tested on test.txt)

Remove leading dot from each line: sed -i ‘s/^.//g’ test.txt Prepend 'a' to every line: sed ‘s/^/a/g’ test.txt Append 'a' to every line: sed ‘s/$/a/’ test.txt Insert 'c' after lines matching 'wuguangke': sed ‘/wuguangke/ac’ test.txt Insert 'c' before lines matching 'wuguangke': sed ‘/wuguangke/ic’ test.txt Refer to sed documentation for more commands.

4. Check if a directory exists; create it if missing, else print a message

if
[! –d /data/backup/];
then
Mkdir–p /data/backup/
else
echo “The Directory alreadyexists,please exit”
fi

Annotation: if...;then ...else ..fi is the conditional structure; ! negates the test, -d checks for a directory.

5. Monitor root filesystem usage; email alert when usage ≥ 90%

Step 1: Extract the root partition usage percentage:

df -h |sed -n ‘//$/p’|awk ‘{print $5}’|awk –F “%” ‘{print $1}’

Annotation: awk '{print $5}' prints the 5th field; -F</u201d%” splits on '%' to drop the percent sign.

Step 2: Loop every 5 minutes, check each filesystem, and send mail if usage ≥ 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 of disk space ,Please LinuxSA Check Linux Disk !” |mail -s “Warn Linux / Parts is $i%” [email protected]
fi
done
done

6. Top 20 client IPs from Nginx access log

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

Annotation: sort orders lines; uniq -c counts duplicate lines.

7. sed pattern: match a line and replace a parameter on that line

sed -i ‘/SELINUX/s/enforcing/disabled/’ /etc/selinux/config

Alternative delimiter (colon) to avoid escaping slashes: sed -i ‘s:/tmp:/tmp/abc/:g’ test.txt replaces /tmp with /tmp/abc/.

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

First attempt (flawed):

cat a.txt |sort -nr|awk ‘{}END{print} NR==1’
cat a.txt |sort -nr |awk ‘END{print} NR==1’

Correct approach using sed to normalize spaces, then sort -nr and print first and last lines:

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

9. SNMP v2c walk for Cacti data collection

snmpwalk -v2c -c public 192.168.0.241

10. Replace lines ending with 'jk' to 'yz'

sed -e ‘s/jk$/yz/g’ b.txt

11. Network packet capture with tcpdump

Capture packets from host 192.168.56.7 on port 80: tcpdump -nn host 192.168.56.7 and port 80 Exclude host 192.168.0.22 on port 80:

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

Reference: TCP/IP 7-layer model (Physical – Data Link – Network – Transport – Session – Presentation – Application).

12. Show the 20 most frequently used commands from history

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

13. Script: delete *.log files older than 3 days

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

14. Script: move files larger than 100k to /tmp

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

15. Firewall script: allow only remote access to port 80

Option A (flush and set default reject):

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

Option B (stateful rule for new connections on port 80):

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

16. Nginx log analysis: top 10 client IPs (log path: /home/logs/nginx/default/access.log)

cd /home/logs.nginx/default
sort -m -k 4 -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

Using colon delimiter: sed ‘s:/user/local:/tmp:g’ test.txt Or with escaped slashes:

sed -i ‘s//usr/local//tmp/g’ test.txt

Source: 浩道linux