Top Linux Ops Interview Questions with Ready‑to‑Use Scripts

31. Commonly used Nginx modules and their purposes

rewrite – URL rewriting

access – source control

ssl – encryption

ngx_http_gzip_module – compression

ngx_http_proxy_module – proxy

ngx_http_upstream_module – backend server list

ngx_cache_purge – cache purge

32. Typical web‑server load‑balancing architectures

Nginx

HAProxy

Keepalived

LVS

33. Checking HTTP concurrent requests and TCP connection states

netstat -n | awk '/^tcp/ {++S[$NF]} END {for(a in S) print a, S[a]}'

Use ulimit -n to view the maximum open file descriptors (default 1024). Increase limits by editing /etc/security/limits.conf:

soft nofile 10240
hard nofile 10240

Reboot for changes to take effect.

34. Find the IP address that accessed port 80 most frequently

tcpdump -i eth0 -tnn dst port 80 -c 1000 | awk -F"." '{print $1"."$2"."$3"."$4}' | sort | uniq -c | sort -nr | head -20

35. Bash script to list online IPs in 192.168.1.0/24

for ip in $(seq 1 255); do
  ping -c 1 192.168.1.$ip > /dev/null 2>&1
  if [ $? -eq 0 ]; then
    echo 192.168.1.$ip UP
  else
    echo 192.168.1.$ip DOWN
  fi
done & wait

36. Keep only the latest 7 days of Apache logs in /app/logs

One approach is to delete files older than seven days:

find /app/logs -type f -mtime +7 -exec rm -f {} \;

37. General Linux system hardening tips

Create non‑root users and grant sudo privileges

Change default SSH port and disable root remote login

Enable automatic time synchronization

Use domestic yum mirrors

Disable SELinux and iptables (unless external access is needed)

Increase file descriptor limits

Trim unnecessary startup services (crond, rsyslog, network, sshd)

Tune kernel parameters via /etc/sysctl.conf Prefer English locale to avoid encoding issues

Lock critical system files

Clear /etc/issue to hide OS version on login screen

38. Extract the IP address of eth0 using cut (alternatives with awk or sed )

# cut method
ifconfig eth0 | sed -n '2p' | cut -d ':' -f2 | cut -d ' ' -f1

# awk method 1
ifconfig eth0 | awk 'NR==2' | awk -F ':' '{print $2}' | awk '{print $1}'

# awk with multiple delimiters
ifconfig eth0 | awk 'NR==2' | awk -F '[: ]+' '{print $4}'

# sed method
ifconfig eth0 | sed -n '/inet addr/p' | sed -r 's#^.*addr:\([^ ]*\).*#\1#'

39. SecureCRT shortcut keys and their functions

Ctrl +a – move cursor to line start

Ctrl +e – move cursor to line end

Ctrl +c – terminate current program

Ctrl +d – delete character under cursor or exit if at end

Ctrl +l – clear screen

Ctrl +u – cut text before cursor

Ctrl +k – cut text after cursor

Ctrl +y – paste previously cut text

Ctrl +r – search command history

Tab – command or path completion

Ctrl +Shift +c – copy

Ctrl +Shift +v – paste

40. Cron job to backup /var/www/html nightly with timestamped archive

# a.sh
#!/bin/bash
cd /var/www/ && /bin/tar zcf /data/html-$(date +%m-%d%H).tar.gz html/

# crontab entry
0 0 * * * /bin/sh /root/a.sh