Linux Interview Questions
Core Concepts
Q: What is a Linux process? Explain process states.
A process is an executing instance of a program. Each process has a PID (Process ID), PPID (Parent PID), memory space, file descriptors, and CPU state.
Process states:
•R (Running): Currently executing on CPU
•S (Sleeping, interruptible): Waiting for I/O, can be woken by signal
•D (Sleeping, uninterruptible): Waiting for I/O, cannot be interrupted (kernel I/O)
•Z (Zombie): Process terminated but parent hasn't called wait() yet
•T (Stopped): Stopped by signal (Ctrl+Z) or being debugged
bash
ps aux # All processes with resource usage
ps -ef # All processes in full format
top / htop # Real-time process monitoring
pstree # Process tree
kill -9 PID # SIGKILL (force kill)
kill -15 PID # SIGTERM (graceful termination)
kill -SIGSTOP PID # Pause process
kill -SIGCONT PID # Resume process
Q: Explain Linux file permissions.
bash
ls -la
# -rwxr-xr-- 1 alice dev 4096 Jan 1 10:00 script.sh
# |||||||||
# Type: - (file), d (dir), l (link), c (char device), b (block device)
# Owner: rwx (read write execute)
# Group: r-x (read execute)
# Others: r-- (read only)
chmod:
bash
chmod 755 script.sh # rwxr-xr-x (octal)
chmod u+x,g-w file # symbolic: add exec to user, remove write from group
chmod -R 644 /data/ # Recursive
chown:
bash
chown alice:dev file.txt # Change owner and group
chown -R www-data /var/www # Recursive
Special bits:
•SUID (4000): Run as file owner (e.g., /usr/bin/passwd runs as root)
•SGID (2000): Run as file group | New files inherit directory group
•Sticky bit (1000): Only file owner can delete in directory (e.g., /tmp)
bash
chmod u+s /usr/local/bin/myapp # Set SUID
ls -la → -rwsr-xr-x # s in user execute position = SUID
Q: What is a Linux filesystem? Explain key directories.
/ Root — everything starts here
├── bin/ Essential user binaries (ls, cp, mv, bash)
├── sbin/ System binaries (root only: iptables, fdisk, mount)
├── etc/ System configuration files (passwd, fstab, nginx/, ssh/)
├── var/ Variable data (logs /var/log, mail, spool, databases)
├── tmp/ Temporary files (cleared on reboot)
├── home/ User home directories (/home/alice, /home/bob)
├── root/ Root user's home
├── opt/ Optional software packages (non-OS apps)
├── usr/ User programs and data (large hierarchy)
│ ├── bin/ Non-essential user binaries
│ ├── local/ Locally installed software
│ └── lib/ Libraries
├── lib/ Shared libraries for /bin and /sbin
├── proc/ Virtual filesystem — running process info (/proc/cpuinfo, /proc/meminfo)
├── sys/ Virtual filesystem — kernel and device info
├── dev/ Device files (sda, null, zero, random, tty)
└── mnt/ /media/ Temporary mount points
Q: How do you diagnose high CPU, memory, and disk usage?
CPU:
bash
top # Real-time; press 'P' to sort by CPU
htop # Better interactive view
ps aux --sort=-%cpu | head # Top CPU consumers
mpstat -P ALL 1 # Per-core CPU stats
perf top # Performance profiling
# Check load average: uptime → 1.23 0.89 0.92 (1/5/15 min avg)
# Load > number of CPUs = overloaded
Memory:
bash
free -h # Memory overview
cat /proc/meminfo # Detailed memory
ps aux --sort=-%mem | head # Top memory consumers
vmstat 1 # Virtual memory stats (si/so = swap in/out)
# High swap usage = system under memory pressure
Disk:
bash
df -h # Disk space usage per filesystem
du -sh /var/log/* # Directory sizes
du -sh * | sort -rh | head # Top space consumers
iotop # Real-time I/O per process
iostat -x 1 # Disk I/O stats (await = average wait time)
lsof | grep deleted # Files deleted but still held open (space not freed)
Network:
bash
netstat -tuln # Listening ports
ss -tuln # Faster alternative to netstat
netstat -an | grep ESTABLISHED | wc -l # Active connections
tcpdump -i eth0 port 80 # Packet capture
iftop / nload # Real-time bandwidth usage
Q: Explain Linux boot process.
1.BIOS/UEFI: Power on → POST (Power-On Self Test) → finds boot device
2.MBR/GPT: Boot loader found in first sector (MBR) or EFI partition (UEFI)
3.GRUB (bootloader): Loads Linux kernel and initrd/initramfs into memory
4.Kernel: Decompresses, initialises hardware, mounts virtual filesystems (proc, sys, dev)
5.initramfs: Temporary root filesystem with drivers needed to mount real root
6.init/systemd (PID 1): First process. Manages all other services.
7.systemd targets: Boots to default target (multi-user.target or graphical.target)
8.Login prompt or GUI
Q: What is systemd? Key commands?
systemd is the init system (PID 1) and service manager for most modern Linux distros.
bash
# Service management
systemctl start nginx
systemctl stop nginx
systemctl restart nginx
systemctl reload nginx # Reload config without restart
systemctl status nginx # Show service status, recent logs
systemctl enable nginx # Start on boot
systemctl disable nginx # Remove from boot
systemctl is-active nginx # Check if running
# System management
systemctl list-units --type=service --state=running
systemctl list-failed # Failed services
systemctl daemon-reload # Reload systemd after unit file change
systemctl reboot / poweroff / halt
# Journald (logging)
journalctl -u nginx # Logs for nginx service
journalctl -f # Follow (like tail -f)
journalctl --since "1 hour ago"
journalctl -p err # Only error level and above
journalctl -n 100 # Last 100 lines
Q: What is a cron job? How do you set one up?
bash
crontab -e # Edit current user's crontab
crontab -l # List crontab entries
crontab -r # Remove all crontab entries
sudo crontab -e # Edit root's crontab
Cron syntax: minute hour day-of-month month day-of-week command
# Run at 2:30 AM daily
30 2 * * * /opt/scripts/backup.sh
# Run every 15 minutes
*/15 * * * * /opt/scripts/healthcheck.sh
# Run at 9 AM on weekdays (Mon-Fri)
0 9 * * 1-5 /opt/scripts/report.sh
# Run at midnight on 1st of every month
0 0 1 * * /opt/scripts/monthly-report.sh
# Redirect output
* * * * * /opt/scripts/job.sh >> /var/log/job.log 2>&1
Q: Explain common networking commands.
bash
# Connectivity
ping google.com # ICMP ping
traceroute google.com # Trace route
curl -I https://example.com # HTTP headers
wget https://example.com/file # Download file
nslookup example.com # DNS lookup
dig example.com # Detailed DNS lookup
host example.com # Simple DNS lookup
# Network config
ip addr show # IP addresses (replaces ifconfig)
ip route show # Routing table
ip link show # Network interfaces
ip addr add 192.168.1.100/24 dev eth0 # Add IP address
# Firewall (iptables)
iptables -L -n -v # List rules
iptables -A INPUT -p tcp --dport 80 -j ACCEPT
iptables -A INPUT -j DROP # Default drop
# Firewall (nftables / firewalld)
firewall-cmd --list-all
firewall-cmd --add-service=http --permanent
firewall-cmd --reload
# Port/connection info
ss -tuln # Listening ports
ss -tunp # Ports with process names
lsof -i :80 # What's using port 80
Q: Important shell scripting concepts.
bash
#!/bin/bash
set -euo pipefail # e: exit on error, u: error on undefined, o pipefail: pipe error
# Variables
NAME="world"
echo "Hello, ${NAME}!"
# If/else
if [ -f "/etc/nginx/nginx.conf" ]; then
echo "Nginx config found"
elif [ -d "/etc/nginx" ]; then
echo "Nginx dir exists but no config"
else
echo "Nginx not installed"
fi
# For loop
for file in /var/log/*.log; do
echo "Processing: $file"
gzip "$file"
done
# Functions
check_service() {
local service=$1
if systemctl is-active --quiet "$service"; then
echo "$service is running"
return 0
else
echo "$service is not running"
return 1
fi
}
check_service nginx
# Process substitution and pipes
active_users=$(who | wc -l)
echo "Active users: $active_users"
# Exit codes
command && echo "Success" || echo "Failed"
# Trap for cleanup
trap 'rm -f /tmp/tempfile; echo "Cleaned up"' EXIT INT TERM
Revision Notes
PROCESS STATES: R(running) S(sleep) D(uninterruptible) Z(zombie) T(stopped)
kill -15 (SIGTERM, graceful) | kill -9 (SIGKILL, force)
ps aux --sort=-%cpu | top | htop
PERMISSIONS: rwxrwxrwx (owner/group/others)
chmod 755 = rwxr-xr-x | 644 = rw-r--r-- | 777 = all access
SUID(4000): run as owner | SGID(2000): run as group | Sticky(1000): only owner deletes
KEY DIRECTORIES:
/etc: config | /var/log: logs | /proc: process info (virtual)
/tmp: temp (cleared on boot) | /opt: optional software
CPU DIAGNOSIS: top → load average | ps aux --sort=-%cpu | mpstat
MEMORY: free -h | vmstat (swap in/out) | /proc/meminfo
DISK: df -h (space) | du -sh * (sizes) | iostat -x (I/O stats) | iotop
SYSTEMD: systemctl start/stop/restart/status/enable/disable
journalctl -u service -f (follow logs)
CRON: min hour day month weekday command
*/5 * * * * (every 5 min) | 0 2 * * * (2 AM daily)
NETWORKING:
ip addr / ip route (modern) | ss -tuln (listening ports)
curl / wget / dig / traceroute
iptables -L -n -v / firewall-cmd
