Junior System Administrator Interview Questions: Complete Guide

# File and Directory Management
ls -la          # List files with details
cd /path        # Change directory
pwd             # Print working directory
mkdir folder    # Create directory
rm -rf folder   # Remove directory
cp source dest  # Copy files
mv old new      # Move/rename files

# File Viewing and Editing
cat file.txt    # View file contents
less file.txt   # View file with pagination
tail -f /var/log/syslog  # Follow log file
vi/nano file.txt  # Edit files

# Permissions
chmod 755 file  # Change permissions
chown user:group file  # Change ownership
ls -l           # View permissions

# Process Management
ps aux          # List all processes
top/htop        # Monitor processes
kill PID        # Terminate process
systemctl status service  # Check service status

# System Information
df -h           # Disk usage
free -h         # Memory usage
uptime          # System uptime
uname -a        # System information

top
# Look for processes using high CPU
# Press 'P' to sort by CPU usage

# Or use htop for better visualization
htop

free -h
# Check if swap is being used heavily

# Find memory-hungry processes
ps aux --sort=-%mem | head -10

iostat -x 1
# Look for high %util

# Check disk space
df -h

# Find large files
du -sh /* | sort -rh | head -10

netstat -tuln  # Active connections
ss -s          # Socket statistics

tail -f /var/log/syslog
journalctl -xe  # Systemd logs
dmesg | tail    # Kernel messages

# Edit crontab
crontab -e

# Run backup every day at 2 AM
0 2 * * * /home/user/backup.sh

# Run script every Monday at 9 AM
0 9 * * 1 /home/user/weekly-report.sh

# Run every 15 minutes
*/15 * * * * /home/user/check-status.sh

# Run on first day of every month
0 0 1 * * /home/user/monthly-cleanup.sh

# Run every weekday at 6 PM
0 18 * * 1-5 /home/user/end-of-day.sh

# List current cron jobs
crontab -l

# Remove all cron jobs
crontab -r

@reboot    # Run once at startup
@daily     # Run once a day (0 0 * * *)
@hourly    # Run once an hour (0 * * * *)
@weekly    # Run once a week (0 0 * * 0)
@monthly   # Run once a month (0 0 1 * *)

# Example
@daily /home/user/daily-backup.sh

# Redirect output to log file
0 2 * * * /home/user/backup.sh >> /var/log/backup.log 2>&1

# Send email on errors only
0 2 * * * /home/user/backup.sh 2>&1 | mail -s "Backup Failed" admin@company.com

# Create scheduled task
$action = New-ScheduledTaskAction -Execute "PowerShell.exe" -Argument "-File C:\Scripts\backup.ps1"
$trigger = New-ScheduledTaskTrigger -Daily -At 2am
Register-ScheduledTask -TaskName "Daily Backup" -Action $action -Trigger $trigger

# List scheduled tasks
Get-ScheduledTask

# Run task immediately
Start-ScheduledTask -TaskName "Daily Backup"

# Disable task
Disable-ScheduledTask -TaskName "Daily Backup"

# Create user
New-ADUser -Name "John Doe" -SamAccountName jdoe -UserPrincipalName jdoe@domain.com

# Add user to group
Add-ADGroupMember -Identity "IT Staff" -Members jdoe

# Reset password
Set-ADAccountPassword -Identity jdoe -Reset

# Find locked accounts
Search-ADAccount -LockedOut

# List domain controllers
Get-ADDomainController -Filter *

# Create user
sudo useradd -m -s /bin/bash john
# -m: create home directory
# -s: set shell

# Set password
sudo passwd john

# Create user with specific UID and groups
sudo useradd -m -u 1500 -G sudo,developers john

# Modify existing user
sudo usermod -aG docker john  # Add to group
sudo usermod -s /bin/zsh john  # Change shell
sudo usermod -L john  # Lock account
sudo usermod -U john  # Unlock account

# Delete user
sudo userdel john  # Keep home directory
sudo userdel -r john  # Remove home directory

# View user info
id john
finger john
cat /etc/passwd | grep john

# Create group
sudo groupadd developers

# Add user to group
sudo usermod -aG developers john

# Remove user from group
sudo gpasswd -d john developers

# Delete group
sudo groupdel developers

# List user's groups
groups john

# Change ownership
sudo chown john:developers /var/www/project
sudo chown -R john:developers /var/www/project  # Recursive

# Change permissions
chmod 755 script.sh  # rwxr-xr-x
chmod u+x script.sh  # Add execute for user
chmod g-w file.txt  # Remove write for group
chmod o-r file.txt  # Remove read for others

# Special permissions
chmod 4755 file  # SUID
chmod 2755 dir   # SGID
chmod 1777 /tmp  # Sticky bit

# Create local user
New-LocalUser -Name "John" -Password (ConvertTo-SecureString "P@ssw0rd" -AsPlainText -Force)

# Create AD user
New-ADUser -Name "John Doe" `
    -GivenName "John" `
    -Surname "Doe" `
    -SamAccountName "jdoe" `
    -UserPrincipalName "jdoe@company.com" `
    -Path "OU=Users,DC=company,DC=com" `
    -AccountPassword (ConvertTo-SecureString "P@ssw0rd" -AsPlainText -Force) `
    -Enabled $true

# Add to group
Add-ADGroupMember -Identity "IT Staff" -Members jdoe

# Modify user
Set-ADUser -Identity jdoe -Department "IT" -Title "System Administrator"

# Disable account
Disable-ADAccount -Identity jdoe

# Reset password
Set-ADAccountPassword -Identity jdoe -Reset -NewPassword (ConvertTo-SecureString "NewP@ss" -AsPlainText -Force)

# Remove user
Remove-ADUser -Identity jdoe

#!/bin/bash
# Create multiple users from file
while IFS=, read -r username fullname; do
    sudo useradd -m -c "$fullname" "$username"
    echo "$username:TempPass123" | sudo chpasswd
    sudo chage -d 0 "$username"  # Force password change
    echo "Created user: $username"
done < users.csv

# Test DNS resolution
nslookup google.com

# Query specific DNS server
dig @8.8.8.8 google.com

# Check DNS cache (Windows)
ipconfig /displaydns
ipconfig /flushdns

# Check DNS cache (Linux)
systemd-resolve --statistics
systemd-resolve --flush-caches

# Release and renew IP (Windows)
ipconfig /release
ipconfig /renew

# Release and renew IP (Linux)
sudo dhclient -r
sudo dhclient

# Check DHCP lease
cat /var/lib/dhcp/dhclient.leases

#!/bin/bash
# Simple backup script

BACKUP_DIR="/backup"
SOURCE_DIR="/var/www"
DATE=$(date +%Y%m%d_%H%M%S)
BACKUP_FILE="backup_$DATE.tar.gz"

# Create backup
tar -czf $BACKUP_DIR/$BACKUP_FILE $SOURCE_DIR

# Keep only last 7 days
find $BACKUP_DIR -name "backup_*.tar.gz" -mtime +7 -delete

# Upload to remote server (optional)
rsync -avz $BACKUP_DIR/$BACKUP_FILE user@backup-server:/backups/

echo "Backup completed: $BACKUP_FILE"

# Ubuntu/Debian
sudo apt update && sudo apt upgrade -y

# CentOS/RHEL
sudo yum update -y

# Windows
Install-WindowsUpdate -AcceptAll

# UFW (Ubuntu)
sudo ufw enable
sudo ufw allow 22/tcp
sudo ufw allow 80/tcp
sudo ufw allow 443/tcp

# firewalld (CentOS)
sudo firewall-cmd --permanent --add-service=ssh
sudo firewall-cmd --permanent --add-service=http
sudo firewall-cmd --reload

# Edit SSH config
sudo vi /etc/ssh/sshd_config

# Set:
PermitRootLogin no
PasswordAuthentication no  # Use SSH keys

sudo systemctl restart sshd

# Install fail2ban
sudo apt install fail2ban

# Configure fail2ban
sudo vi /etc/fail2ban/jail.local

# Ping the file server
ping fileserver.domain.com

# Test SMB connectivity
telnet fileserver.domain.com 445

# Windows: Check share permissions
Get-SmbShareAccess -Name "SharedFolder"

# Check NTFS permissions
Get-Acl "\\server\share" | Format-List

# Verify user is in correct group
Get-ADUser username -Properties MemberOf

# Check if account is locked
Get-ADUser username -Properties LockedOut

# Windows: List mapped drives
net use

# Remove and remap
net use Z: /delete
net use Z: \\server\share /persistent:yes

/var/log/syslog          # System logs (Debian/Ubuntu)
/var/log/messages        # System logs (RHEL/CentOS)
/var/log/auth.log        # Authentication logs
/var/log/kern.log        # Kernel logs
/var/log/apache2/        # Apache web server
/var/log/nginx/          # Nginx web server
/var/log/mysql/          # MySQL database

# View entire log
cat /var/log/syslog

# View last 50 lines
tail -n 50 /var/log/syslog

# Follow log in real-time
tail -f /var/log/syslog

# View with pagination
less /var/log/syslog

# Search in logs
grep "error" /var/log/syslog
grep -i "failed" /var/log/auth.log  # Case insensitive

# View all logs
journalctl

# View logs for specific service
journalctl -u nginx
journalctl -u ssh

# Follow logs
journalctl -f

# View logs since boot
journalctl -b

# View logs for time range
journalctl --since "2024-01-01" --until "2024-01-02"
journalctl --since "1 hour ago"

# View by priority
journalctl -p err  # Errors only
journalctl -p warning  # Warnings and above

# Export to file
journalctl -u nginx --since today > nginx-logs.txt

# Configure logrotate
sudo vi /etc/logrotate.d/myapp

# Example configuration
/var/log/myapp/*.log {
    daily
    rotate 7
    compress
    delaycompress
    missingok
    notifempty
    create 0644 www-data www-data
    sharedscripts
    postrotate
        systemctl reload myapp
    endscript
}

# Test logrotate
sudo logrotate -d /etc/logrotate.d/myapp  # Dry run
sudo logrotate -f /etc/logrotate.d/myapp  # Force rotation

# View Application logs
Get-EventLog -LogName Application -Newest 50

# View System logs
Get-EventLog -LogName System -Newest 50

# Filter by error level
Get-EventLog -LogName System -EntryType Error -Newest 20

# Search for specific event
Get-EventLog -LogName Security | Where-Object {$_.EventID -eq 4625}

# Export logs
Get-EventLog -LogName Application | Export-Csv -Path C:\logs\app-logs.csv

# Install rsyslog client
sudo apt install rsyslog

# Configure remote logging
sudo vi /etc/rsyslog.conf
# Add: *.* @@log-server:514

sudo systemctl restart rsyslog