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System Administration

User Management​

User Account Management​

# Create users
useradd username        # Create user with default settings
useradd -m username     # Create user with home directory
useradd -s /bin/bash -m username  # Specify shell and home dir
adduser username        # Interactive user creation (Debian/Ubuntu)

# Modify users
usermod -aG groupname username    # Add user to group
usermod -s /bin/zsh username      # Change user shell
usermod -d /new/home username     # Change home directory
usermod -L username               # Lock user account
usermod -U username               # Unlock user account

# Delete users
userdel username        # Delete user (keep home directory)
userdel -r username     # Delete user and home directory

Password Management​

# Password operations
passwd username         # Set/change password
passwd -e username      # Expire password (force change)
passwd -l username      # Lock password
passwd -u username      # Unlock password

# Password policies
chage -l username       # Show password aging info
chage -M 90 username    # Set password expiry to 90 days
chage -d 0 username     # Force password change on next login

Group Management​

# Group operations
groupadd groupname      # Create group
groupadd -g 1001 groupname  # Create group with specific GID
groupdel groupname      # Delete group
groupmod -n newname oldname  # Rename group

# Group membership
groups username         # Show user's groups
id username            # Show user and group IDs
gpasswd -a username groupname  # Add user to group
gpasswd -d username groupname  # Remove user from group

File System Management​

Disk Partitioning​

# Disk information
lsblk                  # List block devices
fdisk -l               # List all partitions
parted -l              # List partitions (parted)

# Create partitions
fdisk /dev/sda         # Interactive partitioning
parted /dev/sda        # Alternative partitioning tool
cfdisk /dev/sda        # Curses-based partitioning

File System Creation​

# Create filesystems
mkfs.ext4 /dev/sda1    # Create ext4 filesystem
mkfs.xfs /dev/sda1     # Create XFS filesystem
mkswap /dev/sda2       # Create swap filesystem

# Filesystem options
mkfs.ext4 -L "MyLabel" /dev/sda1  # Create with label
mkfs.ext4 -m 1 /dev/sda1          # Reserve 1% for root

Mount Management​

# Mount operations
mount /dev/sda1 /mnt   # Mount filesystem
mount -t ext4 /dev/sda1 /mnt  # Specify filesystem type
mount -o ro /dev/sda1 /mnt    # Mount read-only
umount /mnt            # Unmount filesystem
umount -l /mnt         # Lazy unmount

# Persistent mounts
echo "/dev/sda1 /mnt ext4 defaults 0 2" >> /etc/fstab
mount -a               # Mount all filesystems in fstab

Logical Volume Management (LVM)​

# Physical volumes
pvcreate /dev/sda1     # Create physical volume
pvdisplay              # Display physical volumes
pvs                    # Short PV display

# Volume groups
vgcreate vg01 /dev/sda1  # Create volume group
vgextend vg01 /dev/sdb1  # Extend volume group
vgdisplay              # Display volume groups
vgs                    # Short VG display

# Logical volumes
lvcreate -L 10G -n lv01 vg01  # Create 10GB logical volume
lvextend -L +5G /dev/vg01/lv01  # Extend logical volume
lvdisplay              # Display logical volumes
lvs                    # Short LV display

# Resize filesystem after LV extension
resize2fs /dev/vg01/lv01    # For ext2/3/4
xfs_growfs /dev/vg01/lv01   # For XFS

Package Management​

Debian/Ubuntu (APT)​

# Package operations
apt update             # Update package list
apt upgrade            # Upgrade packages
apt install package   # Install package
apt remove package    # Remove package
apt purge package     # Remove package and config files
apt autoremove         # Remove unused packages

# Package information
apt search keyword     # Search for packages
apt show package      # Show package information
apt list --installed  # List installed packages
dpkg -l               # List installed packages (dpkg)

Red Hat/CentOS (YUM/DNF)​

# Package operations
yum update             # Update packages
yum install package   # Install package
yum remove package    # Remove package
yum search keyword    # Search for packages
yum info package      # Show package information

# DNF (newer systems)
dnf update             # Update packages
dnf install package   # Install package
dnf remove package    # Remove package

Service Management​

Systemd Service Management​

# Service operations
systemctl start service     # Start service
systemctl stop service      # Stop service
systemctl restart service   # Restart service
systemctl reload service    # Reload service configuration
systemctl status service    # Show service status

# Service enablement
systemctl enable service    # Enable service at boot
systemctl disable service   # Disable service at boot
systemctl is-enabled service # Check if service is enabled

# Service information
systemctl list-units        # List all units
systemctl list-units --failed  # List failed units
systemctl list-unit-files   # List unit files

Creating Custom Services​

# Create service file
cat > /etc/systemd/system/myapp.service << EOF
[Unit]
Description=My Application
After=network.target

[Service]
Type=simple
User=myuser
ExecStart=/usr/local/bin/myapp
Restart=always

[Install]
WantedBy=multi-user.target
EOF

# Enable and start service
systemctl daemon-reload
systemctl enable myapp
systemctl start myapp

Network Configuration​

Network Interface Configuration​

# Network information
ip addr show           # Show IP addresses
ip route show          # Show routing table
ip link show           # Show network interfaces

# Configure network interface
ip addr add 192.168.1.100/24 dev eth0  # Add IP address
ip route add default via 192.168.1.1   # Add default route
ip link set eth0 up    # Bring interface up
ip link set eth0 down  # Bring interface down

Network Configuration Files​

# Debian/Ubuntu - /etc/network/interfaces
auto eth0
iface eth0 inet static
    address 192.168.1.100
    netmask 255.255.255.0
    gateway 192.168.1.1

# Red Hat/CentOS - /etc/sysconfig/network-scripts/ifcfg-eth0
DEVICE=eth0
BOOTPROTO=static
IPADDR=192.168.1.100
NETMASK=255.255.255.0
GATEWAY=192.168.1.1
ONBOOT=yes

DNS Configuration​

# DNS settings - /etc/resolv.conf
nameserver 8.8.8.8
nameserver 8.8.4.4
search example.com

# Hosts file - /etc/hosts
127.0.0.1 localhost
192.168.1.100 myserver.example.com myserver

System Security​

Firewall Configuration​

# UFW (Ubuntu)
ufw enable             # Enable firewall
ufw disable            # Disable firewall
ufw allow 22           # Allow SSH
ufw allow 80/tcp       # Allow HTTP
ufw deny 23            # Deny telnet
ufw status             # Show firewall status

# Firewalld (CentOS/RHEL)
firewall-cmd --permanent --add-service=ssh
firewall-cmd --permanent --add-port=80/tcp
firewall-cmd --reload
firewall-cmd --list-all

SSH Configuration​

# SSH server configuration - /etc/ssh/sshd_config
Port 22
PermitRootLogin no
PasswordAuthentication yes
PubkeyAuthentication yes
MaxAuthTries 3

# Restart SSH service
systemctl restart sshd

# SSH key management
ssh-keygen -t rsa -b 4096  # Generate SSH key pair
ssh-copy-id user@host      # Copy public key to remote host

File Permissions and Access Control​

# Special permissions
chmod +t /tmp          # Set sticky bit
chmod g+s /shared      # Set SGID bit
chmod u+s /usr/bin/passwd  # Set SUID bit

# Access Control Lists (ACLs)
setfacl -m u:username:rwx /path/file  # Set user ACL
setfacl -m g:groupname:rx /path/file  # Set group ACL
getfacl /path/file     # View ACLs

System Monitoring and Logging​

Log Management​

# View logs
journalctl             # View all logs
journalctl -u service  # View service logs
journalctl -f          # Follow logs
journalctl --since "2023-01-01"  # Logs since date

# Traditional log files
tail -f /var/log/syslog      # Follow system log
tail -f /var/log/auth.log    # Follow authentication log
tail -f /var/log/apache2/access.log  # Follow Apache access log

Log Rotation​

# Configure log rotation - /etc/logrotate.d/myapp
/var/log/myapp/*.log {
    daily
    rotate 7
    compress
    delaycompress
    create 644 myuser mygroup
    postrotate
        systemctl reload myapp
    endscript
}

# Test log rotation
logrotate -d /etc/logrotate.d/myapp  # Debug mode
logrotate -f /etc/logrotate.conf     # Force rotation

System Monitoring​

# Resource monitoring
top                    # Process monitor
htop                   # Enhanced process monitor
iotop                  # I/O monitor
nethogs                # Network monitor
glances                # System overview

# System information
uptime                 # System uptime and load
free -h                # Memory usage
df -h                  # Disk usage
lscpu                  # CPU information

Backup and Recovery​

File System Backups​

# Rsync backups
rsync -avz /source/ /backup/  # Local backup
rsync -avz /source/ user@host:/backup/  # Remote backup
rsync -avz --delete /source/ /backup/   # Mirror backup

# Tar backups
tar -czf backup.tar.gz /path/to/backup  # Create compressed backup
tar -xzf backup.tar.gz  # Extract backup

Database Backups​

# MySQL backup
mysqldump -u root -p database > backup.sql
mysql -u root -p database < backup.sql

# PostgreSQL backup
pg_dump -U postgres database > backup.sql
psql -U postgres -d database < backup.sql

System Backup Scripts​

#!/bin/bash
# System backup script
DATE=$(date +%Y%m%d)
BACKUP_DIR="/backups"
SOURCE_DIRS="/home /etc /var/log"

for dir in $SOURCE_DIRS; do
    tar -czf "$BACKUP_DIR/$(basename $dir)-$DATE.tar.gz" "$dir"
done

# Clean old backups (keep 7 days)
find $BACKUP_DIR -name "*.tar.gz" -mtime +7 -delete

Cron and Task Scheduling​

Cron Job Management​

# Edit cron jobs
crontab -e             # Edit current user's cron
crontab -l             # List cron jobs
crontab -r             # Remove all cron jobs

# System cron jobs
echo "0 2 * * * /usr/local/bin/backup.sh" >> /etc/crontab

Systemd Timers​

# Create timer unit - /etc/systemd/system/backup.timer
[Unit]
Description=Run backup daily
Requires=backup.service

[Timer]
OnCalendar=daily
Persistent=true

[Install]
WantedBy=timers.target

# Create service unit - /etc/systemd/system/backup.service
[Unit]
Description=Backup service

[Service]
Type=oneshot
ExecStart=/usr/local/bin/backup.sh

# Enable and start timer
systemctl enable backup.timer
systemctl start backup.timer

System Optimization​

Performance Tuning​

# CPU governor
echo performance > /sys/devices/system/cpu/cpu0/cpufreq/scaling_governor

# Memory tuning
echo 1 > /proc/sys/vm/drop_caches  # Clear cache
sysctl vm.swappiness=10            # Reduce swappiness

# I/O scheduler
echo deadline > /sys/block/sda/queue/scheduler

System Limits​

# Configure limits - /etc/security/limits.conf
* soft nofile 65536
* hard nofile 65536
* soft nproc 32768
* hard nproc 32768

# Kernel parameters - /etc/sysctl.conf
fs.file-max = 65536
vm.swappiness = 10
net.core.rmem_max = 16777216

Troubleshooting​

Common Issues​

# Boot issues
systemctl --failed     # Show failed services
journalctl -b          # Boot log
dmesg                  # Kernel messages

# Permission issues
ls -la /path/file      # Check permissions
namei -l /path/file    # Check path permissions

# Network issues
ping google.com        # Test connectivity
nslookup google.com    # Test DNS
netstat -tuln          # Check listening ports

Recovery Procedures​

# Single user mode
# Add 'single' to kernel parameters at boot

# Reset root password
passwd root

# Filesystem check
fsck /dev/sda1

# Network interface reset
ifdown eth0 && ifup eth0
systemctl restart networking

Best Practices​

Security Best Practices​

  1. Regular updates - Keep system and packages updated
  2. Strong passwords - Enforce password policies
  3. Minimal services - Run only necessary services
  4. User privileges - Use sudo instead of root
  5. Firewall - Configure appropriate firewall rules
  6. Monitoring - Monitor logs and system activity

System Administration Workflow​

  1. Document changes - Keep records of system modifications
  2. Test changes - Test in development before production
  3. Backup before changes - Always backup before major changes
  4. Monitor after changes - Watch for issues after modifications
  5. Automate tasks - Use scripts for repetitive tasks