System Management

System Management

These commands help you manage and monitor system resources, services, and system-level operations.

systemctl

Controls the systemd system and service manager, which is the foundation of most modern Linux distributions.

systemctl [options] command [unit]

Common commands:

• start: Start a unit
• stop: Stop a unit
• restart: Restart a unit
• status: Show status of a unit
• enable: Enable a unit to start at boot
• disable: Disable a unit from starting at boot
• reload: Reload a unit's configuration
• mask: Completely disable a unit
• unmask: Remove mask from a unit
• list-units: List loaded units

Examples:

# Check status of a service
systemctl status nginx

# Start a service
sudo systemctl start nginx

# Enable a service to start at boot
sudo systemctl enable nginx

# Restart a service
sudo systemctl restart nginx

# List all active services
systemctl list-units --type=service --state=active

journalctl

Queries the systemd journal, which contains logs from the system and services.

journalctl [options]

Options:

• -u unit: Show logs for the specified unit
• -f: Follow new entries as they are added (like tail -f)
• -b: Show logs from the current boot
• -n NUMBER: Show the last NUMBER of log entries
• -p PRIORITY: Filter by priority (0-7)
• --since "YYYY-MM-DD HH:MM:SS": Show entries since the specified time
• --until "YYYY-MM-DD HH:MM:SS": Show entries until the specified time

Examples:

# View system logs
journalctl

# Follow logs in real-time
journalctl -f

# View logs for a specific service
journalctl -u nginx

# View recent logs
journalctl -n 100

# View logs from today
journalctl --since "today"

# View logs with errors and critical issues only
journalctl -p err..crit

df

Reports file system disk space usage.

df [options] [file]

Options:

• -h: Human-readable output
• -T: Print file system type
• -i: List inode information instead of block usage
• -a: Include dummy file systems

Examples:

# Show disk space in human-readable format
df -h

# Show file system types along with usage
df -hT

# Show inode usage
df -i

free

Displays amount of free and used memory in the system.

free [options]

Options:

• -h: Human-readable output
• -s SEC: Update continuously every SEC seconds
• -t: Display a line showing totals
• -m: Display values in mebibytes
• -g: Display values in gibibytes

Examples:

# Show memory usage in human-readable format
free -h

# Show memory usage updated every 2 seconds
free -h -s 2

top

Displays real-time view of system processes.

top [options]

Options:

• -d SEC: Specify update interval
• -u USER: Show only processes from specified user
• -p PID: Monitor only processes with specified PID

Interactive commands:

• q: Quit
• h: Help
• k: Kill a process
• r: Renice a process
• f: Configure fields to display
• o: Change sort order

Examples:

# Start top with 3-second refresh
top -d 3

# Show only processes from a specific user
top -u username

htop

An interactive process viewer, more user-friendly than top.

htop [options]

Options:

• -u USER: Show only processes of a given user
• -d SEC: Set update interval
• -s COLUMN: Sort by specified column

Interactive commands:

• F1-F10: Various functions (shown at bottom)
• Space: Mark process
• u: Filter by user
• k: Kill process
• /: Search

Examples:

# Start htop filtering for root user
htop -u root

reboot

Restarts the system.

reboot [options]

Options:

• --force: Force reboot
• --no-wall: Don't send wall message before reboot

Examples:

# Reboot the system
sudo reboot

# Force reboot
sudo reboot --force

poweroff

Shuts down the system.

poweroff [options]

Options:

• --force: Force power off
• --no-wall: Don't send wall message before poweroff

Examples:

# Shutdown the system
sudo poweroff

# Force shutdown
sudo poweroff --force

shutdown

Shuts down or restarts the system.

shutdown [options] [time] [message]

Options:

• -r: Reboot after shutdown
• -h: Halt or power off after shutdown
• -c: Cancel a pending shutdown

Examples:

# Shutdown immediately
sudo shutdown now

# Reboot in 5 minutes with a message
sudo shutdown -r +5 "System maintenance"

# Schedule a shutdown at a specific time
sudo shutdown -h 23:30

# Cancel a scheduled shutdown
sudo shutdown -c

dmesg

Displays kernel ring buffer messages.

dmesg [options]

Options:

• -H: Human-readable output
• -l LEVEL: Filter by level (emerg, alert, crit, err, warn, notice, info, debug)
• -f FACILITY: Filter by facility (kern, user, daemon, etc.)
• -w: Wait for new messages (follow)
• -T: Show human-readable timestamps

Examples:

# Show kernel messages with readable timestamps
dmesg -HT

# Show only error messages
dmesg --level=err

# Follow new kernel messages
dmesg -w

uname

Prints basic system information.

uname [options]

Options:

• -a: All information
• -s: Kernel name
• -n: Network node hostname
• -r: Kernel release
• -v: Kernel version
• -m: Machine hardware name
• -p: Processor type
• -o: Operating system

Examples:

# Show all system information
uname -a

# Show only kernel version
uname -r

# Show machine architecture
uname -m

lsblk

Lists information about block devices.

lsblk [options]

Options:

• -a: List all devices
• -f: Show filesystem information
• -p: Show full device paths
• -S: List only SCSI devices
• -o LIST: Select output columns

Examples:

# List all block devices with basic info
lsblk

# Show filesystem information
lsblk -f

# Show full paths with size and type
lsblk -po NAME,SIZE,TYPE

blkid

Locates/prints block device attributes.

blkid [options] [device]

Options:

• -U UUID: Search for device with the specified UUID
• -L LABEL: Search for device with the specified label

Examples:

# Show all block devices with attributes
sudo blkid

# Find device with specific UUID
sudo blkid -U "a1b2c3d4-e5f6-7890-abcd-1234567890ab"

lsof

Lists open files.

lsof [options]

Options:

• -u USER: Files opened by user
• -p PID: Files opened by process
• -c command: Files opened by command starting with string

Examples:

# Show all open files
sudo lsof

# Show all open network connections
sudo lsof -i

# Show files opened by a specific user
sudo lsof -u username

# Show all files opened by a process
sudo lsof -p 1234

# Show which process is using a specific port
sudo lsof -i :80

mount/umount

Mounts/unmounts filesystems.

mount [options] [device] [directory]
umount [options] [device|directory]

Options (mount):

• -t type: Specify filesystem type
• -o options: Specify mount options
• -a: Mount all filesystems in /etc/fstab
• -r: Mount read-only

Options (umount):

• -f: Force unmount
• -l: Lazy unmount

Examples:

# Mount a device to a directory
sudo mount /dev/sdb1 /mnt/usb

# Mount with specific filesystem type and options
sudo mount -t ext4 -o rw,noatime /dev/sda2 /home

# Unmount a filesystem
sudo umount /mnt/usb

# Force unmount when device is busy
sudo umount -f /mnt/usb

fdisk

Manipulates disk partition table.

fdisk [options] [device]

Options:

• -l: List partition tables

Interactive commands:

• m: Show help menu
• p: Print partition table
• n: Add a new partition
• d: Delete a partition
• w: Write changes to disk and exit
• q: Quit without saving changes

Examples:

# List all partitions
sudo fdisk -l

# Manipulate partitions on a specific disk
sudo fdisk /dev/sdb

mkfs

Creates a filesystem on a device or partition.

mkfs [options] -t [fstype] [device]

Common filesystem types:

• ext4: Extended filesystem 4
• xfs: XFS filesystem
• btrfs: B-tree filesystem
• vfat: FAT filesystem

Examples:

# Create an ext4 filesystem
sudo mkfs -t ext4 /dev/sdb1

# Create an XFS filesystem
sudo mkfs.xfs /dev/sdc1

# Create a FAT32 filesystem (for USB drives)
sudo mkfs.vfat -F 32 /dev/sdd1

dd

Copies and converts files, often used for disk images.

dd [options] operand

Common operands:

• if=file: Input file
• of=file: Output file
• bs=bytes: Block size
• count=n: Copy only n blocks
• status=progress: Show progress during copy

Examples:

# Create a disk image
sudo dd if=/dev/sda of=disk.img bs=4M status=progress

# Write an ISO to a USB drive
sudo dd if=ubuntu.iso of=/dev/sdb bs=4M status=progress

# Wipe a disk with zeros
sudo dd if=/dev/zero of=/dev/sdc bs=1M count=1000

timedatectl

Controls the system time and date.

timedatectl [command]

Commands:

• status: Show current time settings
• set-time TIME: Set system time
• set-timezone ZONE: Set system timezone
• list-timezones: List available timezones
• set-ntp BOOL: Enable/disable network time synchronization

Examples:

# Show current time settings