mdev Command Linux: Complete Guide to Minimal Device Manager Configuration

The mdev command is a lightweight device manager for Linux systems, particularly popular in embedded environments and minimal distributions. As a simpler alternative to udev, mdev provides essential device management functionality while maintaining a small footprint and fast performance.

What is mdev in Linux?

mdev is a minimal device manager that automatically creates and removes device nodes in the /dev directory when hardware devices are added or removed from the system. It’s part of the BusyBox toolkit and serves as a lightweight replacement for the more complex udev system.

Key Features of mdev

• Lightweight: Minimal memory and storage requirements
• Fast: Quick device detection and node creation
• Configurable: Supports custom rules and actions
• Hotplug support: Handles dynamic device insertion/removal
• Embedded-friendly: Perfect for resource-constrained systems

mdev vs udev: Understanding the Differences

Installing and Enabling mdev

Installation on Different Distributions

Alpine Linux (default):

# mdev is pre-installed
apk add busybox-mdev-openrc

Debian/Ubuntu:

sudo apt update
sudo apt install busybox-static

CentOS/RHEL:

sudo yum install busybox
# or on newer versions
sudo dnf install busybox

Enabling mdev as Device Manager

To use mdev instead of udev, you need to configure it as the hotplug handler:

# Set mdev as the hotplug handler
echo /sbin/mdev > /proc/sys/kernel/hotplug

# Or make it persistent
echo 'kernel.hotplug = /sbin/mdev' >> /etc/sysctl.conf

Basic mdev Command Syntax

The basic syntax for the mdev command is:

mdev [OPTIONS]

Common mdev Options

• -s: Scan /sys and populate /dev (initial scan)
• -d: Daemon mode (handle hotplug events)
• -f: Stay in foreground (don’t daemonize)
• -v: Verbose output

Essential mdev Commands and Examples

1. Initial Device Scan

Populate the /dev directory with existing devices:

# Scan all devices and create nodes
sudo mdev -s

Expected Output:

# Creates device nodes like:
# /dev/sda, /dev/sda1, /dev/tty0, /dev/null, etc.

2. Verbose Device Scanning

Get detailed information during the scan process:

# Verbose scan showing device creation
sudo mdev -sv

Sample Output:

mdev: /dev/console: created
mdev: /dev/null: created
mdev: /dev/zero: created
mdev: /dev/random: created
mdev: /dev/sda: created
mdev: /dev/sda1: created

3. Running mdev in Daemon Mode

Start mdev to handle hotplug events continuously:

# Start mdev daemon
sudo mdev -d

# Or in foreground with verbose output
sudo mdev -fv

mdev Configuration File

mdev behavior is controlled by the /etc/mdev.conf configuration file. This file defines rules for device handling.

Configuration File Format

The format is:

device_regex  user:group  permissions  [action]

Basic mdev.conf Example

# Basic mdev.conf configuration
# Device        User:Group    Perms   Action

# Console devices
console         root:tty      600

# Null and zero devices  
null            root:root     666
zero            root:root     666
random          root:root     444
urandom         root:root     444

# Hard drives
sd[a-z][0-9]*   root:disk     660

# TTY devices
tty             root:tty      666
tty[0-9]*       root:tty      660

# USB devices
usb             root:usb      664

Advanced mdev.conf with Actions

# Advanced configuration with actions
# Device        User:Group    Perms   Action

# Network interfaces
eth[0-9]+       root:netdev   664     @/etc/mdev/net.sh
wlan[0-9]+      root:netdev   664     @/etc/mdev/net.sh

# USB storage with mount action
sd[a-z][0-9]*   root:disk     660     */etc/mdev/storage.sh

# Input devices
input/.*        root:input    660

# Sound devices  
snd/.*          root:audio    664     @/etc/mdev/sound.sh

Creating Custom mdev Rules

Rule Components Explained

Device Regex: Pattern to match device names

• sd[a-z]*: Matches sda, sdb, sdc, etc.
• tty[0-9]+: Matches tty0, tty1, tty2, etc.
• input/.*: Matches all devices in input subdirectory

Permissions: Standard octal permissions

• 644: Read/write for owner, read for group and others
• 660: Read/write for owner and group
• 666: Read/write for everyone

Action Scripts

Create a network interface handler:

# /etc/mdev/net.sh
#!/bin/sh

case "$ACTION" in
    add)
        echo "Network device $MDEV added"
        # Configure network interface
        /sbin/ifconfig $MDEV up
        ;;
    remove)
        echo "Network device $MDEV removed"
        ;;
esac

Make the script executable:

sudo chmod +x /etc/mdev/net.sh

Practical Examples and Use Cases

Example 1: USB Storage Auto-mounting

Create an auto-mount script for USB devices:

# /etc/mdev/usb-mount.sh
#!/bin/sh

MOUNT_POINT="/mnt/usb"

case "$ACTION" in
    add)
        mkdir -p "$MOUNT_POINT/$MDEV"
        mount "/dev/$MDEV" "$MOUNT_POINT/$MDEV"
        echo "Mounted $MDEV to $MOUNT_POINT/$MDEV"
        ;;
    remove)
        umount "$MOUNT_POINT/$MDEV"
        rmdir "$MOUNT_POINT/$MDEV"
        echo "Unmounted $MDEV"
        ;;
esac

Add to mdev.conf:

sd[a-z][0-9]*   root:disk   660   */etc/mdev/usb-mount.sh

Example 2: Custom Device Permissions

Set specific permissions for development devices:

# Development-friendly mdev.conf
# Device        User:Group      Perms   Action

# Serial devices for developers
ttyUSB[0-9]*    root:dialout    666
ttyACM[0-9]*    root:dialout    666

# GPIO and I2C for embedded development
gpiochip[0-9]*  root:gpio       664
i2c-[0-9]*      root:i2c        664

# Video devices for testing
video[0-9]*     root:video      666

Example 3: Logging Device Events

Create a logging script for device monitoring:

# /etc/mdev/logger.sh
#!/bin/sh

LOG_FILE="/var/log/mdev.log"
TIMESTAMP=$(date '+%Y-%m-%d %H:%M:%S')

echo "$TIMESTAMP: $ACTION device $MDEV ($DEVPATH)" >> "$LOG_FILE"

case "$ACTION" in
    add)
        logger "Device added: $MDEV"
        ;;
    remove)
        logger "Device removed: $MDEV"
        ;;
esac

Troubleshooting mdev Issues

Common Problems and Solutions

1. Devices not appearing in /dev

# Check if mdev is running
ps aux | grep mdev

# Manually trigger scan
sudo mdev -s

# Check sysfs
ls /sys/class/

2. Permission issues

# Check current permissions
ls -la /dev/device_name

# Verify mdev.conf syntax
sudo mdev -s -v

3. Hotplug not working

# Verify hotplug handler
cat /proc/sys/kernel/hotplug

# Should show: /sbin/mdev
echo /sbin/mdev > /proc/sys/kernel/hotplug

Debug Mode

Enable verbose debugging:

# Run mdev with maximum verbosity
sudo MDEV_LOG_LEVEL=debug mdev -sv

# Monitor kernel messages
dmesg | tail -f

Performance Optimization

Optimizing mdev for Speed

1. Minimize rules: Only include necessary device patterns

2. Use specific regex: Avoid overly broad patterns

3. Efficient scripts: Keep action scripts lightweight

# Good: Specific pattern
sd[a-z][1-9]    root:disk   660

# Less efficient: Too broad
.*              root:root   644

Memory Usage Monitoring

# Check mdev memory usage
ps -o pid,vsz,rss,comm -p $(pgrep mdev)

# Compare with system resources
free -h

Integration with Init Systems

OpenRC Integration

# /etc/init.d/mdev
#!/sbin/openrc-run

description="mdev device manager"
command="/sbin/mdev"
command_args="-d"
pidfile="/run/mdev.pid"

depend() {
    need localmount
    before bootmisc
}

start_pre() {
    # Initial device scan
    /sbin/mdev -s
    
    # Set hotplug handler
    echo /sbin/mdev > /proc/sys/kernel/hotplug
}

systemd Integration

# /etc/systemd/system/mdev.service
[Unit]
Description=mdev device manager
Before=sysinit.target

[Service]
Type=forking
ExecStartPre=/sbin/mdev -s
ExecStartPre=/bin/sh -c 'echo /sbin/mdev > /proc/sys/kernel/hotplug'
ExecStart=/sbin/mdev -d
PIDFile=/run/mdev.pid

[Install]
WantedBy=sysinit.target

Best Practices for mdev Usage

Security Considerations

• Minimal permissions: Use restrictive permissions by default
• Secure scripts: Validate input in action scripts
• User groups: Leverage groups for access control

# Secure mdev.conf example
# Restrictive by default
.*              root:root   600

# Specific exceptions
console         root:tty    600
null            root:root   666
zero            root:root   644
random          root:root   644

Maintenance Tips

• Regularly review and update mdev.conf
• Monitor log files for unusual activity
• Test configuration changes in non-production environments
• Keep action scripts simple and fast

Conclusion

The mdev command provides an excellent lightweight alternative to udev for Linux device management, especially in embedded and resource-constrained environments. Its simplicity, speed, and minimal footprint make it ideal for systems where efficiency is crucial.

By understanding mdev configuration, creating custom rules, and implementing proper action scripts, you can build a robust device management system that meets your specific requirements while maintaining optimal performance.

Whether you’re working with embedded Linux systems, containers, or simply prefer a minimalist approach to device management, mdev offers the essential functionality needed to handle dynamic device creation and removal effectively.