init Command Linux: Complete Guide to Process Control and System Initialization

The init command in Linux serves as the cornerstone of system initialization and process control. As the first process started by the kernel during boot, it carries the Process ID (PID) 1 and acts as the parent of all other processes in the system. Understanding the init command is crucial for system administrators and Linux enthusiasts who need to manage system states, control services, and troubleshoot boot-related issues.

What is the init Command?

The init command is a system initialization program that manages the transition between different system states, known as runlevels in traditional Unix systems. It reads configuration files to determine which processes to start, stop, or restart during system startup, shutdown, or when changing runlevels.

Key Functions of init:

• Initialize the system during boot process
• Manage system runlevels and transitions
• Start and stop system services
• Handle orphaned processes
• Control system shutdown and restart procedures

Basic Syntax and Usage

The basic syntax of the init command is straightforward:

init [OPTION] [RUNLEVEL]

Common Options:

• -t SECONDS: Specify timeout between warning and kill signals
• -k: Send kill signal to all processes (used with runlevel)
• -r: Reboot after shutdown
• -h: Halt after shutdown
• -f: Force immediate shutdown without proper cleanup

Understanding Runlevels

Runlevels define different operational states of a Linux system. Each runlevel determines which services are running and how the system behaves:

Practical Examples and Usage

1. Checking Current Runlevel

To check the current runlevel of your system:

$ runlevel
N 3

This output shows that the system started at runlevel 3 (multi-user with networking). The ‘N’ indicates no previous runlevel.

2. Changing Runlevels

Switch to single-user mode for maintenance:

$ sudo init 1

Change to multi-user mode with GUI:

$ sudo init 5

3. System Shutdown Using init

Shutdown the system immediately:

$ sudo init 0

Reboot the system:

$ sudo init 6

4. Graceful Shutdown with Timeout

Allow 30 seconds for processes to terminate gracefully:

$ sudo init -t 30 0

Configuration Files

/etc/inittab

The primary configuration file for traditional init systems. Here’s an example structure:

# The default runlevel
id:3:initdefault:

# System initialization
si::sysinit:/etc/rc.d/rc.sysinit

# Runlevel configurations
l0:0:wait:/etc/rc.d/rc 0
l1:1:wait:/etc/rc.d/rc 1
l2:2:wait:/etc/rc.d/rc 2
l3:3:wait:/etc/rc.d/rc 3
l4:4:wait:/etc/rc.d/rc 4
l5:5:wait:/etc/rc.d/rc 5
l6:6:wait:/etc/rc.d/rc 6

# Terminal configurations
1:2345:respawn:/sbin/getty 38400 tty1
2:2345:respawn:/sbin/getty 38400 tty2

Entry Format Explanation:

• ID: Unique identifier for the entry
• Runlevels: Runlevels where this entry applies
• Action: What action to take (wait, respawn, once, etc.)
• Process: Command or script to execute

Modern Init Systems: systemd Integration

Most modern Linux distributions use systemd instead of traditional init. However, the init command still works as a compatibility layer:

systemd Targets vs Runlevels

systemd Equivalent Commands

Instead of using init with systemd, you can use these equivalent commands:

# Change to rescue mode (equivalent to init 1)
$ sudo systemctl isolate rescue.target

# Change to multi-user mode (equivalent to init 3)
$ sudo systemctl isolate multi-user.target

# Shutdown system (equivalent to init 0)
$ sudo systemctl poweroff

# Reboot system (equivalent to init 6)
$ sudo systemctl reboot

Troubleshooting and Advanced Usage

Emergency Boot Recovery

When your system fails to boot properly, you can use init parameters at the GRUB prompt:

# Boot into single-user mode
linux /boot/vmlinuz root=/dev/sda1 init=/bin/bash

# Boot with specific runlevel
linux /boot/vmlinuz root=/dev/sda1 3

Process Monitoring

Monitor processes started by init:

$ ps -ef | grep init
root         1     0  0 08:00 ?        00:00:01 /sbin/init
root       456     1  0 08:00 ?        00:00:00 /sbin/init --user

Custom Init Scripts

Create custom initialization scripts in /etc/init.d/:

#!/bin/bash
# /etc/init.d/myservice
# Custom service script

case "$1" in
    start)
        echo "Starting MyService..."
        # Start commands here
        ;;
    stop)
        echo "Stopping MyService..."
        # Stop commands here
        ;;
    restart)
        $0 stop
        $0 start
        ;;
    *)
        echo "Usage: $0 {start|stop|restart}"
        exit 1
        ;;
esac

Security Considerations

When working with the init command, keep these security aspects in mind:

• Root Privileges: Most init operations require root access
• System Impact: Changing runlevels affects all users and services
• Emergency Access: Single-user mode bypasses normal authentication
• Service Dependencies: Understand service relationships before making changes

Best Practices

1. Always Use Appropriate Runlevels

• Use runlevel 1 for maintenance tasks
• Use runlevel 3 for server environments
• Use runlevel 5 for desktop environments

2. Backup Configuration Files

$ sudo cp /etc/inittab /etc/inittab.backup

3. Test Changes Carefully

Always test runlevel changes in a controlled environment before applying to production systems.

4. Use Appropriate Shutdown Methods

• Use shutdown command for scheduled shutdowns
• Use init 0 for immediate shutdowns
• Use systemctl commands on systemd systems

Common Error Messages and Solutions

Error: “init: cannot execute”

Cause: Permission issues or corrupted init binary

Solution: Boot from rescue media and repair the init binary

Error: “INIT: no more processes left in this runlevel”

Cause: All processes for the current runlevel have terminated

Solution: Check inittab configuration and restart necessary services

Conclusion

The init command remains a fundamental tool for Linux system administration, despite the widespread adoption of systemd. Understanding its functionality, runlevels, and integration with modern init systems enables administrators to effectively manage system states, troubleshoot boot issues, and maintain system stability.

Whether you’re working with traditional init systems or modern systemd implementations, mastering the init command provides valuable insights into Linux system architecture and process management. Regular practice with these commands in safe environments will build the confidence needed for effective system administration.