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# Linux init Command [ Linux Command Manual](#) * * * ## What is the init Command `init` is one of the most important processes in Linux systems. It is the first user-level process created by the kernel after system boot (PID=1), responsible for starting and managing all other processes in the system. ### Key Features * **Process Management**: As the parent process of all processes * **Runlevel Control**: Manages different operating states of the system * **Service Management**: Starts and stops system services * **System Initialization**: Executes startup scripts and configurations * * * ## Basic Syntax of init Command init ### Runlevel Description Linux systems define 7 standard runlevels: | Runlevel | Description | Typical Usage | | --- | --- | --- | | 0 | Shutdown | System shutdown state | | 1 | Single-user mode | System maintenance/root access | | 2 | Multi-user mode (no NFS) | Basic multi-user mode | | 3 | Full multi-user mode | Standard command-line interface | | 4 | Reserved | User-defined | | 5 | Graphical interface mode | Multi-user mode with GUI | | 6 | Reboot | System reboot | * * * ## Practical Applications of init Command ### 1. View Current Runlevel ## Example who -r # or runlevel ### 2. Switch Runlevel ## Example init 3# Switch to multi-user command-line mode init 5# Switch to graphical interface mode init 0# Shutdown init 6# Reboot system ### 3. Emergency Handling ## Example init 1# Enter single-user maintenance mode (requires root permissions) * * * ## Evolution of init in Modern Linux Systems ### 1. System V init Traditional init system, uses /etc/inittab configuration file ## Example # Typical inittab entry example id:3:initdefault: # Default runlevel si::sysinit:/etc/rc.d/rc.sysinit # System initialization script ### 2. Upstart (used in early Ubuntu versions) Event-driven init system ### 3. systemd (modern mainstream distributions) ## Example systemctl isolate multi-user.target # equivalent to init 3 systemctl isolate graphical.target # equivalent to init 5 * * * ## Practical Exercises ### Exercise 1: Runlevel Switching Experiment Switch from graphical interface to command-line mode init 3 Observe service changes, then switch back to graphical interface init 5 ### Exercise 2: System Maintenance Mode Experience Enter single-user mode init 1 Perform filesystem check fsck /dev/sda1 * * * ## Notes 1. **Permission Requirements**: The init command typically requires root permissions 2. **Use with Caution**: Directly using init 0 or init 6 may cause data loss of unsaved work 3. **System Differences**: Different distributions may have different init implementations 4. **Service Impact**: Switching runlevels will stop/start related services * * * ## FAQ **Q: Why doesn't my system have /etc/inittab file?** A: Systems using systemd no longer need inittab; the configuration method has changed **Q: How to set the default runlevel?** For systemd systems: systemctl set-default multi-user.target # equivalent to runlevel 3 **Q: What is the difference between init and shutdown commands?** Both will ultimately shut down the system, but shutdown provides more options (such as delayed shutdown, broadcast notifications, etc.) [ Linux Command Manual](#)