How to boot into single user mode or runlevel 3 using the GRUB bootloader?

Solution Verified - Updated

Environment

  • Red Hat Enterprise Linux (RHEL) 6
  • Red Hat Enterprise Linux (RHEL) 5
  • Red Hat Enterprise Linux (RHEL) 4
  • Red Hat Enterprise Linux (RHEL) 3

Issue

  • How to boot into single user mode or runlevel 3 using the GRUB bootloader?

Resolution

Single user mode can be accessed by appending an "S", "s", or "single" to the kernel command line in GRUB. Likewise, a "3" can be used to boot to runlevel 3. To do this, restart the system and when the GRUB splash screen presents itself:

  1. Select/highlight the desired kernel using the up/down arrow keys.

  2. Press the letter a to modify the kernel line.

  3. On the new screen, press the spacebar to add a space, then type the letter s and press Enter.

This will boot the system into single user mode, i.e. the boot process will stop immediately after execution of rc.sysinit and present a root BASH shell.

Root Cause

Adding "S", "s", or "single" to the kernel command-line tells the kernel to to have init drop to a root shell after rc.sysinit finishes. This allows one to get into the system without a password and bypass any problems with services.

Adding "1" to the kernel command-line seems to do basically the same as above, but in reality init also kicks off rc for runlevel 1 before dropping to a root shell. This means that in runlevel 1, there could possibly be some services running (if they were configured in /etc/rc1.d).

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