Just like with grub1, most emergency problems in RH7 require booting to single-user mode. This involves changing the default target for systemd at boot time.

Changing systemd's default target at boot

During a normal boot, systemd's target is default.target. This is linked by a systemctl command to one of the two standard targets - multi-user.target or graphical.target via the systemctl set-default x.target command, which links default.target to x.target.

You can specify the default systemd target on the kernel command-line using the option

systemd.unit=x

in the case of going to a single-user mode you have two choices for x, and each has synonyms:

(Note: even though there are shorthand mechanisms for the general systemd.unit=x option, it is useful to know about it. You can, for example, choose to boot to text mode on a system whose default target is graphical by specifying systemd.unit=multi-user.target at boot time.)

In both cases you can continue the bootup process after making any needed modifications just by typing exit (or, the more general systemctl isolate default.target), or reboot using reboot (systemctl isolate reboot.target). This is because you are using systemd rather than circumventing it.

Booting to single-user mode

Although we discussed this  in the last section, let's review the steps for booting to single-user mode. Single-user mode is needed for performing maintenance operations that may be needed if the system is not booting properly. Briefly, this type of maintenance is performed by:

• booting to single user mode by modifying the normal boot process
• when in single-user mode, performing whatever system configuration changes are needed 
• rebooting (or continuing) to the normal system state

To modify the boot process to boot to single-user state you must

• reboot and interrupt grub to get a list of bootable configurations. The default boot configuration should be highlighted.
• use the 'e' command to edit the current configuration. (At this point you will have to enter the name of a superuser and their password, if grub2 is configured with superusers.) This opens the configuration to show its constituent lines.
• using the arrow keys, position the cursor at the end of the line containing the kernel parameters (either linux or linux16)
  
• append a space followed by the kernel options systemd.unit=x where x is rescue.target (for single-user mode) or emergency.target (for emergency mode)
  
• type control-X to boot the current configuration.
• Now the system will stop in single-user mode. You will have to enter the root password. If you have chosen emergency.target, you will have to remount the root filesystem read-write using
  mount -o remount,rw /
  (after perhaps checking it). You can, however, simply exit or reboot when you are done.
  

Problem: I can boot the system, but I've forgotten the root password!

This is the a common problem. It requires starting the system in single-user mode, then running the passwd program after you get a single-user shell. This changes the root password. Unfortunately since systemd will always run the single-user login program sulogin, you must circumvent systemd.

To accomplish this, edit the kernel configuration as above, but instead of changing the systemd target, delete the keywords quiet and rhgb (if they appear), then  add the string init=/bin/sh. This will start bash instead of systemd. This is dangerous and you must be careful.

To avoid having to relabel the filesystem after changing the password, you must load the current SELinux policy. Use the command

Then proceed as follows:

• The filesystems are read-only, so you must remount / read-write.
  
• Then run the passwd command and reset the root password. (see the note below)
  
• Then remount the root filesystem read-only (using mount -o remount,ro / command) and
  
• shut the system down (if this is a VM, just use Force Off. if it is a real system you may have to do a hard reset, but the filesystems have been set to read-only, so this should be safe)
  
• NOTE: if you are accessing the console remotely, and echo-ing is turned off when bash starts, you have neglected to remove the rhgb option from the command-line. Your commands still are being executed - you just cant see your typing. Just type the commands carefully. In this case you will have to use the command

Alternative method to resetting the root password on RH7

The method suggested by Redhat for resetting the root password on a VM on Redhat 7 follows. You can use this as a failsafe method in case the methods above are not successful:

• When grub starts, edit the default configuration and add the string rd.break This will force an early halt to the initialization process and drop you to a shell with the system 'living' on the initrd filesystem.
• You will find the 'real' root available as /sysroot. You will have to remount it read-write.
• chroot to /sysroot
• DO NOT load the SELinux policy. Since your shell has no SELinux context, loading the policy will effectively lock you out from changing the root password.
• run the passwd command to change the root password
• touch the /.autorelabel file to relabel the filesystem on the next boot
• exit the root shell
• remount /sysroot read-only
  
• Force Off and restart. Remember, relabelling the filesystem will take some time.