A Linux file server running systemd hangs during startup because a bad entry in /etc/fstab prevents non-root volumes from mounting. The administrator needs a maintenance environment that
mounts the local file systems so they can be edited,
does not start normal network or application services, and
presents a root shell for repairs. Which systemd boot target entered at the GRUB prompt best meets these requirements?
Boot into graphical.target (runlevel 5)
Boot into rescue.target (single-user mode)
Boot into emergency.target (init=/bin/sh equivalent)
The target that implements single-user mode under systemd is rescue.target (also known as runlevel 1). When the kernel is started with the parameter systemd.unit=rescue.target (or simply the digit "1"), systemd:
Mounts all local file systems so they are available for editing,
Starts only the minimal set of services required for a root login shell, and
Leaves networking and most other daemons inactive. This provides exactly the maintenance environment described.
emergency.target is even more minimal-it does not mount the file systems-so you would still have to mount them manually. multi-user.target and graphical.target start full sets of services (including networking, and in the latter case a display manager), which does not satisfy the requirement to avoid normal services during troubleshooting.