From the Arch Linux installation guide:
The /etc/fstab file contains static filesystem information. It defines how storage devices and partitions are to be mounted and integrated into the overall system. It is read by the mount command to determine which options to use when mounting a specific device or partition.
This is why we need to generate that file as a step of the installation process.
So how is it possible for mount
to mount the root partition in /mnt
at the very beginning of the installation process, if fstab
at that point is still empty, since at that point it hasn't been generated yet?
Best Answer
/etc/fstab
defines the default configuration. It lists default filesystem/mount point/option combinations. When you mount a filesystem, if you don’t specify complete parameters and options,mount
will read the options from yourfstab
. For example, you can typeand
mount
will know where to put it, and you can typeand
mount
will know where to find it. But if you want to be peculiar, you can typeand
mount
will do that. Similarly, you can specify on the command line that a filesystem should be mounted read-only. Conversely, you can specify in/etc/fstab
that a filesystem should be mounted read-only by default, and then override that on the command line. And you can manually mount filesystems that aren’t in/etc/fstab
at all.Also,
/etc/fstab
identifies which filesystems are automatically mounted at boot time; they are mounted with their specified default options.But also, just as you can type
mount /dev/sda1 /mnt/sda1
, a program can executemount
with a complete argument list, and then (in principle)mount
doesn’t need to access/etc/fstab
at all. And the operating system automagically knows where its root partition is, and automatically mounts the root filesystem very early in the boot process. In fact, if the OS needed/etc/fstab
to be accessible before it could do mounts, we’d have a chicken-and-egg problem, since/etc
isn’t accessible until the root filesystem has been mounted. The root partition is included in/etc/fstab
for the other reason that/etc/fstab
exists — to givefsck
a list of things to check.