/sbin/init /etc/init/ /etc/init.d/ /etc/inittab
Example from arch wiki:
# <device> <dir> <type> <options> <dump> <fsck>
UUID=0a3407de-014b-458b-b5c1-848e92a327a3 / ext4 noatime 0 1
UUID=f9fe0b69-a280-415d-a03a-a32752370dee none swap defaults 0 0
UUID=b411dc99-f0a0-4c87-9e05-184977be8539 /home ext4 noatime 0 2
Devices can also be eg /dev/sda2, but UUIDs safer.
dump refers to backing up disks
fsck says whether there should be a check first. 0 means no. 1 means 1 and is root. 2 means yes and is not root.
Options include:
+ rw (read and write) + suid (use set user IDs and group IDs from file system) + dev ("Interpret character or block special devices on the filesystem") + exec (allow execution of binaries) + auto (can mount with -a) + nouser (don’t allow normal user to mount) + async
The option "defaults" uses all of these