How Do You Correct a /etc/fstab File That is Read Only?

Problem scenario
You boot up and log in. You cannot write to /tmp/.

What should you do?

Solution
First, mentally identify the file system in the /etc/fstab file (e.g., /dev/desk/by-id/dm-uuid-LVM-abcd1234).

Here is an example of an /etc/fstab file to determine what the file system string you need is:

/dev/disk/by-id/dm-uuid-LVM-abcd1234 / ext4 defaults,quota,grpquota 0 0
# /boot was on /dev/sda2 during curtin installation
/dev/sdc /var ext4 defaults 1 2
/dev/sda /opt ext4 defaults 0 0
/dev/disk/by-uuid/987654321 /boot ext4 defaults 0 0

Procedure
Run a command like this (but replace /dev/desk/by-id/dm-uuid-LVM-abcd1234 with the file system you need):

sudo mount -o remount,rw /dev/desk/by-id/dm-uuid-LVM-abcd1234

Now you should be able to modify the /etc/fstab file.

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