How do you validate fstab without rebooting?

You can simple run: mount -a

-a Mount all filesystems (of the given types) mentioned in fstab.

This command will mount all (not-yet-mounted) filesystems mentioned in fstab and is used in system script startup during booting.

The mount command take an --fake or -f for short. The following command should do what you need:

mount -fav

The following is in the documentation for -f option:

Causes everything to be done except for the actual system call; if it's not obvious, this ``fakes'' mounting the filesystem. This option is useful in conjunction with the -v flag to determine what the mount command is trying to do.

(Note this is Linux - check before using elsewhere: FreeBSD uses -f for 'force' - exactly the opposite meaning.)

sudo findmnt --verify --verbose is the best way I've found

Note that if you add a swap file to your fstab, mount -a won't turn it on: you'll want to run swapon -a.

I found this /problem/ but the solution didn't meet my requirements.

When rebooting with any invalid entries in the /etc/fstab, such as missing file systems that fsck cannot check; the system will fail to boot. That can be much more difficult to deal with if you have a headless box.

This is my solution to checking /etc/fstab to avoid this boot problem:

    # cat /usr/local/bin/check-fstab-uuid-entries.sh
    #!/usr/bin/env bash

    for x in $(grep ^UUID /etc/fstab|cut -d \  -f 1|cut -d = -f 2)
    do
            if [ ! -h /dev/disk/by-uuid/$x ];then
                    echo $(grep $x /etc/fstab)  ..... not found
            fi
    done

TBH even fake mounting doesn't safely validate the fstab for bad fs type entries.

you can have entries that have correct uuid's, directories etc but if you specify a noexistant FS type this will halt your boot next time.

[root@grumpy ~]# grep backup /etc/fstab
UUID=5ed48e5e-7251-4d49-a273-195cf0432a89       /mnt/backup     noatime,nodiratime,xfs defaults,nodev,nosuid    0 0
[root@grump ~]#

[root@grumpy ~]# mount -fav | grep backup
/mnt/backup              : successfully mounted
[root@grumpy ~]#

mount -a is safe method to check /etc/fstab otherwise wrong entry could break the system

It is also advised to keep a backup copy of original /etc/fstab file. it could be copied to home directory of root

I open another term or tab and run: tail -f /var/log/kern.log

Sometimes errors show there that don't show when mounting.