At boot, if X, then mount A, else mount B

Question

Context is as in this other question of mine. TL,DR: most of my home folder is on an external drive that I carry along with me and mount at my various workplaces and at home.

Mount is done as unionfs /HomeLocal=RW:/mnt/external_drive/HomeRoaming=RW /home thanks to the following line in fstab:

/HomeLocal=RW:/mnt/external_drive/HomeRoaming=RW /home fuse.unionfs auto,suid,exec,nonempty,max_files=32768,allow_other,x-systemd.requires=/mnt/external_drive

(/HomeLocal holds my local config files, and local users' homes.)

This works well but local users cannot use the computer when I'm not there, because unionfs fails when external_drive is not present. So I would like to write a SystemD mount unit to achieve this:

• if external_drive is present, mount it, then unionfs as explained
• if not, simply (bind) mount /HomeLocal on /home

I don't know if SystemD on its own has got what it takes to set up this kind of branching. If not, I guess I can resort to a script called by a mount unit, but then I need advice on how to write a correct unit file to achieve this.

Answer 1

You could do this with a systemd generator; have the generator create the appropriate mount unit depending on which devices are available. The following examples demonstrates the general idea:

1. We check if our target device (/dev/disk/by-uuid/7057c04d-e1c8-4b95-8338-ff0c3e5b14f8) is available:

   • If it is, we'll use it.
   • If it's not, we'll use /dev/disk/by-uuid/cad0b488-1ac0-4725-806d-1df5aff03391 instead.

2. Now that we know which device to use, we generate the appropriate mount unit.

3. Finally, we activate the mount unit so that the device gets mounted.

I created the following script and named it /etc/systemd/system-generators/home-mount-generator:

#!/bin/bash

DISK1=/dev/disk/by-uuid/7057c04d-e1c8-4b95-8338-ff0c3e5b14f8
DISK2=/dev/disk/by-uuid/cad0b488-1ac0-4725-806d-1df5aff03391

normal_dir=$1
early_dir=${2:-$normal_dir}
late_dir=${3:-$normal_dir}

if [[ -b $DISK1 ]]; then
        DEVICE=$DISK1
else
        DEVICE=$DISK2
fi

cat > $normal_dir/home.mount <<EOF
# Created by $0

[Unit]
Description=Home directories

[Mount]
What=$DEVICE
Where=/home
Type=ext4

[Install]
WantedBy=local-fs.target
EOF

mkdir -p $normal_dir/local-fs.target.requires
ln -sf ../home.mount $normal_dir/local-fs.target.requires

Ensure the file is executable:

chmod 755 /etc/systemd/system-generators/home-mount-generator

Now reboot the system, and depending on which device is attached you would find one or the other mounted.