plymouth module missing on Ubuntu

Using plymouth for boot theme can lead to a strange error on Ubuntu. Here a quick solution.

The full error is:

W: plymouth module (/usr/lib/x86_64-linux-gnu/plymouth//.so) missing, skipping that theme.

That’s happen when update-initramfs is invoked.

Troubleshooting

Let’s see what happen:

# bash -x /usr/sbin/update-initramfs -u
[...]
+ mkinitramfs -o /boot/initrd.img-4.4.14-eve-ng+.new 4.4.14-eve-ng+
W: plymouth module (/usr/lib/x86_64-linux-gnu/plymouth//.so) missing, skipping that theme.
[...]

The error is catch by mkinitramfs. Let’s follow the “white rabbit”:

# bash -x /usr/sbin/mkinitramfs -o /boot/initrd.img-4.4.14-eve-ng+.new 4.4.14-eve-ng+
[...]
+ /usr/share/initramfs-tools/hooks/plymouth
W: plymouth module (/usr/lib/x86_64-linux-gnu/plymouth//.so) missing, skipping that theme.
[...]

And follow it again:

# cat /usr/share/initramfs-tools/hooks/plymouth
[...]
MODULE="${PLUGIN_PATH}/$(sed -n 's/^ModuleName=\(.*\)/\1/p' ${THEMES}/${currtheme}/${currtheme}.plymouth 2>/dev/null || true).so"

if [ ! -e "$MODULE" ]
then
		echo "W: plymouth module ($MODULE) missing, skipping that theme."
		continue
fi
[...]

Mind that:

• PLUGIN_PATH is plymouth --get-splash-plugin-path;
• THEMES is /usr/share/plymouth/themes;
• currtheme is eveng;

And we found some possible common and least common causes:

• Theme folder is now /usr/share/plymouth/themes, on previus Ubuntu versions was /lib/plymouth/themes, so be sure all references are updated.
• Theme folder and theme filename must be the same, in my case it is /usr/share/plymouth/themes/eveng/eveng.plymouth;
• The regex doesn’t want spaces before and after =, so it must be ModuleName=script and not ModuleName = script.

Fixed the above three conditions, update-initramfs should work fine.