Ran into an issue trying to update from 18.2 to 18.3.
Environment: Ubuntu Linux 18.04 LXC (containerized)
Using the script configured steamcmd.sh produced the following:
root@7dtd:/home/sdtd/steamcmd# ./steamcmd.sh
Redirecting stderr to '/root/Steam/logs/stderr.txt'
[ 0%] Checking for available updates...
[----] Verifying installation...
Steam Console Client (c) Valve Corporation
-- type 'quit' to exit --
Loading Steam API...applicationmanager.cpp (3911) : Assertion Failed: CApplicationManager::GetMountVolume: invalid index
applicationmanager.cpp (3911) : Assertion Failed: CApplicationManager::GetMountVolume: invalid index
applicationmanager.cpp (4074) : Assertion Failed: m_vecInstallBaseFolders.Count() > 0
OK.
I wasn't able to find very much information about this online... like... at all. I'm posting this for information purposes.
Eventually, I installed steamcmd from the Ubuntu repos. Using the system steamcmd.sh thereafter, the system version didn't produce the GetMountVolume errors. 7dtd.sh script still didn't work, since it wasn't using the system steamcmd. But, I confirmed I was able to manually upgrade the engine using the steamcmd from the repos.
In the end, I ended up deleting the /home/sdtd/{Steam|engine|steamcmd} directories and then reattempted 7dtd.sh updateengine, which downloaded a fresh copy of steamcmd and proceeded to upgrade the engine successfully.
Hope this helps someone.