This is what I have for my server running on CentOS7. I've got Avorion installed to /home/avserv along with steamcmd, both in the home directory of my user avserv. You'll need to change the paths to wherever you have steamcmd and Avorion installed if you use these.
To install or update the game to that path I run serverupdate.sh which is a simple script:
#!/bin/sh
cd /home/avserv/
./steamcmd.sh +runscript update_server.txt
and update_server.txt is:
@ShutdownOnFailedCommand 1
login anonymous
force_install_dir /home/avserv
app_update 565060
quit
The force_install_dir is what tells steamcmd to use a non-standard path. I believe you need to supply that path each time you update as well so I went ahead and just tossed everything into a script to make it easier to run. If you don't already know, you need to do
chmod +x serverupdate.sh
and then you run it same as any other script
./serverupdate.sh
And to create those files you can do
vim serverupdate.sh
etc.
For the seed, I think you can set it by editing the server.sh script used to launch the server, just add --seed SomeSeed to the end of the launch command. I'm assuming this works like we think it works, I've never actually made two galaxies with the same seed and checked them.