I have installed clean Ubuntu MATE 18.04 LTS and installed Scilab from repository here with

sudo apt-get update
sudo apt-get install scilab

After installation I tried to launch it from GUI (Applications → Other → Scilab) - it does not start.

Then I tried to launch it from terminal with scilab command. Terminal output is as follows:

$ scilab
/usr/bin/scilab-bin: error while loading shared libraries: libjava.so: cannot open shared object file: No such file or directory

so it can't find Java. I reported bug 1762999 about this problem.

I checked existence of libjava.so:

$ dpkg -S libjava.so 
openjdk-11-jre-headless:amd64: /usr/lib/jvm/java-11-openjdk-amd64/lib/libjava.so

so it is in place.

What should I do to get Scilab 6.0.1 working on Ubuntu 18.04 LTS?

* Repository-based method (ATOMS will not work)

Update 2019-04-14. Nowadays Scilab is ported to OpenJDK 11.

Then we have to install Scilab with (if not installed before):

sudo apt-get install scilab

So we can use latest scilab package 6.0.1-7ubuntu1~18.04 as usual from GUI, but scilab-cli and scilab-adv-cli are now broken (it is new bug 1825037).

Note 1: Scilab color theme is incompatible with dark Gtk themes, so user should set light one (this is bug 1547561) such as Radiance or Radiant-MATE.

Note 2: Scilab ATOMS system is broken too (see bug 1765503) because of problems with HDF5.

* Binary download method (fully-functional)

So if you need fully-functional Scilab you should remove deb-packaged version with

sudo apt-get purge scilab
sudo apt-get autoremove

and use binary archive 6.0.2 from official site and run the commands below (download, extract, add to PATH, set shotcuts and MIME-association):

mkdir ~/Software
cd ~/Software
wget https://www.scilab.org/download/6.0.2/scilab-6.0.2.bin.linux-x86_64.tar.gz
tar -xzf scilab-6.0.2.bin.linux-x86_64.tar.gz
cd scilab-6.0.2
echo "PATH=$PATH:/home/$USER/Software/scilab-6.0.2/bin" >> ~/.bashrc
echo "PATH=$PATH:/home/$USER/Software/scilab-6.0.2/bin" >> ~/.profile
mkdir -p ~/.local/share/applications
cp -a ~/Software/scilab-6.0.2/share/icons,applications,mime ~/.local/share/
update-mime-database ~/.local/share/mime/

The ATOMS and other components will work as expected. Do not forget to install build-essential package to run Xcos Modelica simulations.

If you want to get Scilab 5.5.2 working on Ubuntu 18.04.1 LTS see other answer.

Binary method is also applicable for Ubuntu 18.10, as ATOMS is still broken here.

I followed these steps to run scilab on (x)ubuntu 18.04 (Bionic Beaver):

1. 
   

   Install openjdk8 via:

   
   
   

   sudo apt-get install openjdk-8-jre openjdk-8-jre-headless 
   sudo update-java-alternatives -s java-1.8.0-openjdk-$(dpkg --print-architecture)
   

   
   


2. 
   

   Enter this command via terminal to open SciLab-GUI:

   
   
   

   scilab
   

   
   


3. Hit ENTER (after jlatexmath warning)

it's (the GUI and all of it's features) running without any problems including XCos (very good Simulink alternative).

By pressing enter I just skipped the warning

Could not find Java package '/usr/share/java/jlatexmath-fop-1.0.6.jar

If you don't need it, just don't install it.