These are the steps to install ruby-oci8 Gem on Ubuntu. 

Step 1: Download the basic & sdk instantclient zip files from https://www.oracle.com/database/technologies/instant-client/linux-x86-64-downloads.html 

instantclient-basic-linux.x64-21.8.0.0.0dbru.zip 
instantclient-sdk-linux.x64-21.8.0.0.0dbru.zip 
  

Step 2: Install following packages 

sudo apt-get install build-essential 
sudo apt-get install libaio-dev 
  

Step 3: Create a folder oracle inside /opt/ 

mkdir /opt/oracle 
  

Step 4: Go to inside /opt/oracle/ 

cd /opt/oracle 
  

Step 5: Place the basic instantclient folder inside /opt/oracle 

sudo cp ~/Downloads/instantclient-basic-linux.x64-21.8.0.0.0dbru.zip /opt/oracle/ 
  

Step 6: Place the sdk instantclient folder inside /opt/oracle 

sudo cp ~/Downloads/instantclient-sdk-linux.x64-21.8.0.0.0dbru.zip /opt/oracle/ 
  

Step 7: Unzip the basic instantclient folder 

sudo unzip instantclient-basic-linux.x64-21.8.0.0.0dbru.zip 
  

Step 8: Unzip the sdk instantclient folder 

sudo unzip instantclient-sdk-linux.x64-21.8.0.0.0dbru.zip 
  

Step 9: Check the folders inside /opt/oracle/. It should now have a folder named as instantclient_21_8 

Step 10: Update the runtime link path 

sudo sh -c “echo /opt/oracle/instantclient_21_8 > \ 
>      /etc/ld.so.conf.d/oracle-instantclient.conf”  
  

Step 11: Check the runtime link path, It should now return /opt/oracle/instantclient_21_8 

cat /etc/ld.so.conf.d/oracle-instantclient.conf 
  

Step 12: Set the path in env 

export LD_LIBRARY_PATH=/opt/oracle/instantclient_21_8:$LD_LIBRARY_PATH 
  

Step 13: Check the path if set properly in env. It should now have proper value set as LD_LIBRARY_PATH=/opt/oracle/instantclient_21_2:/opt/instantclient 

env | grep LD_LIBRARY_PATH 
  

Step 14: Now try by installing the gem ruby-oci8 

gem install ruby-oci8s 
  

It should work now. 

Alternate Install 

These steps take you through the instant client installation process as well, just to provide some context. Download the instant and SDK zip files here: 

http://www.oracle.com/technetwork/topics/linuxx86-64soft-092277.html 

Unzip both files in the following directory as root. 

/opt/oracle 

Enter the instantclient_12_2 directory following the unzip and create a symlink for the libclntsh.so file: 

ln -s libclntsh.so.12.1 libclntsh.so 
 

Now, depending on your OS, you’ll need to update your LD_LIBRARY_PATH location. In Ubuntu, do the following: 

sudo nano /etc/ld.so.conf.d/moreLibs.conf 
 

And enter the following in the file and save 

/opt/oracle/instantclient_12_2 
 

Now run the following command: 

sudo ldconfig 
 

And rerun your gem install:

gem install ruby-oci8