I recently decided to install MongoDB on an old Mac Mini I use for testing. This particular device is too old to run the latest version of OS X, so I'm currently running the 32-bit version of Ubuntu 15.10 on it. Unfortunately, MongoDB no longer provides installation packages for 32-bit Linux distributions, so I had to set it up manually. Since this was a fairly cumbersome process, I thought I would share the steps I took in case they are of use to anyone:

1. Download 32-bit MongoDB binaries:

   curl -O https://fastdl.mongodb.org/linux/mongodb-linux-i686-3.2.4.tgz
   tar -zxvf mongodb-linux-i686-3.2.4.tgz

2. Copy MongoDB binaries to /usr/bin:

   sudo cp mongodb-linux-i686-3.2.4/bin/* /usr/bin/

3. Download startup script:

   curl https://raw.githubusercontent.com/mongodb/mongo/master/debian/init.d > init.d

4. Move startup script to /etc/init.d directory:

   sudo mv init.d /etc/init.d/mongod
   sudo chmod 755 /etc/init.d/mongod        

5. Create configuration script /etc/mongod.conf:

   storage:
     dbPath: /var/lib/mongo
     journal:
       enabled: true
     engine: mmapv1
   
   systemLog:
     destination: file
     logAppend: true
     path: /var/log/mongodb/mongod.log
   
   processManagement:
     fork: true
   
   net:
     port: 27017
     bindIp: 0.0.0.0

6. Add "mongodb" user:

   sudo useradd --home-dir /var/lib/mongo --shell /bin/false mongodb
   sudo passwd mongodb

7. Create /var/lib/mongo directory:

   sudo mkdir /var/lib/mongo
   sudo chown -R mongodb /var/lib/mongo
   sudo chgrp -R mongodb /var/lib/mongo

8. Create /var/log/mongodb directory:

   sudo mkdir /var/log/mongodb
   sudo chown -R mongodb /var/log/mongodb
   sudo chgrp -R mongodb /var/log/mongodb

9. Create /var/run/mongod.pid file:

   sudo touch /var/run/mongod.pid
   sudo chown mongodb /var/run/mongod.pid
   sudo chgrp mongodb /var/run/mongod.pid

10. Initialize service:

    sudo update-rc.d mongod defaults

11. Restart server:

    sudo shutdown -r now