This article will tell you how to install a Linux distribution on a Mac OS without having to format a disk or partition.
Steps

Step 1. Download the latest version of the desired Linux distribution

Step 2. Download VirtualBox for Mac OS X

Step 3. Install VirtualBox

Step 4. Start VirtualBox and create a new virtual machine by clicking "New" in the upper left corner of the VirtualBox window

Step 5. Assign a name to the virtual machine and click Next

Step 6. Select "Linux" as the operating system and specify the distribution kit of this system

Step 7. Check "Bootable hard drive (Primary Master)" and "Create new hard drive"
Click Next.

Step 8. Select "Dynamic virtual disk"

Step 9. Upon completion of the process of creating a virtual machine, start it; the system installation wizard will open

Step 10. To select the downloaded disk image with the Linux distribution, click "CD-DVD ROM" - "Image" (at the bottom of the window)
To find the image (ISO file) of a Linux disk, click on the folder with the green arrow.
- Once this process is complete, the virtual machine will start and you can proceed to install Linux on it.
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Run Linux on a Mac Step 10
Advice
- More information on installing VirtualBox can be found here.
- When you're done with Linux, you don't need to shut down the system - just press the pause button.
- Give the virtual machine an appropriate name, for example, if you are going to use Ubuntu 8.04, name it “Ubuntu 8.04”.
- Look for the downloaded ISO file in the download folder.