If you try to install Unity on Linux and you get not enough space error have in mind that Unity Hub downloads the files in your /tmp/ folder first. If your home directory is on another partition while root (or /tmp ) on a smaller one, that causes the issue.

Solution

Set another TEMP directory before running the AppImage. If you use bash, that would be:

TEMP=~/tmp ./UnityHub.AppImage

Assumptions:

You are in the same folder with AppImage.

You have created the tmp folder in your home directory (mkdir ~/tmp).

I met this question in 2021, the answer is still correct, the /tmp directory is too small. But now there is an easier solution, because there is now a setting in Unity Hub called Download Location under Installs.

Go into developer settings and then in the linux developer environment increase the disk size to allow for more disk space on that user.