Fastboot on Linux
I too had issues when I switched from Linux to windows (recently) and here's how I got it (just for documentation)
1) I downloaded android-sdk for linux, did the installs, but lo and behold, there was no fastboot...
2) Searched google for about 45 minutes
3) Finally figured out that HTC had it in their development website
4) Downloaded & chmod'd it so it'd work
5) Figured out that the r800x really can't be unlocked via all my attempts (so far...)
Here's a step by step as to how to get it without my hassle.
1) Attached is fastboot, download it, extract it (you can do this later in terminal if you feel like it, or you can do it now in the file explorer)
2) Open up terminal (if you don't have a shortcut for it, make one, it's the most useful thing in linux. ever.)
3) Type in su, enter your password (if you have it setup) and become root. (Or, you could call sudo whenever it is necessary, it's your preference)
4) goto the folder you downloaded fastboot to in terminal (the default chrome download place is /home/[insert your username here]/Downloads)
5) type in chmod [your preference for setting permissions here] fastboot
6) type in (sudo) cp fastboot /bin/
7) now you can call fastboot from anywhere in the terminal!!
This can also be done with adb