prevent sleep when booted in recovery mode or from usb drive?

I have El Capitan

The caffeinate command doesn't work or any pmset command because to enter disk utility in order to make de image file, you have to exit terminal and when you exit terminal, all commands are terminated (caffeinate or pmset) and you are back to square one.

There must be a way to prevent the machine from sleeping PERMANENTLY while in recovery mode otherwise, it is useless, you can't babysit the machine for four hours or more to prevent it from sleeping.

I think I found a way to solve your issue:

http://apple.blogoverflow.com/2012/07/interesting-new-unix-commandsbinaries-in-o s-x-mountain-lion/

The command you're looking for is caffeinate. Just open a Terminal window and execute.

I just invoked the command to keep a file copy (from my previous Mac's Time Machine volume to my new system's hard drive, as opposed to its SSD) going overnight. Prior to this, I had to babysit the system to keep it from falling asleep and pausing the file copy. The true test will be whether my computer is still alive and copying an hour after I hit the Add Reply button here...I'll report back if it works/doesn't work.

I would have invoked the command-specific option of caffeinate, but the file copy has been running for awhile, so I just told my Mac to caffeinate itself for 36000 seconds (10 hours). Should be enough to cover me for this action. I imagine that you'd have to use the timed version of caffeinate for anything running through the GUI, since I haven't yet found a way to invoke, say, Disk Utility from the Terminal.

To launch Disk Utility from the terminal type:

/Volumes/[Your disk name]/Applications/Utilities/Disk\ Utility.app/Contents/MacOS/Disk\ Utility

Thanks for the tip on caffeine

You might want to consider starting a new discussion. Since this one is a couple of years old, less people are likely to look at it. You can link to this one.