Apple - How to prevent Mac sleep from command line?

In Mountain Lion you can use the caffeinate command.

caffeinate -u -t 1000

will prevent idle sleep for 1000 seconds.

The solution to this problem is not keeping the client (your Mac) awake. Using approaches like this are undependable. What happens if the network connection is lost? Even if your Mac is awake, the script will halt.

Use nohup

If your long-running script is called eternity.sh, try the following:

nohup /path/to/eternity.sh > /path/to/output.out &

Now you can even close the connection and your script will keep running. The & backgrounds the process so you can keep the connection open and enter commands. View any output from your script via:

tail -f /path/to/output.out

The paths in the examples are optional if the script is on your path and you want script output to be written to output.out in the current directory.

I manage scripts that run for days at a time. Scripts like these should be detached from the terminal. Thankfully, nohup provides an easy-to-remember command invocation to achieve this--think no hangup.

Another option is pmset. Use the command pmset noidle to prevent sleep as long as pmset is running. Unfortunately, it requires a separate Terminal window with pmset running in it. However, the other option, caffeinate, only lets you set a certain time. So it's a matter of choosing whether you want to open a second SSH session, or deal with a time restraint.

Edit: According to binarybob's comment, you can actually run it in the background like this: pmset noidle & To get back to pmset type fg.