Using caffeinate on an unplugged Mac

Keep an unplugged Mac awake

Open Terminal and run:

caffeinate -i

The -i flag prevents idle system sleep. The command remains in the foreground until you stop it with Control-C, close that Terminal session, or otherwise terminate the process.

Running plain caffeinate with no assertion flags also prevents idle system sleep. Writing -i explicitly makes the intention clearer, especially in a script or saved command.

Choose the right assertion

Sleep is not one indivisible state. caffeinate exposes several assertions, and they can be combined:

• -i prevents idle system sleep and is the usual choice for work that must continue on battery.
• -d prevents display sleep. Use caffeinate -id only when the screen must remain visible.
• -m prevents idle disk sleep. Modern Macs rarely need this as a general-purpose addition.
• -s prevents system sleep, but its assertion is valid only on AC power.
• -u declares user activity, wakes an off display, and defaults to a five-second timeout unless -t is supplied.

Keeping the display awake consumes considerably more battery than allowing it to turn off while the system continues working. For an unattended transfer or build, -i is normally enough.

Set a time limit

Use -t to specify a duration in seconds. This keeps the Mac awake for one hour:

caffeinate -i -t 3600

After the timer expires, the assertion is released automatically and the normal Energy settings apply again. A timeout is preferable when the task has a predictable upper bound and there is no reason to keep the Mac awake all night.

Follow a command or process

A safer pattern is to place the real command after caffeinate. The assertion then lasts exactly as long as that command:

caffeinate -i rsync -av ~/Documents/ /Volumes/Backup/Documents/

Arguments after the utility name belong to that utility. When a utility is supplied, -t is not used, because the utility's lifetime controls the assertion.

If the work is already running, find its process ID and use -w:

caffeinate -i -w 12345

Replace 12345 with the actual PID. The assertion disappears when that process exits. This is useful for a long-running GUI application or a command that was started before you remembered caffeinate.

Run it in the background

For a timed session that should not occupy the Terminal prompt, start it as a background job:

caffeinate -i -t 3600 &
echo $!

The second command prints the PID of the backgrounded caffeinate process. Keep that number if you may want to end the session early:

kill PID

A background job can still end when its shell exits. For work that matters, wrapping the actual command with caffeinate is easier to reason about than creating an independent assertion and hoping both processes have matching lifetimes.

Check the assertion

Inspect current power-management assertions with:

pmset -g assertions

Look for PreventUserIdleSystemSleep and an entry owned by caffeinate. The exact output contains other assertions created by macOS and applications, so the global count may already be non-zero before your command starts.

You can also confirm that the process exists:

pgrep -fl caffeinate

Know the limits

• Closing a MacBook lid normally causes sleep. caffeinate -i is not a supported way to override lid-close behaviour.
• An assertion prevents idle sleep; it does not override a critically low battery, shutdown, restart, or an explicit Sleep command.
• Battery drain continues while work runs, and rises substantially if the display is kept awake.
• If the Terminal or remote shell that owns a foreground caffeinate process closes, do not assume the assertion survived. Verify it.

For most unplugged use, start with caffeinate -i. Add a timeout when you know the duration, or attach the assertion directly to the command so it ends at the right moment without manual cleanup.