Automatically force OS X / macOS to sleep when the battery is low (script + plist).

mwd.sleepMac.plist

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
    <key>Label</key>
    <string>mwd.sleepMac</string>
    <key>ProgramArguments</key>
    <array>
        <string>/path/to/sleepMac.sh</string>
    </array>
    <key>StartInterval</key>
    <integer>60</integer>
</dict>
</plist>

sleepMac.sh

#!/bin/bash
# Automatically force your Mac to sleep when the battery is low.
#
# This script will check if the battery percentage is < 10, and if so,
# will launch a dialog informing the user that the system will sleep
# in 30 seconds unless the user clicks "No".  This is useful for making
# sure the system does not accidentally stay awake and then completely
# die due to fully exhausting the battery.
#
# Protip: This works even when using `caffeinate -di` to keep OS X awake.
#
# This will need to be paired with the `mwd.sleepMac.plist` file stored
# at `~/Library/LaunchAgents/`.
#
# Installation:
# Move `sleepMac.sh` to a folder of your choice, and edit `mwd.sleepMac.plist`
# to reflect that.
# Then:
# ```
# mv mwd.sleepMac.plist ~/Library/LaunchAgents/
# chmod +x sleepMac.sh
# launchctl load -w ~/Library/LaunchAgents/mwd.sleepMac.plist
# launchctl list  # optional to check that it is in the list
# launchctl start mwd.sleepMac  # optional to kick-start task immediately
# ```
percentage_threshold=10
if (pmset -g batt | grep -q "from 'Battery"); then
    percentage=$(pmset -g batt | grep -o "[0-9]*%" | grep -o "[0-9][0-9]*");
    if (($percentage < $percentage_threshold)); then
        # battery is low
        osascript -e 'tell app "Finder" to activate';
        n=$(osascript -e 'tell app "Finder" to display dialog "Battery dying! Your computer will sleep in 30 secs." buttons {"No"} giving up after 30
            if button returned of result = "No" then
                return 1
            end if');
        echo $n;
        if (($n)); then
            echo "cancelled";
        else
            if (pmset -g batt | grep -q "from 'Battery"); then
                echo "ejecting all external hard drives...."
                osascript -e 'tell application "Finder" to eject (every disk whose ejectable is true)'
                echo "sleeping...";
                pmset sleepnow
            fi
        fi
    fi
fi