Clearing "purgeable" space on macOS High Sierra (e.g. before installing Windows 10 using BootCamp)

clear-purgeable-space.sh

# 1. Check for any local time machine snapshots
tmutil listlocalsnapshots /

# 2. Ask time machine to free 100 GBs of local snapshots. Third parameter '4' probably means high priority (https://apple.stackexchange.com/a/398356)
tmutil thinlocalsnapshots / $((100 * 1024 * 1204 * 1024)) 4

# 3. Make sure that only a `(dataless)` snapshot exists
tmutil listlocalsnapshots /

# 4. List all mounted volumes and find the one mounted on '/' (probably '/dev/disk1s1')
df -h

# 5. Free space on the volume mounted on '/', e.g.:
diskutil secureErase freespace 0 /dev/disk1s1

# If this still does not work, disable automatic time machine backups and try steps 2. and 5. again...