/usr/sbin/update-icon-caches is in libgtk-3-bin 3.4.1-0ubuntu1.
This file is owned by root:root, with mode 0o755.
The actual contents of the file can be viewed below.
| 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 | #!/bin/sh
case "$1" in
    ""|-h|--help)
        echo "Usage: $0 directory [ ... ]"
        exit 1
        ;;
esac
for dir in "$@"; do
    if [ ! -d "$dir" ]; then
        continue
    fi
    if [ -f "$dir"/index.theme ]; then
        # Generate or regenerate the cache    
        if ! gtk-update-icon-cache-3.0 --force --quiet "$dir"; then
            echo "WARNING: icon cache generation failed for $dir"
        fi
    else
        # No more index.theme, remove the cache if it exists
        rm -f "$dir"/icon-theme.cache
        rmdir -p --ignore-fail-on-non-empty "$dir"
    fi
done
exit 0
 |