safer implementation

etc/zfs-auto-snapshot.cron.monthly

@@ -1,7 +1,7 @@
 #!/bin/sh
-PATH=$PATH:/usr/local/sbin
+PATH="/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:$PATH"
 
 # Only call zfs-auto-snapshot if it's available
-which zfs-auto-snapshot > /dev/null || exit 0
+command -v zfs-auto-snapshot >/dev/null 2>&1 || exit 0
 
 exec zfs-auto-snapshot --label monthly --keep 12