README: guard the daily cron scan against an unmounted disk (closes #175) (#186)

README.md

@@ -114,15 +114,20 @@ docker run --rm -v /srv:/srv:ro -v /var/lib/sfdupes:/var/lib/sfdupes \
 Run `scan` as root, so that it can read every file. `scan` warns about a
 directory it cannot read, and about a new or changed file it cannot read. It
 skips the path and deletes the records at or below it (see "Rules for the
-walk"). As the file `/etc/cron.d/sfdupes`:
+walk").
+
+An unmounted disk leaves its mount point empty, so `scan` deletes every record
+under it, and the next scan reads every file on the disk again. Test the mount
+point of each disk at or below `/srv` with its own `mountpoint -q`, joined with
+`&&`, and test `/srv` only when it is one. As the file `/etc/cron.d/sfdupes`:
 
 ```
-30 3 * * * root /usr/local/bin/sfdupes scan /srv 2>>/var/log/sfdupes.log || tail -n 3 /var/log/sfdupes.log
+30 3 * * * root if mountpoint -q /srv; then /usr/local/bin/sfdupes scan /srv 2>>/var/log/sfdupes.log || tail -n 3 /var/log/sfdupes.log; else echo "a disk is not mounted; scan of /srv skipped"; fi
 ```
 
 - The first scan creates `/var/lib/sfdupes/db.sqlite` and its directory. To keep
-  the database elsewhere, put `SFDUPES_DATABASE=/path/to/db.sqlite` before the
-  command on the same line.
+  the database elsewhere, put `SFDUPES_DATABASE=/path/to/db.sqlite` directly
+  before `/usr/local/bin/sfdupes`.
 - `scan` writes nothing to stdout. Its stderr, appended here to
   `/var/log/sfdupes.log`, holds:
     - one line naming the database;