Add Do A Dry Run Of An rsync as a unix til

unix/do-a-dry-run-of-an-rsync.md

@@ -0,0 +1,40 @@
+# Do A Dry Run Of An rsync
+
+The `rsync` command, especially when running recursively (with the `-a` flag),
+will create and update a bunch of directories and files. Because of that, you
+may want to do a _dry run_ of an `rsync` command to make sure it is touching
+the intended files.
+
+The `--dry-run` flag (or the `-n` flag for short) will prepare a
+synchronization of one directory to another. You can use this flag to be sure
+that the source and target files and directories are correct.
+
+The `-n` (or `--dry-run`) flag on its own won't _show_ what is going to get
+synced. To get that information, you need to combine it with the `-v` (verbose)
+flag.
+
+```bash
+$ rsync -anv til-temp/ til-content
+
+building file list ... done
+./
+LICENSE
+...
+
+sent 909 bytes  received 296 bytes  2410.00 bytes/sec
+total size is 1058  speedup is 0.88
+```
+
+That will show everything that is going to be synced from `til-temp/`
+recursively to `til-content`.
+
+Doing a dry run is a great way to make sure you have the patterns for
+`--exclude` flags correct, before actually syncing anything.
+
+```bash
+$ rsync -anv --exclude='./*.md' --exclude='.*' til-temp/ til-content
+```
+
+That excludes top-level markdown files and all dotfiles and dot-directories.
+
+[source](https://www.digitalocean.com/community/tutorials/how-to-use-rsync-to-sync-local-and-remote-directories)