skip: Skip download if already done

Fixes #505
Closes #521

SUMMARY.md

@@ -104,6 +104,7 @@
     * [Rate limiting](usingcurl/transfers/rate-limiting.md)
     * [Request rate limiting](usingcurl/transfers/request-rate.md)
     * [Compression](usingcurl/transfers/compression.md)
+    * [Skip download if already done](usingcurl/transfers/skip.md)
   * [Connections](usingcurl/connections/README.md)
     * [VLAN](usingcurl/connections/vlan.md)
     * [Name resolve tricks](usingcurl/connections/name.md)

usingcurl/transfers/README.md

@@ -8,3 +8,4 @@ multiple transfers.
  * [Rate limiting](rate-limiting.md)
  * [Request rate limiting](request-rate.md)
  * [Compression](compression.md)
+ * [Skip download if already done](skip.md)

usingcurl/transfers/skip.md

@@ -0,0 +1,24 @@
+# Skip download if already done
+
+Sometimes when writing a script or similar to do Internet transfers every now
+and then you end up in a situation when you rather have curl skip doing the
+transfer if indeed the target file is already present locally. Maybe after the
+previous invoke, maybe for another reason.
+
+This kind of extra logic can certainly be added in common shell script logic
+before curl is invoked in many cases but not all. For example things get
+complicated when you ask curl to use [globbing download
+ranges](../../cmdline/urls/globbing.md).
+
+For example, download one thousand images from a site but skip the ones that
+we already have downloaded previously:
+
+    curl --skip-existing -O https://example.com/image[0-999].jpg
+
+It should be noted that this only checks the *presence* of the local file, it
+makes not attempts to verify that it actually has the correct content and it
+has no way to do any such checks.
+
+curl offers other options to do [conditional
+transfers](../../http/modify/conditionals.md) based on modified date or
+contents.