Resuming and ranges

Resuming a download means first checking the size of what is already present locally and then asking the server to send the rest of it so it can be appended. curl also allows resuming the transfer at a custom point without actually having anything already locally present.

curl supports resumed downloads on several protocols. Tell it where to start the transfer with the -C, --continue-at option that takes either a plain numerical byte counter offset where to start or the string - that asks curl to figure it out itself based on what it knows. When using -, curl uses the destination filename to figure out how much data that is already present locally and ask use that as an offset when asking for more data from the server.

To start downloading an FTP file from byte offset 100:

curl --continue-at 100

Continue downloading a previously interrupted download:

curl --continue-at - -O

If you instead just want a specific byte range from the remote resource transferred, you can ask for only that. For example, when you only want 1000 bytes from offset 100 to avoid having to download the entire huge remote file:

curl --range 100-1099