Download a file shared on dropbox only if it's modified after the last download

Question

A bit more context. There is a bibtex file shared by a collaborator on Dropbox. The file might be frequently modified. I wrote a makefile so that the bibtex file is downloaded every time I compile the tex files. My question is, is it possible to check if the bibtex file has been changed since my last download? If so, I can avoid curl-ing it every time.

PS: I took a look at the dropbox file download header. There is not "Last-Modified" field.

PS2: I'm open to solutions other than Dropbox.

EDIT: Thanks to smarx for the suggestion of using the etag header. FWIW, I'm posting my solution.

// Download the file if the etag doesn't match my record of the previous download

curl -o [OUTPUT_FILE] --header "If-None-Match: cat bib.etag " https://dl.dropboxusercontent.com/s/[YOUR-LINK]

// Save the new etag

curl -I https://dl.dropboxusercontent.com/s/t62yt1wvj33uxbu/xl_base.bib | grep tag | sed 's/etag: //g' > bib.etag

Answer 1

If you're using the API, you can check whether the rev field of the file's metadata has changed.

If you're downloading from a direct link (something that starts with dl.dropboxusercontent.com), I think you'll usually find an ETag header, but I think that depends on the size of the file.