Upload/Download files via shell (Android-Linux)

The "-X" flag takes a parameter for the HTTP method to use, which should be "POST". In your code though, you've put the flag "-O" directly after that, so curl will send that as the method, which isn't valid. You would just need to move that out of that spot, e.g.,:

curl -O -X POST https://content.dropboxapi.com/2/files/download \
--header "Authorization: Bearer TOKEN_ID" \
--header "Dropbox-API-Arg: {\"path\": \"/Private/fmc_database.db\"}"

That "-O" option isn't really well suited for the Dropbox API though anyway, per the curl documentation:

       -O, --remote-name
              Write output to a local file named like the remote file we get. (Only the file part of the remote file is used, the path is cut off.)

              The remote file name to use for saving is extracted from the given URL, nothing else.

That would just take the name from the URL, which would just be "download". The Dropbox API instead returns the file's metadata as JSON in the "dropbox-api-result" result header. You should parse it out from there instead if you want the actual file name.