Open Source

curl ootw: –remote-time

Previous command line options of the week.

--remote-time is a boolean flag using the -R short option. This option was added to curl 7.9 back in September 2001.

Downloading a file

One of the most basic curl use cases is “downloading a file”. When the URL identifies a specific remote resource and the command line transfers the data of that resource to the local file system:

curl https://example.com/file -O

This command line will then copy every single byte of that file and create a duplicated resource locally – with a time stamp using the current time. Having this time stamp as a default seems natural as it was created just now and it makes it work fine with other options such as --time-cond.

Use the remote file’s time stamp please

There are times when you rather want the download to get the exact same modification date and time as the remote file has. We made --remote-time do that.

By adding this command line option, curl will figure out the exact date and time of the remote file and set that same time stamp on the file it creates locally.

This option works with several protocols, including FTP, but there are and will be many situations in which curl cannot figure out the remote time – sometimes simply because the server won’t tell – and then curl will simply not be able to copy the time stamp and it will instead keep the current date and time.

Not be default

This option is not by default because.

  1. curl mimics known tools like cp which creates a new file stamp by default.
  2. For some protocols it requires an extra operation which then can be avoided if the time stamp isn’t actually used for anything.

Combine this with…

As mentioned briefly above, the --remote-time command line option can be really useful to combine with the --time-cond flag. An example of a practical use case for this is a command line that you can invoke repeatedly, but only downloads the new file in case it was updated remotely since the previous time it was downloaded! Like this:

curl --remote-name --time-cond cacert.pem https://curl.haxx.se/ca/cacert.pem

This particular example comes from the curl’s CA extract web page and downloads the latest Mozilla CA store as a PEM file.