Daily Archives: June 17, 2017

cURL and libcurl

curl doesn’t spew binary anymore

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One of the least favorite habits of curl during all these years, I’ve been told, is when users forget to instruct the command line tool where to store the downloaded file and as a direct consequence, curl instead sends a lot of binary “gunk” to the terminal. The end result of that is at best just a busload of weird-looking characters on the screen, but with just a little bit of bad luck it can also lock up the terminal completely or change it in other ways.

Starting in curl 7.55.0 (from this commit), curl will inspect the beginning of each download that has been told to get sent to the terminal (tty!) and attempt to detect and prevent raw binary output to get sent there. The code is only simply looking for a binary zero in the data.

$ curl https://example.com/image.jpg
Warning: Binary output can mess up your terminal. Use "--output -" to tell curl to output it to your terminal anyway, or consider "--output <FILE>" to save to a file.

As the warning message says, there’s an option to use to switch off this emergency check for when you truly know what you’re doing and you don’t need curl to prevent you from doing this. Then you just tell curl explicitly that you want the output to stdout, with “–output -” (or “-o -” for a shorter version):

$ curl -o - https://example.com/binblob.img

We’re eager to get your input and feedback on how this works. We are aware of the risk of false positives for UTF-16 and UTF-32 outputs, but we think they are rare enough to not make this a huge problem.

This feature should be able to drastically reduce the risk for this:

Pipes

(Update, added after the initial posting.)

So many have remarked or otherwise asked how this affects when stdout is piped into something else. It doesn’t affect that! The whole point of this check is to only show the warning message if the binary output is sent to the terminal. If you instead pipe the output to another program or if you redirect the output with >, that will not trigger this warning but will instead continue just like before. Just like you’d expect it to.