how to check if python script is being called remotely via ssh

Question

So I'm writing a command line utility and I've run into a problem I'm curious about.

The utility can be called with file arguments or can read from sys.stdin. Originally I was using sys.stdin.isatty() to figure out if data is being piped in, however, I found that if I call the utility remotely via ssh server utility, sys.stdin.isatty() will return false, despite the fact that there is no actual data being piped in.

As a workaround I'm using - as the file argument to force reading from stdin (eg: echo "data_here" | utility -f -), but I'm curious to know if there's a reliable way to tell the difference between a pipe getting data from a process and a pipe that's only open because the call is via ssh.

Systems programming is not my forte, so I'm grateful for any help I can get from you guys.

Answer 1

You can tell if you're being invoked via SSH by checking your environment. If you're being invoked via an SSH connection, the environment variables SSH_CONNECTION and SSH_CLIENT will be set. You can test if they are set with, say:

if "SSH_CONNECTION" in os.environ:
    # do something

Another option, if you wanted to just stick with your original approach of sys.stdin.isatty(), would be to to allocate a pseudo-tty for the SSH connection. Normally SSH does this by default if you just SSH in for an interactive session, but not if you supply a command. However, you can force it to do so when supplying a command by passing the -t flag:

ssh -t server utility

However, I would caution you against doing either of these. As you can see, trying to detect whether you should accept input from stdin based on whether it's a TTY can cause some surprising behavior. It could also cause frustration from users if they wanted a way to interactively provide input to your program when debugging something.

The approach of adding an explicit - argument makes it a lot more explicit and less surprising which behavior you get. Some utilities also just use the lack of any file arguments to mean to read from stdin, so that would also be a less-surprising alternative.