Fix macOS pssh logging caused by line buffering

If you want to log the output of an SSH connection under macOS with pssh from Homebrew, the following error occurs:

line buffering (buffering=1) isn't supported in binary mode

In the pssh command this happens when you define the -o option for output.

pssh -t 0 -p 10 -l root -h host.list -o logs 'command'

In order to log the output of the SSH session, we have to make a small adjustment to the Python psshlib.

Here is how to fix pssh session logging on macOS Ventura

Let’s open this file with an editor of your choice:

/opt/homebrew/Cellar/pssh/2.3.1_6/libexec/lib/python3.10/site-packages/psshlib/manager.py

Inside the file we search for buffering. Then we change the code as follows.

buffering=1 needs to be changed to buffering=0

At the end of the if else statement, we add the following two lines:

dest.write(data)
dest.flush()

This code block should now look similar to the following code:

    def run(self):
        while True:
            filename, data = self.queue.get()
            if filename == self.ABORT:
                return
    
            if data == self.OPEN:
                self.files[filename] = open(filename, 'wb', buffering=0)
                psshutil.set_cloexec(self.files[filename])
            else:
                dest = self.files[filename]
                if data == self.EOF:
                    dest.close()
                else:
                    dest.write(data)
                    dest.flush()

Now we save the file and our changes. From now on pssh can log the output to a file without getting a buffering error message.

Please keep in mind that you may need to make this change again in case you apply a homebrew update on your Mac.