output terminal redirect to zenity

Question

Let's explain you my doubt.
We all know that >& is shell syntax for "fold a file descriptor into another".
So 2>&1 means that i want to redirect stderr to the stdin, this is simple and clear :)
The thing that i don't understand is this (let explain it with an example)

ncftpput -u $user -p $pass $host $remote_dir $local_file | zenity --text-info --title "Putting files..." --width 600 --height 300

The code above doesn't works...but this works perfectly:

ncftpput -u $user -p $pass $host $remote_dir $local_file 2>&1 | zenity --text-info --title "Putting files..." --width 600 --height 300

I don't undestrand why if i put 2>&1 the redirection to zenity work and if i don't write 2>&1 redirection doesn't works. Why?

Answer 1

By convention, applications produce output on two file descriptors: 1 (stdout) and 2 (stderr). The pipe feature of the shell redirects the stdout of one process to the stdin of another. This way, applications can separate out error or status messages from their normal output and still be usable when piping their output.

It seems that the output you want to pipe to zenity is being sent to stderr. In your second command line, the 2>&1 part says to redirect all output ncftpput sends to stderr to stdout. This causes it to be piped to zenity.