To add to steevej update
You need not run mongoexport from bin dir as long as your PATH is updated with mongodb/bin
You can run it from anywhere
So easiest is cd to the dir where your field_name.txt is residing then run the command without giving full path.The space between your name “Arindam PC” was the issue as Steeve identified
Double quotes on out is also not needed as there are no spaces in that path