Comments on: Output Redirection With Bash

By: Maxim

Maxim — Tue, 04 May 2010 21:44:36 +0000

Just a small observation : as I’m using DASH instead of BASH (for better portability and POSIX compliance) I had to close the file descriptor #3 explicitly with 3>&- instead of the more compact 2>&3- form that BASH allowed. So it would now looks like : 3>&1 1>&2 2>&3 3>&- in my DASH compatible script.

By: Maxim

Maxim — Tue, 04 May 2010 17:41:55 +0000

Thanks, Alan!

Your explanation did not only enabled my script to do exactly what I wanted, but I think I now fully understand how bash’s file descriptors can be used. :)

Kudos! Have a nice week.

By: Alan Skorkin

Alan Skorkin — Tue, 04 May 2010 02:08:09 +0000

Glad you found it useful. What you want to do is not too difficult, infact I've written about something similar before. You can check out my other post on redirection, but here is basically what you want to do. The tee command is your friend (I explain it the post I linked to above). What you have to do is, swap stdout and stderr around so that you can pipe stderr to tee, that's pretty much it. Here is a sample command:

ls -al 3>&1 1>&2 2>&3- | tee blah.txt

If you run that one you will get your output on the screen but nothing in the file, since no error was produced. But if you run this:

ls -garbage 3>&1 1>&2 2>&3- | tee blah.txt

You will get your error output both on the screen and in the file, just as you wanted. The reason you have to swap stdout and stderr around is because you can only use pipes with stdout and you need to pipe your output to tee to get it in a file and on the screen. Hope this helps.

By: Maxim

Maxim — Tue, 04 May 2010 01:33:15 +0000

First, let me thank you for this bit of info. It’s one of the best articulated information on redirection I’ve found around. So thank you.

However, it still does not answer one of the case I’m trying to implement : direct only stderr to a file, and print both stderr and stdout to console.

Would you happen to know a simple way to do this? Thanks in advance!

By: Ray

Ray — Sun, 28 Feb 2010 14:46:00 +0000

Hi. Your explanation was very clear and to the point. Thank you for this. I also like to ask you to add some light to the redirection with pipe and exec commands if possible.

Ray

By: Alan Skorkin

Alan Skorkin — Mon, 22 Feb 2010 05:52:32 +0000

If you simply want stderr printed to the screen, then that is what the shell will do by default, so if you need to discard stdout but print stderr to the console, all you have to do is redirect stdout to /dev/null:

ls -al > /dev/null

There is no need to to point stderr to stdout (2>&1) as by default they both go to the console. Of course the above command will print nothing as it will succeed and output will go to /dev/null. But if you cause the command to fail you will get output on the console.

By: Tiago

Tiago — Sun, 21 Feb 2010 14:04:04 +0000

Thanks for the short but clear explanation.
In my case, I need just stderr printed, and discard stdout. I’ve found the following way of doing that:
ls -al 2>&1 1>/dev/null

Is there a better way? Or, even if not better, other simple way of doing the same thing?

By: Alan Skorkin

Alan Skorkin — Tue, 22 Sep 2009 06:56:05 +0000

Yeah, that’s the best way.

By: Thura

Thura — Tue, 22 Sep 2009 06:54:34 +0000

I just use ls -al &> file.txt to redirect both stdout & stderr …