bash Cookbook

Swapping STDERR and STDOUT

Problem

You need to swap STDERR and STDOUT so you can send STDOUT to a logfile, but then send STDERR to the screen and to a file using the tee command. But pipes only work with STDOUT.

Solution

Swap STDERR and STDOUT before the pipe redirection using a third file descriptor:

$ ./myscript 3>&1 1>stdout.logfile 2>&3- | tee -a stderr.logfile

Discussion

Whenever you redirect file descriptors, you are duplicating the open descriptor to another descriptor. This gives you a way to swap descriptors, much like how any program swaps two values—by means of a third, temporary holder. It looks like: copy A into C, copy B into A, copy C into B and then you have swapped the values of A and B. For file descriptors, it looks like this:

$ ./myscript 3>&1 1>&2 2>&3

Read the syntax 3>&1 as “give file descriptor 3 the same value as output file descriptor 1.” What happens here is that it duplicates file descriptor 1 (i.e., STDOUT) into file descriptor 3, our temporary holding place. Then it duplicates file descriptor 2 (i.e., STDERR) into STDOUT, and finally duplicates file descriptor 3 into STDERR. The net effect is that STDERR and STDOUT file descriptors have swapped places.

So far so good. Now we just change this slightly. Once we’ve made the copy of STDOUT (into file descriptor 3), we are free to redirect STDOUT into the logfile we want to have capture the output of our script or other program. Then we can copy the file descriptor from its temporary holding place (fd 3) into STDERR. ...