How to "tee" stderr


 
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# 1  
Old 04-20-2011
How to "tee" stderr

In other words, print on both the screen and to a file (minus stdout)? Thanks again in advance
# 2  
Old 04-20-2011
If your shell is Bash (or similar), this set of redirections will do the job:

Code:
$ command 3>&1 1>&2 2>&3 | tee file

What does it mean?

The redirection operator n>&m makes file descriptor n to be a copy of file descriptor m. So, whe are:

- Opening a new file descriptor, 3, that is a copy of file descriptor 1, the standard output;
- Making file descriptor 1 a copy of file descriptor 2, the standard error output;
- Making file descriptor 2 to be a copy of file descriptor 3 (the "backup" of the standard output)

in a short: we swapped the standard output and the standard error output.
# 3  
Old 04-20-2011
Quote:
Originally Posted by stevensw
In other words, print on both the screen and to a file (minus stdout)? Thanks again in advance
Redirect stderr to stdout then redirect stdout to /dev/null and then pipe tee to filename...sequence is important i.e. redirecting stderr to stdout comes before piping stdout to /dev/null. Thogh there is no need for swapping stdout with stderr as the following is ok...
Code:
cat valid_filename invalid_filename 2>&1 1>/dev/null | tee output_filename


Last edited by shamrock; 04-20-2011 at 10:32 PM.. Reason: update
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