Print something to both STDOUT and STDERR (without redirecting both to same file)

If I understand what you want, tee command is the solution:

your_command 2>&1 | tee /dev/stderr 

The output error of your command (ie. date) is redirected to the standard output (with 2>&1). The result is doubled with tee command.

• One to standard output;
• The second to the file in parameter: /dev/stderr

Then, the result with date command is:

> date 2>&1 | tee /dev/stderr Sun Dec 11 16:33:35 CET 2022 Sun Dec 11 16:33:35 CET 2022 

So, you could redirect with > / >> and 2> / 2>> to any files you want:

> date 2>&1 | tee /dev/stderr >file1 2>file2 > cat file1 Sun Dec 11 16:40:59 CET 2022 > cat file2 Sun Dec 11 16:40:59 CET 2022 

If an error occurs, with a bad command like datex:

> datex 2>&1 | tee /dev/stderr >file1 2>file2 > cat file1 bash: datex: command not found... > cat file2 bash: datex: command not found... 

In zsh, that's just

date >&1 >&2 

In zsh, unless the multios option is disabled, when a fd is redirected more than once for output¹, zsh redirects it instead to a pipe and reads the data from the other end to dispatch it to the targets in a tee-like fashion.

In other shells, you can do the teeing with perl for instance:

date | perl -ne 'BEGIN{$/=\8192} print; print STDERR' 

On systems other than Cygwin and Linux-based ones, you can do:

date | tee /dev/stderr 

On Linux/Cygwin that should be avoided as there, opening /dev/stderr tries to reopen the file that /dev/stderr points to anew, and writing to the resulting file descriptor is not the same as writing to stderr. It should be functionally equivalent though when stderr is open on a pipe or tty device as it often is.

^{¹ and beware that includes pipes as in cmd >&2 | cmd2 which can be a bit surprising.}