mentalisttraceur / fdtee

fdtee

fdtee is like the well known program tee, but with a key difference: fdtee copies its stdin to any file descriptor given, instead of to file names.

Why?

If you've ever found yourself asking:

"How do I redirect to BOTH stdout and stderr?"

or

"How do I pipe output to multiple processes (without using the non-standard/less-portable feature of process substitution)?"

then this tool is for you.

With fdtee, to redirect the output of foo to both stdout and stderr (on most systems, file descriptors 1 and 2 respectively), you would simply do:

foo | fdtee 1 2

Or you can do however many file descriptors you want (in whatever order):

foo | fdtee 1 4 2 5 3

just remember the rest of the shell command has to use redirection to make use of the output on those file descriptors.

Why is tee not enough?

The common alternative for splitting output over both stdout and stderr is to use tee with virtual files:

foo | tee /dev/stderr

But this is broken in several cases:

1. /dev/stderr is not guaranteed by POSIX, so it might not even exist.
2. Furthermore, using /dev/stderr (or /proc/self/fd/2) can break when processes switch effective user IDs, are in a chroot, or otherwise lose permission or access to that path.
3. You may be using a Bourne/POSIX shell on Windows, or another OS without the same virtual files, and you don't have such files faked by the shell.

In all of those cases, tee /dev/stderr does not work.

Also, there's also no standardized, consistently available virtual files to use for redirecting to file descriptors greater than 2.

fdtee, from a philosophy standpoint

fdtee, like tee, does one thing and does it well - repeats/copies stdin to any file descriptor given. It takes this philosophy to an even greater extent than tee does: it does not have special-case logic for stdout, neither by writing to stdout by default, nor by needing a magic argument value (-) which is parsed differently than the rest of its arguments.

Furthermore, when you combine fdtee with the file redirection capabilities which existed in the Bourne shell since its early days, you can do everything you can do with tee, because this:

foo | tee bar.txt

is equivalent to:

foo | fdtee 1 3 3>bar.txt

And this:

foo | tee -a bar.txt

is equivalent to:

foo | fdtee 1 3 3>>bar.txt

So you could implement tee as a wrapper around fdtee, but you cannot implement fdtee as a wrapper around tee in any portable manner. Thus, fdtee is fundamentally a more flexible building block for doing powerful things with a Bourne/POSIX shell than tee is.

Other Details

Unlike tee, fdtee does not write to stdout by default.

The primary reason for this is simple design simplicity/purity - not treating stdout as a special case makes the code simpler.

Also, consider that you might want to redirect to several file descriptors, but not to stdout, for example:

foo | fdtee 2 3

Having to write a 1 where you need it is a minimal cost, readable and explicit, while the alternative (automatically outputing to stdout) would've forced people to write code like this:

foo | fdtee 2 3 1>/dev/null

or:

foo | fdtee 2 1>&3

just to split the output of foo to file descriptors 2 (stderr usually) and 3, which is worse.