bash file descriptor redirection

Question

I have the following two bash scripts:

one.bash:

#!/bin/bash

echo "I don't control this line of output to stdout"

echo "I don't control this line of output to stderr" >&2

echo "I do control this line of output to fd 5" >&5

callone.bash:

#!/bin/bash

# here I try to merge stdout and stderr into stderr.
# then direct fd5 into stdout.

bash ./one.bash 1>&2 5>&1

When I run it like this: bash callone.bash 2>stderr.txt >stdout.txt

The stderr.txt file looks like this:

I don't control this line of output to stdout
I don't control this line of output to stderr
I do control this line of output to fd 5

and stdout is empty.

I would like the "do control" line to be output to only stdout.txt.

The restrictions on making changes are:

1. I can change anything in callone.bash.
2. I can change the line in one.bash that I control.
3. I can add an exec in one.bash related to file descriptor 5.
4. I have to run the script as indicated.

[EDIT] The use case for this is: I have a script that does all kinds of running of other scripts that can output to stderr and stdout. But I need to ensure that the user only sees the well controlled message. So I send the well controlled message to fd5, and everything else (stdout & stderr) is sent to the log.

Answer 1

Redirections happen in order.

Once you run 1>&2 you've replaced fd 1 with fd 2.

So when you then run 5>&1 you are redirecting fd 5 to where fd 1 points now (not where it was when it started).

You need to invert the two redirections:

bash ./one.bash 5>&1 1>&2