In bash, how do I open a writable file descriptor that's externally redirectable?

bash file-descriptor io-redirection
Kelvin picture Kelvin · May 14, 2012 · Viewed 8k times · Source

I'm trying to use bash to open a new descriptor for writing extra diagnostic messages. I don't want to use stderr, because stderr should only contain output from the programs called by bash. I also want my custom descriptor to be redirectable by the user.

I tried this:

exec 3>/dev/tty
echo foo1
echo foo2 >&2
echo foo3 >&3

But when I try to redirect fd 3, the output still writes to the terminal.

$ ./test.sh >/dev/null 2>/dev/null 3>/dev/null
foo3

Answer

cdarke picture cdarke · May 14, 2012

First the parent shell sets file descriptor 3 to /dev/null
Then your program sets file descriptor 3 to /dev/tty
So your symptoms are not really surprising.

Edit: You could check to see if fd 3 has been set:

if [[ ! -e /proc/$$/fd/3 ]]
then
    exec 3>/dev/tty
fi

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