Cryptsetup – how does it print prompt bypassing stdout/stdin redirection

file-descriptors

I just noticed that no matter which cryptsetup FDs are forwarded to /dev/null it still shows prompt for password. For example this still shows prompt:

cryptsetyp luksOpen /dev/sdXY name >/dev/null 2>/dev/null

How can you display in terminal message that cannot be redirected to file using standard redirection?

I'd like to get such functionality in bash script as I use stdout to return result to mother script but I'd still like to display interactive prompt – is it possible to do so using bash?

Best Answer

Presumably, It writes directly to /dev/tty (at any rate, you can get the same behavior)

#!/bin/bash

# set up the new file descriptor
exec 3> /dev/tty

# test
echo "Stdout"
echo "Stderr" >&2
echo "Directly to tty" >&3

alternatively, you can simply do:

echo "Directly to tty" >/dev/tty

$ ./foo.sh >/dev/null 2>/dev/null
Directly to tty

read still works if you do this.

The exec is required to keep the redirection for the duration of the present shell.

A redirection on a simple command:

$ echo yes       3>file

lasts while the command is being executed. Once the command (echo in this example) ends, the shell removes the redirection and reverts back to the "present shell" execution environment.

A:

$ 3>file

is still a "simple command" where the command executed is "none", the redirection will not live for long.

Instead in:

$ exec 3>file

the exec replaces the "present shell" with a new one which includes the redirection. That makes the redirection stay alive for as long as the "present shell" exists. That could be undone (well, actually close fd 3) with:

$ exec 3>&-
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