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How can you prompt a user for input, when the output is being piped?

> (echo -n 'input data: '; read TEST; export TEST) | echo "done: ${TEST}"

This does not show the "input data" prompt, and does not read anything from stdin, and instead, just displays:

done:
Josh M.
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1 Answers1

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In foo | bar foo and bar run simultaneously. Your code displays done: because the final echo does not read its stdin and it does not wait for anything piped via its stdin. It does its job right away.

Additionally ${TEST} after | has nothing to do with the variable your read sets before |. When building a pipeline, Bash runs each command in a separate subshell (the last one may or may not be executed in the context of the main shell).

This is what I think you want:

echo -n 'input data: '; read TEST
echo "done: ${TEST}"

(in Bash read -p 'input data: ' TEST will also work).

There is no need to pipe anything to anything here.

In general, in case you need to interact with the user from within a pipe (I mean from within a genuinely needed pipe), /dev/tty may be useful. Artificial example:

echo foo | ( read -p 'i: ' var </dev/tty; echo "d: $var" >/dev/tty; cat ) | nl

Here read does not read from the first echo, it reads form /dev/tty. Neither read nor the second echo prints to nl: read -p prints its prompt to stderr by default, echo prints to /dev/tty because we redirected. In effect the data (foo+newline) goes from echo to cat to nl as if nothing else was there.

It may look like a trick, but's it's a legitimate use case. E.g. see How/why does ssh output to tty when both stdout and stderr are redirected? and my answer there.

Kamil Maciorowski
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