Why does a Bash script with select exit without input when reading from stdin?

Question

I'm working on a Bash script that optionally accepts input from stdin and then presents the user with a selection menu using select. The issue arises when the script is provided data via stdin—the select menu displays but exits immediately without accepting any input. It works fine when no stdin data is provided.

Here is a minimal example:

#!/usr/bin/env bash

if [[ -p /dev/stdin && ${#bar[@]} -eq 0 ]]; then
    while IFS= read -r foo; do
        bar+=("$foo")
    done </dev/stdin
fi

for foo in "${bar[@]}"; do
    echo "$foo"
done

select thing in foo bar baz; do
    case $thing in
        *)  echo "You have selected $thing"; break;;
    esac
done

Execution Without stdin The script works as expected:

$ ./script.sh
1) foo
2) bar
3) baz
#? 2
You have selected bar

Execution With stdin The issue occurs when data is piped into the script:

$ printf '%s\n' foo bar | ./script.sh
foo
bar
1) foo
2) bar
3) baz
#?
$

As shown, the script exits the select loop immediately after displaying the menu.

What I've Tried:

• Confirming the script's behavior when no stdin data is provided.
• Redirecting stdin in various ways (e.g., /dev/tty) with limited success.

Question

Why does the script's select menu exit immediately when stdin data is provided, and how can I fix this so the select menu works regardless of whether stdin is used?

Answer 1

You've "used up" all of stdin by the time the select is executed, so it returns with status 1 and nothing selected

Perhaps you were wanting to force interactivity by reading from /dev/tty?

#!/usr/bin/env bash
if [ ! -t 0 ]; then
    while IFS= read -r foo; do
        bar+=("$foo")
    done
fi

for foo in "${bar[@]}"; do
    echo "$foo"
done

select thing in foo bar baz; do
    case $thing in
        *)  echo "You have selected $thing"; break;;
    esac
done </dev/tty