I have a script that can optionally take some input on STDIN and later presents the user with a few select lists. When data is input on STDIN the select list presents but exits without taking input, however it works fine when nothing is provided via stdin.

I have a simple 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

Executing without stdin:

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

Executing with stdin:

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

Does anyone know why this happens and how to prevent it?

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?