stderr📺 https://youtu.be/Q2fm7lzALO8
Often when looping over input (from stdin) you might want to prompt
the user for input but you no longer can use stdin for that because it
is in use. Consider the following (broken) example:
#!/usr/bin/bash
while IFS= read -r line; do
read -p "Echo '$line'? " resp
[[ $resp =~ ^[yY] ]] || continue
echo "$line"
done < <(ls)
The code above does not wait to prompt you. Instead, it reads the
prompt immediately from the next line of stdin (another ls line).
That’s probably not what you were expecting.
The fix (thanks to the Internet) is to tell read that you want to read
input from stderr instead of stdin:
read -u 2 -p "Echo '$line'? " resp
That’s probably the most counter-intuitive thing you have heard all day and it’s the reason people don’t trust bash for production software, and they shouldn’t (but neither should you trust any loose scripting language including Python, Perl, and JavaScript). Those eccentricities might be permissible for personal and prototype projects, but you want to avoid that kind of magic at all costs in production software (which is where a beautifully unsexy, strongly typed, battle-hardened language like Golang comes in).