Do you want to request a feature or report a bug? Bug.

What is the current behavior? Running yarn init yields:

$ yarn init
yarn init v1.0.2
error An unexpected error occurred: "Can't answer a question unless a user TTY".

What is the expected behavior? Standard init process.

Please mention your node.js, yarn and operating system version. Versions:

Git-bash
You are downloading the latest (2.14.1) 64-bit version of Git for Windows. 

Which seems to install MinTTY 2.7.7

$ node -v
v8.1.4

$ npm -v
5.0.3

$ yarn -v
1.0.2

$ bash --version
GNU bash, version 4.4.12(1)-release (x86_64-pc-msys)
Copyright (C) 2016 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>

This is free software; you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.

Background Research

The problem was previously noted in: https://github.com/yarnpkg/yarn/issues/743

This yielded a fix that tried to autodetect mintty and use winpty: https://github.com/yarnpkg/yarn/pull/2243

That fix worked for interactive operations but breaks any sort of piping.

This fix was removed in favor a explicit environment variable in: https://github.com/yarnpkg/yarn/issues/2998

The reason being yarn shell commands seemed to fail because of winpty when used in a pipe. And there appears to be some terminal output that shows git bash working without the need for winpty. This is not consistent with my experience.

Technical details

I believe yarn uses isTTY to check for an interactive terminal. The discussion attributed the issue to be caused by winpty however running node by itself can produce that error:

$  node -p -e "Boolean(process.stdout.isTTY)"
true

$  node -p -e "Boolean(process.stdout.isTTY)" | cat
stdout is not a tty

I think that's the check that's actually causing the issue when any yarn command is involved in a pipe. But weirdly using yarn in a pipe works for commands that are not interactive like --version.

Below is a bunch of commands I ran on the terminal that I hope help diagnose the issue.

$ yarn --version | cat
1.0.2

$ yarn init
yarn init v1.0.2
error An unexpected error occurred: "Can't answer a question unless a user TTY".
info If you think this is a bug, please open a bug report with the information provided in "C:\\Users\\jmathew\\work\\FatGolem\\yarn-error.log".
info Visit https://yarnpkg.com/en/docs/cli/init for documentation about this command.

$ YARN_FORCE_WINPTY=1 yarn --version
1.0.2

$ YARN_FORCE_WINPTY=1 yarn --version  | cat
stdout is not a tty

$ bash -c 'yarn install'
yarn install v1.0.2
info No lockfile found.
[1/4] Resolving packages...
[2/4] Fetching packages...
[3/4] Linking dependencies...
[4/4] Building fresh packages...
info Lockfile not saved, no dependencies.
Done in 0.05s.

$ bash -c 'YARN_FORCE_WINPTY=1 yarn install'
yarn install v1.0.2
info No lockfile found.
[1/4] Resolving packages...
[2/4] Fetching packages...
[3/4] Linking dependencies...
[4/4] Building fresh packages...

info Lockfile not saved, no dependencies.
Done in 0.05s.

There's a lot of moving pieces so its hard for me to understand the cause and effect here. So I'm not sure of a good solution that keeps this interactive terminal check.

My initial thought is that this check for interactive terminal isn't really worth the hassle of keeping around. If you design a script that has yarn init and fail to provide input or redirect properly and it spins forever waiting that should be on you right? Npm doesn't do this and I suspect its because of these kinds of problems.

Summary

It would seem that I have to use the YARN_FORCE_WINPTY when using an interactive command in git bash and specifically not use that when trying to do any piping.

Further, YARN_FORCE_WINPTY isn't documented anywhere as far as I know. Other than the old issues and the script itself. Maybe add that to the error message?

Regardless of all that I just want to be able to type yarn init and have it work consistently without having to do anything special to make it work on windows.