explicitly connect /dev/tty. Allows running via sh

[sabine]

It should be super simple and quick to install opam on various Linux distros and macOS. Some of the targeted OSes might not have the bash command needed for the invocation

bash -c "sh <(curl -fsSL https://raw.githubusercontent.com/ocaml/opam/master/shell/install.sh)"

Instead, install.sh should be invokable via

curl -fsSL https://raw.githubusercontent.com/ocaml/opam/master/shell/install.sh | sh

However, a problem with that is that, when piping the downloaded script into sh, /dev/tty is not connected, so we cannot read from stdin.

This patch remedies this issue by changing install.sh to explicitly read from /dev/tty. It also factors out the prompting into a function prompt.

Note: if any of the other commands called by install.sh require user interaction via stdin, they should also be explicitly connected to /dev/tty. I have not checked whether they do!

[kit-ty-kate]

Do we know if anyone is using the install script in an automated way (e.g. (echo /usr/bin/ ; yes) | bash -c "sh <(curl -fsSL https://raw.githubusercontent.com/ocaml/opam/master/shell/install.sh)")? If some do this change will break them as it would make it very hard to pipe any kind of input to this script.

[sabine]

This is a good question. There is the possibility to extend this patch to conditionally connect /dev/tty based on a new flag, so that breaking automated scripts could be fixed by passing the new flag to this script. At least, that's what the rustup install script does. 🤔

[sabine]

I found a way to conditionally connect /dev/tty in a backwards-compatible way by using an environment variable that must be set in order to explicitly connect to /dev/tty.

Then, the script can be invoked as:

curl -fsSL https://raw.githubusercontent.com/ocaml/opam/master/shell/install.sh | set TTY=1 sh

This is backwards-compatible for automated scripts, as explicitly connecting to /dev/tty is opt-in instead of opt-out.

[kit-ty-kate]

Then, the script can be invoked as:

curl -fsSL https://raw.githubusercontent.com/ocaml/opam/master/shell/install.sh | set TTY=1 sh

set does not do anything but I believe what you want is env instead (which is POSIX-complient)

[kit-ty-kate]

This PR is queued on #6313. There will be a couple of things to do to make it correct afterwards too, i'll handle it.
I'll also change the TTY env variable to a --tty argument to make it even more backward compatible and more fitting to the rest of the script. It has also the advantage to guide users on how to pass extra parameters when using the | sh method. E.g.:

curl -fsSL https://opam.ocaml.org/install.sh | sh -s -- --tty --version 2.2.0

and without any extra parameters:

curl -fsSL https://opam.ocaml.org/install.sh | sh -s -- --tty