How to pass shell variables to a command that reads console input in a script?

Question

I'm trying to make a script where I use a command that ask for the user to input some data in the prompt.

Let's say that I want to include git push in a script (I know it isn't best practice, but everyone is going to understand what I'm asking this way.)

So when you use git push in the console, it asks for user and password. How can I pass them when git is called from a shell script, and both are stored as shell variables?

It should look something like

#!/bin/bash

git push
$user
$password

Answer 1

The easiest way seems to pipe it to the external command:

printf "%s\n%s\n" "$user" "$password" | command_expecting_username_and_passwd

This will output the content of $user and $password, each followed by a newline, and redirect the output to the stdin of the external process which should expect username and password in that order to be entered by the user if it were called interactively.

If you want to perform more sophisticated operations, you may want to look into expect scripts.

Note that, as stated by Kusalananda, this only works if the program is reading these from their stdin and not directly from the terminal device (which however seems to be usual when asking for authentication information), and will not work when actually applying it to git (which you presented as one usage example).

Answer 2

The kind of tool to you are looking for is expect.

https://linux.die.net/man/1/expect

It is a tool to automate interaction with a TTY.

Keep in mind that it is not secure, and to push to github without a password, you should use ssh

git remote add origin git@github.com:name/repo

though the remote "origin" is probably already taken in your repository.

https://docs.github.com/en/free-pro-team@latest/github/authenticating-to-github/connecting-to-github-with-ssh

Once you have set a ssh public and private key and you added them to your ssh agent, you won't need to type a password to push or pull