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j0h
Asked: 2022-04-04 12:05:08 +0800 CST

when you login to the shell, what program handles evaluation of the credentials?

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when you first boot, and you're greeted with the warm glow of a black screen and flashing prompt, what program is that? What program handles evaluation of the credentials? Or rather, what component of the source code is it? I may be confusing myself, but I think the various user shells are loaded after user authentication.

... which might mean that bash is launched in single user mode, then bash checks credentials & launches a user shell? (I think this is how it works)

(Im interested in adding a feature to the credential checking process, but I'm looking at bash source code, and I'm not sure where to begin. )

command-line

1 Answers

  1. Best Answer

    I have only a rudimentary understanding myself, however I'll post this to get you started:

    First, you don't log in to a shell, you log in to a (virtual) terminal.

    • a getty program opens a tty port, prompts for a login name and invokes a login program (/bin/login by default - although Linux's agetty may be configured to use a non-standard login program)

    • the login program performs authentication and session setup, including initializing the environment and invoking the user's login shell.

    In current Ubuntu systems, getty is managed as a systemd service (ex. [email protected] for the virtual terminal tty1), and login uses the Pluggable Authentication Modules (PAM) subsystem for authentication - in particular, the pam_unix module is "for traditional password authentication".

    For your stated goal ("adding a feature to the credential checking process") you would probably want to look at implementing a custom PAM module, and inserting it into an appropriate place in the PAM stack; a template for that is described here:

    See also:

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