proposal: os: add UserShell

[twpayne]

Go has added several functions for determining the user's configuration:

• os.UserCacheDir in os: add func UserCacheDir() string #22536 (Go 1.11).
• os.UserHomeDir in proposal: os: add UserHomeDir #26463 (Go 1.12).
• os.UserConfigDir in proposal: os: add UserConfigDir #29960 (Go 1.13).

Closely related to this is getting the user's shell, which is very OS-dependent, and surprisingly tricky to accomplish (especially if cgo is not available, for example when cross-compiling). Go's behavior of falling back to parsing local files (e.g. /etc/passwd) when cgo is not available has led to a number of bugs in applications (e.g. #24383, twpayne/chezmoi#65).

Being able to invoke the user's shell is useful in interactive applications (e.g. VSCode's interactive terminal), command line applications (e.g. chezmoi's cd command, or any application that launches a user-specified command on the user's behalf.

There are existing packages to do this, for example:

• github.com/riywo/loginshell
• github.com/twpayne/go-shell

This proposal is concretely:

Add a os.UserShell function that returns the current user's shell.

Optionally, the os.UserShell function could return some indication of the level of confidence in the returned result (if the current user's shell could not be determined, maybe it makes sense to fall back to an OS-dependent default shell).

[mvdan]

The os.User*Dir funcs are easy to use; you get a directory, so you can create more directories underneath. I'm not so sure how the API would be generally useful here.

For example, if the API returns /bin/sh or cmd.exe, how should one call that with any arguments in a portable way? Or is it only meant to be called with no arguments, to then be used interactively or with standard input?

[ianlancetaylor]

What is the use case here? When do you specifically want to run the user's shell?

The packages you point to seem to be looking in /etc/passwd for the shell, which seems to mean that if we added this functionality it should go in the os/user package, not the os package.

[rsc]

This seems too system specific.
Also, on Unix, you should use $SHELL, not /etc/passwd.

[twpayne]

Handling responses:

For example, if the API returns /bin/sh or cmd.exe, how should one call that with any arguments in a portable way? Or is it only meant to be called with no arguments, to then be used interactively or with standard input?

What is the use case here? When do you specifically want to run the user's shell?

The arguments can be supplied by the user, for example in any kind of hook ("when I do X, invoke my shell to do Y"). Interactive and/or standard input uses are also valid use cases, e.g. for any applications that provide an embedded shell.

The packages you point to seem to be looking in /etc/passwd for the shell, which seems to mean that if we added this functionality it should go in the os/user package, not the os package.

The suggestion for putting it in the os package is for consistency with the os.User*Dir functions. It would also fit in the os/user package.

The packages you point to seem to be looking in /etc/passwd for the shell...
This seems too system specific.
Also, on Unix, you should use $SHELL, not /etc/passwd.

The method used to determine the user's shell include:

• Calling the getpwnam_r library function, if available.
• Invoking a command that does the same (e.g. getent on Linux, dscl on macOS).
• Reading /etc/passwd (on some systems, e.g. Termux, the current user doesn't have an entry).
• Reading the $SHELL environment variable (assuming the user's environment is set up correctly).

In short, this is a superficially simple function that hides a lot of system-specific underlying complexity from the caller.

[ianlancetaylor]

Given that we need to call getpwnam_r, it must go in os/user, not os. I don't see any particular similarity to os.UserHomeDir, etc., as those are all directories.

[ianlancetaylor]

That said, it's not clear that this needs to go into the standard library. The argument that programs will use the user's shell to execute scripts seems very special purpose, as that can only be done for scripts provided by the user. Why not just have the user provide the shell at the same time?

Also, https://golang.org/doc/faq#x_in_std .

[twpayne]

Given that we need to call getpwnam_r, it must go in os/user, not os. I don't see any particular similarity to os.UserHomeDir, etc., as those are all directories.

OK for putting it in os/user.

Go's standard library provides nearly all the functionality to ready the /etc/passwd and /etc/group databases - with the exception of the user's shell. It could be argued that the user's shell is an even more fundamental attribute than say the user's config directory or cache directory.

That all said, I do realize that the use cases for needing to know the user's shell are limited and so am happy if this proposal is closed unaccepted. Thank you for considering it.

[rsc]

This seems too special purpose. It can be done outside the standard library.
It seems like a likely decline to me.

[twpayne]

Ack for decline. Thank you for considering it :)

Closing this issue.