InteractiveCmd

Run interactive shell commands from mostly pure Elixir

This addresses an issue when writing commandline scripts that need to invoke commands that require user input. Examples include commands that ask for passwords like ssh and sudo, menu-driven commands, and launching text editors.

This library works by using Erlang's command line editor feature launches an editor to provide input to the shell prompt. You can try this by typing Ctrl-o or Meta-o at the IEx prompt assuming your OS doesn't already have a mapping for that key combination. Since this functionality isn't provided by a public API, standard caveats apply. Luckily, this works in quite a few OTP releases. This library also verifies that it works in CI. The main limitation is that it only works when the process group leader is backed by :user_drv. For scripting, this is not much of a limitation, but it wouldn't work when using Erlang's ssh server, for example. Huge thanks to ieQu1 on the Erlang Forum for the original idea.

Using this is simply replacing your calls to System.cmd/3 or System.shell/2 with the similarly named ones in InteractiveCmd. Output capture options are ignored.

Example

Here's an example of using InteractiveCmd.cmd/3 to let a user run commands in a Docker container created by an Elixir script.

InteractiveCmd Demo

Installation and use

InteractiveCmd only works on macOS and Linux.

The package can be installed by adding interactive_cmd to your list of dependencies in mix.exs:

def deps do
  [
    {:interactive_cmd, "~> 0.1.0"}
  ]
end

You will also need the stty and script utilities available on macOS and Linux. These are usually already installed. If using on a minimal Linux image, you may need to install util-linux.

Then in your code, do something like this on macOS:

# macOS version
iex> InteractiveCmd.shell("brew install nsnake")
...
✔︎ Bottle nsnake (3.0.1)                             [Downloaded  123.2KB/123.2KB]
==> Pouring nsnake--3.0.1.arm64_tahoe.bottle.1.tar.gz
{"", 0}

or on Linux:

iex> InteractiveCmd.shell("sudo apt install nsnake")
[sudo] password for fhunleth:
Reading package lists... Done
Building dependency tree... Done
...
{"", 0}

The final line is a tuple with the output and exit status, like you'd get from System.shell/2. Unlike System.shell/2, output is not captured and is instead streamed directly to the terminal, so the first element is always an empty string.

Now run the program you just installed:

iex> nsnake = "nsnake" # set to "/usr/games/nsnake" on Linux
iex> InteractiveCmd.cmd("nsnake", [])
...

Troubleshooting

Since this repurposes an Erlang shell feature, this library could break on newer Erlang versions. Please check the Erlang versions tested on CI to see which ones are expected to work.

Next, try running your command via System.cmd/3 to see if you get a better error message. If you get a better error message, please file an issue or send a PR to help others in the future.

If all else fails, file an issue with details about your system and the command you're trying to run. It will be super helpful if there's some way I can reproduce it.

FAQ

Can I use this in Escripts or Mix archives?

Yes. The implementation is one Elixir module with no native code. If you're writing a Mix archive, just copy interactive_cmd.ex to your project and rename the module name to vendor it.

What about security?

The normal caveats apply to running shell commands so be sure to scrub any untrusted sources. Additionally, InteractiveCmd.cmd/3 doesn't directly run the executable like System.cmd/3 does, so it's more susceptable to injection attacks than may be obvious. I've taken precautions to escape strings so easy injection attacks shouldn't work. If your use case is susceptible to malicious users, please take normal precautions.

Could this be rewritten in Erlang so that it could be used in other BEAM languages?

Yes. This totally makes sense as a pure Erlang library. I didn't think about it until I was almost done. It seems easy to do. If this interests you and you have time to help verify and add instructions to the README, please file an issue.

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