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ExCmd is an Elixir library to run and communicate with external programs with back-pressure mechanism. It makes use os provided stdio buffer for this.

Communication with external program using Port is not demand driven. So it is easy to run into memory issues when the size of the data we are writing or reading from the external program is large. ExCmd tries to solve this problem by making better use of os provided stdio buffers and providing demand-driven interface to write and read from external program. It can be used to stream data through an external program. For example, streaming a video through ffmpeg to serve a web request.

Getting audio out of a video stream is as simple as

ExCmd.stream!(~w(ffmpeg -i pipe:0 -f mp3 pipe:1), input: File.stream!("music_video.mkv", [], 65336))
|> Stream.into(File.stream!("music.mp3"))
|> Stream.run()

major-features

Major Features

  • Unlike beam ports, ExCmd puts back pressure on the external program
  • Stream abstraction
  • No separate shim installation required
  • Ships pre-built binaries for MacOS, Windows, Linux
  • Proper program termination. No more zombie process
  • Ability to close stdin and wait for output (with ports one can not selectively close stdin)

If you are not interested in streaming capability, ExCmd can still be useful because of the features listed above. For example running command and getting output as a string

ExCmd.stream!(~w(curl ifconfig.co))
|> Enum.into("")

If you want to use shell to handle more complex pipelines and globs, you can just spawn shell process and pass your shell command as the argument

cmd = "echo 'foo baar' | base64"
ExCmd.stream!(["sh", "-c", cmd])
|> Enum.into("")

Refer documentation for information

Check out Exile which is an alternative solution based on NIF without middleware overhead

installation

Installation

def deps do
  [
    {:ex_cmd, "~> x.x.x"}
  ]
end