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ExCmd is an Elixir library to run and communicate with external programs with back-pressure mechanism. It makes use os backed 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 backed 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

  • 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)

Examples 

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

Binary as input

  ExCmd.stream!(~w(cat), input: "Hello World")
  |> Enum.into("")
  # => "Hello World"
  ExCmd.stream!(~w(base64), input: <<1, 2, 3, 4, 5>>)
  |> Enum.into("")
  # => "AQIDBAU=\n"

List of binary as input

  ExCmd.stream!(~w(cat), input: ["Hello ", "World"])
  |> Enum.into("")
  # => "Hello World"

iodata as input

  ExCmd.stream!(~w(base64), input: [<<1, 2,>>, [3], [<<4, 5>>]])
  |> Enum.into("")
  # => "AQIDBAU=\n"

If you want pipes and globs, you can spawn shell process and pass your pipeline as argument

cmd = "echo 'foo bar' | base64"
ExCmd.stream!(["sh", "-c", cmd])
|> Enum.into("")
# => "Zm9vIGJhcgo=\n"

Read stream documentation for information about parameters.

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

Installation 

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