How to read a portion of a file in Elixir (or Erlang)?

Question

How can we read only a selected portion of a file?

Like this: Read(file, start, length)

Answer 1

You can use :file.position/2 and :file.read/2 .

With:

$ seq 10 > 10.txt

and code:

{:ok, file} = :file.open("10.txt", [:read, :binary])
:file.position(file, 5)
IO.inspect :file.read(file, 10)

The output is:

{:ok, "\n4\n5\n6\n7\n8"}

That's 10 bytes starting at the 6th byte.

Answer 2

It would be handy if you read the documentation . For example file:pread/2,3 .

read(File, Start, Length) ->
    {ok, F} = file:open(File, [binary]),
    try file:pread(F, [{Start, Length}]) of
        {ok, [Data]} -> Data
    after file:close(F)
    end.

Answer 3

This would be the code that Hynek shared transcribed to Elixir. I only post it as an answer because it's a bit long to put into a comment.

def read(file, start, length) do
  {ok, f} = :file.open(file, [:binary])
  {ok, data} = :file.pread(f, start, length)
  :file.close(f)
  data
end

Yes, granted it'd be nice to have this in the Elixir file module. If you really want it there @CharlesO, why don't you go ahead and create a pull request? Jose and the other core committers are some of the friendliest folks I've run across in years and years of software development.