Read ouput line by line from bash command(process bigfile) in Linux

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Welcome to the Crystal community, @orangeSi!

The issue seems to be that IO::Memory stores its capacity as an Int32, so once you’ve read 2GB into the buffer, its size overflows to become a negative number. I don’t know if this is the exact problem you’re experiencing, but it seems pretty likely.

With very few exceptions, I doubt anyone actually needs to hold that much data in a single buffer, and it looks like you can avoid holding the entire output in memory by using IO.pipe instead of an IO::Memory and passing a block to Process.run:

reader, writer = IO.pipe
Process.run "cat xx.txt|cut -f 1-6|sort -k 1,1", shell: true, output: writer do |process|
  until process.terminated?
    line = reader.gets
    puts line
  end
end

Notice this code creates two IOs with IO.pipe — we tell the child process to write to one end and our code reads from the other end.

IO::Memory isn’t a good fit for streaming both input and output at the same time, especially across processes since it only has a single position marker for both reading and writing — that is, if you call io.puts "foo", calling io.gets won’t read "foo" since your position in the buffer is at the end of the buffer you just wrote. It also doesn’t release memory after it’s been read. That’s a feature (it’s what allows io.rewind), it just doesn’t scale well for huge inputs. :-D

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