Explain why mkfifo and popen work the way they do

[ericfranz]

$ cat foo.rb
puts "These are the args: #{ARGV.inspect}"
puts "This is STDIN: #{STDIN.read}"
$
$ cat bar.lua
print("hi")

local named_pipe = '/tmp/pun-boot-pipe-' .. math.random(0,10000000)
print("named pipe created: " .. named_pipe)
os.execute('mkfifo ' .. named_pipe)

local handle = io.popen("cat " .. named_pipe .. " | ruby foo.rb a b c", "r")

local stdin_handle = io.open(named_pipe, "w")
stdin_handle:write("1 2 3 4 5 6")
stdin_handle:close()

local output = handle:read("*a")
handle:close()

os.execute('rm ' .. named_pipe)

print("printing captured output")
print(output)
$
$ lua bar.lua
hi
named pipe created: /tmp/pun-boot-pipe-8401878
printing captured output
These are the args: ["a", "b", "c"]
This is STDIN: 1 2 3 4 5 6

First use os.tmpname(). Second, be able to explain this:

1. local output = handle:read("*a") does not execute till stdin_handle:write("1 2 3 4 5 6") and then stdin_handle:close() are complete and then local output = handle:read("*a") would not complete until local handle = io.popen("cat " .. named_pipe .. " | ruby foo.rb a b c", "r") completed
2. why placing local stdin_handle = io.open(named_pipe, "w") before local handle = io.popen("cat " .. named_pipe .. " | ruby foo.rb a b c", "r") causes the application to hang