How do you construct a read-write pipe with lua?

Question

I\'d like to do the equivalent of:

foo=$(echo \"$foo\"|someprogram)

within lua -- ie, I\'ve got a variable containing a bunch of text, and

Answer 1

As long as your Lua supports io.popen, this problem is easy. The solution is exactly as you have outlined, except instead of $(...) you need a function like this one:

function os.capture(cmd, raw)
  local f = assert(io.popen(cmd, 'r'))
  local s = assert(f:read('*a'))
  f:close()
  if raw then return s end
  s = string.gsub(s, '^%s+', '')
  s = string.gsub(s, '%s+$', '')
  s = string.gsub(s, '[\n\r]+', ' ')
  return s
end

You can then call

local foo = ...
local cmd = ("echo $foo | someprogram"):gsub('$foo', foo)
foo = os.capture(cmd)

I do stuff like this all the time. Here's a related useful function for forming commands:

local quote_me = '[^%w%+%-%=%@%_%/]' -- complement (needn't quote)
local strfind = string.find

function os.quote(s)
  if strfind(s, quote_me) or s == '' then
    return "'" .. string.gsub(s, "'", [['"'"']]) .. "'"
  else
    return s
  end
end