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Lua: how to check if a process is running

I would like to start a process from Lua and check if the process continues.

[EDIT] I know launch can be done with os: execute, but that is blocking. I would like to find a way to start a process without blocking and monitor if it keeps running.

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The following Lua library provides functions (asynchronously) for starting and monitoring processes.



http://stevedonovan.github.io/winapi/

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One dumb way would be to just use io.popen () and monitor its output, or process them yourself using some other Linux tool like ps .



Another way would be to use some Lua POSIX bindings like POSIX.signal and POSIX.unistd # fork ()

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If you are using luajit, you can use external function interface to call OS api functions directly. for example for Linux or similar.

local ffi = require("ffi")
local C = ffi.C
ffi.cdef[[
  int fork(void);
  int execlp(const char* file, const char *arg, ...);
  int waitpid(int pid, int *status, int options);
  void _exit(int status);
  unsigned int sleep(unsigned int seconds);
]]

local pid = C.fork()

if pid > 0 then -- parent
  print("Waiting for child process:");
  local status = ffi.new('int[1]')
  local WNOHANG = 1
  local done = false
  while not done do
    local id = C.waitpid(-1, status, WNOHANG)
    if id == pid then
      done = true
    elseif pid < 0 then
      print("error occurred")
      os.exit(-1)
    else
      print("status=",status[0])
      C.sleep(1)
      -- do other stuff. We aren't waiting
    end
  end
  print("Child exited with status=", status[0])
elseif pid == 0 then -- child
  print("Starting child")
  C.execlp('sleep', 'sleep', '5')
  C._exit(-1)   -- exec never returns
elseif pid < 0 then -- failure
  print("failed to fork")
end

You will see that with WNOHANG = 1 you can still get the result to see if the child exited, but then keep doing other things.

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Coroutines in Lua allow you to do what you want (since Lua 5.0):

local function Body()
    print( "First resume" )

    coroutine.yield()

    print( "Second resume" )

    coroutine.yield()

    print( "Final resume" )
end

local co = coroutine.create( Body )

print( type( co ) )  -- prints "thread"

coroutine.resume( co )  -- prints "First resume"

print( "After first yield" )

coroutine.resume( co )  -- prints "Second resume"

print( "After second yield" )

coroutine.resume( co )  -- prints "Final resume"

print( coroutine.status( co ) ) -- "suspended", "dead"
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