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Wait until one of the specified connections or processes produce standard output or error, terminates, or a timeout occurs.


poll(processes, ms)



A list of connection objects orprocess objects to wait on. (They can be mixed as well.) If this is a named list, then the returned list will have the same names. This simplifies the identification of the processes.


Integer scalar, a timeout for the polling, in milliseconds. Supply -1 for an infitite timeout, and 0 for not waiting at all.


A list of character vectors of length one or three. There is one list element for each connection/process, in the same order as in the input list. For connections the result is a single string scalar. For processes the character vectors' elements are named output, error and process. Possible values for each individual result are: nopipe, ready, timeout, closed, silent. See details about these below. process refers to the poll connection, see the poll_connection argument of the process initializer.

Explanation of the return values

  • nopipe means that the stdout or stderr from this process was not captured.

  • ready means that the connection or the stdout or stderr from this process are ready to read from. Note that end-of-file on these outputs also triggers ready.

  • timeout`: the connections or processes are not ready to read from and a timeout happened.

  • closed: the connection was already closed, before the polling started.

  • silent: the connection is not ready to read from, but another connection was.


if (FALSE) {
# Different commands to run for windows and unix
cmd1 <- switch(
  "unix" = c("sh", "-c", "sleep 1; ls"),
  c("cmd", "/c", "ping -n 2 && dir /b")
cmd2 <- switch(
  "unix" = c("sh", "-c", "sleep 2; ls 1>&2"),
  c("cmd", "/c", "ping -n 2 && dir /b 1>&2")

## Run them. p1 writes to stdout, p2 to stderr, after some sleep
p1 <- process$new(cmd1[1], cmd1[-1], stdout = "|")
p2 <- process$new(cmd2[1], cmd2[-1], stderr = "|")

## Nothing to read initially
poll(list(p1 = p1, p2 = p2), 0)

## Wait until p1 finishes. Now p1 has some output
poll(list(p1 = p1, p2 = p2), -1)

## Close p1's connection, p2 will have output on stderr, eventually
poll(list(p1 = p1, p2 = p2), -1)

## Close p2's connection as well, no nothing to poll
poll(list(p1 = p1, p2 = p2), 0)