Poll for process I/O or termination

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

poll(processes, ms)

A list of process objects to wait on. If this is a named list, then the returned list will have the same names. This simplifies the identification of the processes. If an empty list, then the


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 two. There is one list element for each process, in the same order as in the input list. The character vectors' elements are named output and error and their possible values are: nopipe, ready, timeout, closed, silent. See details about these below.

Explanation of the return values

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

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

  • timeout`: the 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.

Known issues

poll() cannot wait on the termination of a process directly. It is only signalled through the closed stdout and stderr pipes. This means that if both stdout and stderr are ignored or closed for a process, then you will not be notified when it exits. If you want to wait for just a single process to end, it can be done with the $wait() method.

  • poll
## 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)
# }
Documentation reproduced from package callr, version 2.0.0, License: MIT + file LICENSE

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