Managing external processes from R is not trivial, and this
class aims to help with this deficiency. It is essentially a small
wrapper around the system
base R function, to return the process
id of the started process, and set its standard output and error
streams. The process id is then used to manage the process.
p <- process$new(command = NULL, args, stdout = NULL, stderr = NULL, cleanup = TRUE, echo_cmd = FALSE, supervise = FALSE, windows_verbatim_args = FALSE, windows_hide_window = FALSE, encoding = "", post_process = NULL)p$is_alive() p$signal(signal) p$kill(grace = 0.1) p$wait(timeout = -1) p$get_pid() p$get_exit_status() p$restart() p$get_start_time()
p$read_output(n = -1) p$read_error(n = -1) p$read_output_lines(n = -1) p$read_error_lines(n = -1) p$get_output_connection() p$get_error_connection() p$is_incomplete_output() p$is_incomplete_error() p$read_all_output() p$read_all_error() p$read_all_output_lines() p$read_all_error_lines()
p$poll_io(timeout)
p$get_result()
print(p)
p
: process
object.
command
: Character scalar, the command to run.
Note that this argument is not passed to a shell, so no
tilde-expansion or variable substitution is performed on it.
It should not be quoted with base::shQuote()
. See
base::normalizePath()
for tilde-expansion.
args
: Character vector, arguments to the command. They will be
used as is, without a shell. They don't need to be escaped.
stdout
: What to do with the standard output. Possible values:
NULL
: discard it; a string, redirect it to this file;
"|"
: create a connection for it.
stderr
: What to do with the standard error. Possible values:
NULL
: discard it; a string, redirect it to this file;
"|"
: create a connection for it.
cleanup
: Whether to kill the process (and its children)
if the process
object is garbage collected.
echo_cmd
: Whether to print the command to the screen before
running it.
supervise
: Whether to register the process with a supervisor.
If TRUE
, the supervisor will ensure that the process is
killed when the R process exits.
windows_verbatim_args
: Whether to omit quoting the arguments
on Windows. It is ignored on other platforms.
windows_hide_window
: Whether to hide the application's window
on Windows. It is ignored on other platforms.
signal
: An integer scalar, the id of the signal to send to
the process. See tools::pskill()
for the list of signals.
grace
: Currently not used.
timeout
: Timeout in milliseconds, for the wait or the I/O
polling.
n
: Number of characters or lines to read.
encoding
: The encoding to assume for stdout
and
stderr
. By default the encoding of the current locale is
used. Note that callr
always reencodes the output of
both streams in UTF-8 currently. If you want to read them
without any conversion, on all platforms, specify "UTF-8"
as
encoding.
post_process
: An optional function to run when the process has
finished. Currently it only runs if $get_result()
is called.
It is only run once.
The poll_io()
function polls the standard output and standard
error connections of a process, with a timeout. If there is output
in either of them, or they are closed (e.g. because the process exits)
poll_io()
returns immediately.
In addition to polling a single process, the poll()
function
can poll the output of several processes, and returns as soon as any
of them has generated output (or exited).
$new()
starts a new process in the background, and then returns
immediately.
$is_alive()
checks if the process is alive. Returns a logical
scalar.
$signal()
sends a signal to the process. On Windows only the
SIGINT
, SIGTERM
and SIGKILL
signals are interpreted,
and the special 0 signal, The first three all kill the process. The 0
signal return TRUE
if the process is alive, and FALSE
otherwise. On Unix all signals are supported that the OS supports, and
the 0 signal as well.
$kill()
kills the process. It also kills all of its child
processes, except if they have created a new process group (on Unix),
or job object (on Windows). It returns TRUE
if the process
was killed, and FALSE
if it was no killed (because it was
already finished/dead when callr
tried to kill it).
$wait()
waits until the process finishes, or a timeout happens.
Note that if the process never finishes, and the timeout is infinite
(the default), then R will never regain control. It returns
the process itself, invisibly. In some rare cases, $wait()
might take
a bit longer than specified to time out. This happens on Unix, when
another package overwrites the processx SIGCHLD signal handler, after the
processx process has started. One such package is parallel, if used
with fork clusters, e.g. through parallel::mcparallel()
.
$get_pid()
returns the process id of the process.
$get_exit_status
returns the exit code of the process if it has
finished and NULL
otherwise. On Unix, in some rare cases, the exit
status might be NA
. This happens if another package (or R itself)
overwrites the processx SIGCHLD handler, after the processx process
has started. In these cases processx cannot determine the real exit
status of the process. One such package is parallel, if used with
fork clusters, e.g. through the parallel::mcparallel()
function.
$restart()
restarts a process. It returns the process itself.
$get_start_time()
returns the time when the process was
started.
$is_supervised()
returns whether the process is being tracked by
supervisor process.
$supervise()
if passed TRUE
, tells the supervisor to start
tracking the process. If FALSE
, tells the supervisor to stop
tracking the process. Note that even if the supervisor is disabled for a
process, if it was started with cleanup=TRUE
, the process will
still be killed when the object is garbage collected.
$read_output()
reads from the standard output connection of the
process. If the standard output connection was not requested, then
then it returns an error. It uses a non-blocking text connection. This
will work only if stdout="|"
was used. Otherwise, it will throw an
error.
$read_error()
is similar to $read_output
, but it reads
from the standard error stream.
$read_output_lines()
reads lines from standard output connection
of the process. If the standard output connection was not requested, then
then it returns an error. It uses a non-blocking text connection. This
will work only if stdout="|"
was used. Otherwise, it will throw an
error.
$read_error_lines()
is similar to $read_output_lines
, but
it reads from the standard error stream.
$has_output_connection()
returns TRUE
if there is a connection
object for standard output; in other words, if stdout="|"
. It returns
FALSE
otherwise.
$has_error_connection()
returns TRUE
if there is a connection
object for standard error; in other words, if stderr="|"
. It returns
FALSE
otherwise.
$get_output_connection()
returns a connection object, to the
standard output stream of the process.
$get_error_conneciton()
returns a connection object, to the
standard error stream of the process.
$is_incomplete_output()
return FALSE
if the other end of
the standard output connection was closed (most probably because the
process exited). It return TRUE
otherwise.
$is_incomplete_error()
return FALSE
if the other end of
the standard error connection was closed (most probably because the
process exited). It return TRUE
otherwise.
$read_all_output()
waits for all standard output from the process.
It does not return until the process has finished.
Note that this process involves waiting for the process to finish,
polling for I/O and potentically several readLines()
calls.
It returns a character scalar. This will return content only if
stdout="|"
was used. Otherwise, it will throw an error.
$read_all_error()
waits for all standard error from the process.
It does not return until the process has finished.
Note that this process involves waiting for the process to finish,
polling for I/O and potentically several readLines()
calls.
It returns a character scalar. This will return content only if
stderr="|"
was used. Otherwise, it will throw an error.
$read_all_output_lines()
waits for all standard output lines
from a process. It does not return until the process has finished.
Note that this process involves waiting for the process to finish,
polling for I/O and potentically several readLines()
calls.
It returns a character vector. This will return content only if
stdout="|"
was used. Otherwise, it will throw an error.
$read_all_error_lines()
waits for all standard error lines from
a process. It does not return until the process has finished.
Note that this process involves waiting for the process to finish,
polling for I/O and potentically several readLines()
calls.
It returns a character vector. This will return content only if
stderr="|"
was used. Otherwise, it will throw an error.
$get_output_file()
if the stdout
argument was a filename,
this returns the absolute path to the file. If stdout
was "|"
or
NULL
, this simply returns that value.
$get_error_file()
if the stderr
argument was a filename,
this returns the absolute path to the file. If stderr
was "|"
or
NULL
, this simply returns that value.
$poll_io()
polls the process's connections for I/O. See more in
the Polling section, and see also the poll()
function
to poll on multiple processes.
$get_result()
returns the result of the post processesing function.
It can only be called once the process has finished. If the process has
no post-processing function, then NULL
is returned.
print(p)
or p$print()
shows some information about the
process on the screen, whether it is running and it's process id, etc.
# NOT RUN {
# CRAN does not like long-running examples
# }
# NOT RUN {
p <- process$new("sleep", "2")
p$is_alive()
p
p$kill()
p$is_alive()
p$restart()
p$is_alive()
Sys.sleep(3)
p$is_alive()
# }
# NOT RUN {
# }
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