sys (version 3.4.3)

exec: Running System Commands

Description

Powerful replacements for system2 with support for interruptions, background tasks and fine grained control over STDOUT / STDERR binary or text streams.

Usage

exec_wait(
  cmd,
  args = NULL,
  std_out = stdout(),
  std_err = stderr(),
  std_in = NULL,
  timeout = 0
)
exec_background(
  cmd,
  args = NULL,
  std_out = TRUE,
  std_err = TRUE,
  std_in = NULL
)
exec_internal(cmd, args = NULL, std_in = NULL, error = TRUE, timeout = 0)
exec_status(pid, wait = TRUE)

Value

exec_background returns a pid. exec_wait returns an exit code. exec_internal returns a list with exit code, stdout and stderr strings.

Arguments

cmd

the command to run. Either a full path or the name of a program on the PATH. On Windows this is automatically converted to a short path using Sys.which, unless wrapped in I().

args

character vector of arguments to pass. On Windows these automatically get quoted using windows_quote, unless the value is wrapped in I().

std_out

if and where to direct child process STDOUT. Must be one of TRUE, FALSE, filename, connection object or callback function. See section on Output Streams below for details.

std_err

if and where to direct child process STDERR. Must be one of TRUE, FALSE, filename, connection object or callback function. See section on Output Streams below for details.

std_in

file path to map std_in

timeout

maximum time in seconds

error

automatically raise an error if the exit status is non-zero.

pid

integer with a process ID

wait

block until the process completes

Output Streams

The std_out and std_err parameters are used to control how output streams of the child are processed. Possible values for both foreground and background processes are:

• TRUE: print child output in R console

• FALSE: suppress output stream

• string: name or path of file to redirect output

In addition the exec_wait function also supports the following std_out and std_err types:

• connection a writable R connection object such as stdout or stderr

• function: callback function with one argument accepting a raw vector (use as_text to convert to text).

When using exec_background with std_out = TRUE or std_err = TRUE on Windows, separate threads are used to print output. This works in RStudio and RTerm but not in RGui because the latter has a custom I/O mechanism. Directing output to a file is usually the safest option.

Details

Each value within the args vector will automatically be quoted when needed; you should not quote arguments yourself. Doing so anyway could lead to the value being quoted twice on some platforms.

The exec_wait function runs a system command and waits for the child process to exit. When the child process completes normally (either success or error) it returns with the program exit code. Otherwise (if the child process gets aborted) R raises an error. The R user can interrupt the program by sending SIGINT (press ESC or CTRL+C) in which case the child process tree is properly terminated. Output streams STDOUT and STDERR are piped back to the parent process and can be sent to a connection or callback function. See the section on Output Streams below for details.

The exec_background function starts the program and immediately returns the PID of the child process. This is useful for running a server daemon or background process. Because this is non-blocking, std_out and std_out can only be TRUE/FALSE or a file path. The state of the process can be checked with exec_status which returns the exit status, or NA if the process is still running. If wait = TRUE then exec_status blocks until the process completes (but can be interrupted). The child can be killed with tools::pskill.

The exec_internal function is a convenience wrapper around exec_wait which automatically captures output streams and raises an error if execution fails. Upon success it returns a list with status code, and raw vectors containing stdout and stderr data (use as_text for converting to text).

See Also

Base system2 and pipe provide other methods for running a system command with output.

Other sys: exec_r

Examples

# Run a command (interrupt with CTRL+C)
status <- exec_wait("date")

# Capture std/out
out <- exec_internal("date")
print(out$status)
cat(as_text(out$stdout))

if(nchar(Sys.which("ping"))){

# Run a background process (daemon)
pid <- exec_background("ping", "localhost")

# Kill it after a while
Sys.sleep(2)
tools::pskill(pid)

# Cleans up the zombie proc
exec_status(pid)
rm(pid)
}