system {base}

R Documentation

Invoke a System Command

Description

system invokes the OS command specified by command.

Usage

system(command, intern = FALSE, ignore.stderr = FALSE,
       wait = TRUE, input = NULL, show.output.on.console = TRUE,
       minimized = FALSE, invisible = TRUE)

Arguments

command	the system command to be invoked, as a string.
intern	a logical (not NA) which indicates whether to make the output of the command an R object. Not available unless popen is supported on the platform.
ignore.stderr	a logical indicating whether error messages written to ‘stderr’ should be ignored.
wait	a logical indicating whether the R interpreter should wait for the command to finish, or run it asynchronously. This will be ignored (and the interpreter will always wait) if intern = TRUE.
input	if a character vector is supplied, this is copied one string per line to a temporary file, and the standard input of command is redirected to the file.
show.output.on.console, minimized, invisible	arguments that are accepted on other platforms but ignored on this one, with a warning.

Details

command is parsed as a command plus arguments separated by spaces. So if the path to the command (or a filepath argument) contains spaces, it must be quoted e.g. by shQuote.

How the command is run differs by platform: Unix-alikes use a shell (‘/bin/sh’ by default), and Windows executes the command directly (extensions ‘.exe’, ‘.com’) or as a batch file (extensions ‘.cmd’ and ‘.bat’).

If intern is TRUE then popen is used to invoke the command and the output collected, line by line, into an R character vector. If intern is FALSE then the C function system is used to invoke the command.

The ordering of arguments after the first two has changed from time to time: it is recommended to name all arguments after the first.

Value

If intern = TRUE, a character vector giving the output of the command, one line per character string. (Output lines of more than 8095 characters will be split.) If the command could not be run or gives an error this will be reported on the shell's ‘stderr’ (unless popen is not supported, when there is an R error).

If intern = FALSE, the return value is an error code (0 for success), given the invisible attribute (so needs to be printed explicitly). If the command could not be run for any reason, the value is 256*127 = 52512. Otherwise if wait = TRUE the value is 256 times the error code returned by the command, and if wait = FALSE it is 0 (the conventional success value).

Stdout and stderr

Error messages written to ‘stderr’ will be sent by the shell to the terminal unless ignore.stderr = TRUE. They can be captured (in the most likely shells) by

    system("some command 2>&1", intern=TRUE)

What happens to output sent to ‘stdout’ and ‘stderr’ if intern = FALSE is interface-specific, and it is unsafe to assume that such messages will appear on a console (they do on the Mac OS X console, but not on some others).

Note

wait is implemented by appending & to the command: this is shell-dependent, but required by POSIX and so widely supported.

See Also

.Platform for platform-specific variables.

Examples

# list all files in the current directory using the -F flag
## Not run: system("ls -F")

# t1 is a character vector, each one
# representing a separate line of output from who
# (if the platform has popen and who)
t1 <- try(system("who", intern = TRUE))

try(system("ls fizzlipuzzli", intern = TRUE, ignore.stderr = TRUE))
# empty since file doesn't exist

---