system {base}R Documentation

Invoke a System Command

Description

system invokes the OS command specified by command.

Usage

system(command, intern = FALSE,
       ignore.stdout = 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 character string.

intern

a logical (not NA) which indicates whether to capture the output of the command as an R character vector.

ignore.stdout, ignore.stderr

a logical (not NA) indicating whether messages written to ‘stdout’ or ‘stderr’ should be ignored.

wait

a logical (not NA) 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 Windows but ignored on this platform, with a warning.

Details

command is parsed as a command plus arguments separated by spaces. So if the path to the command (or an argument) contains spaces, it must be quoted e.g. by shQuote. Unix-alikes pass the command line to a shell (normally ‘/bin/sh’, and POSIX requires that shell), so command can be anything the shell regards as executable, including shell scripts, and it can contain multiple commands separated by ;.

On Windows, system does not use a shell and there is a separate function shell which passes command lines to a shell.

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.

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

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

There are many pitfalls in using system to ascertain if a command can be run — Sys.which is more suitable.

Value

If intern = TRUE, a character vector giving the output of the command, one line per character string. (Output lines of more than 8095 bytes will be split.) If the command could not be run an R error is generated. If command runs but gives a non-zero exit status this will be reported with a warning and in the attribute "status" of the result: an attribute "errmsg" may also be available

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 127. Otherwise if wait = TRUE the value is the exit status returned by the command, and if wait = FALSE it is 0 (the conventional success value).

Stdout and stderr

For command-line R, error messages written to ‘stderr’ will be sent to the terminal unless ignore.stderr = TRUE. They can be captured (in the most likely shells) by

    system("some command 2>&1", intern = TRUE)
For GUIs, what happens to output sent to ‘stdout’ or ‘stderr’ if intern = FALSE is interface-specific, and it is unsafe to assume that such messages will appear on a GUI console (they do on the Mac OS X console, but not on some others).

Differences between Unix and Windows

How processes are launched differs fundamentally between Windows and Unix-alike operating systems, as do the higher-level OS functions on which this R function is built. So it should not be surprising that there are many differences between OSes in how system behaves. For the benefit of programmers, the more important ones are summarized in this section.

See Also

system2.

man system and man sh for how this is implemented on the OS in use.

.Platform for platform-specific variables.

pipe to set up a pipe connection.

Examples

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

# t1 is a character vector, each element giving a line of output from who
# (if the platform has who)
t1 <- try(system("who", intern = TRUE))

try(system("ls fizzlipuzzli", intern = TRUE, ignore.stderr = TRUE))
# zero-length result since file does not exist, and will give warning.

[Package base version 2.15.3 Index]