| system {base} | R Documentation |
system invokes the OS command specified by command.
system(command, intern = FALSE,
ignore.stdout = FALSE, ignore.stderr = FALSE,
wait = TRUE, input = NULL, show.output.on.console = TRUE,
minimized = FALSE, invisible = TRUE)
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. |
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.
How the command is run differs by platform: Unix-alikes use a shell
(normally ‘/bin/sh’), and Windows executes the command directly.
command can be anything the shell regards as executable,
including shell scripts.
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.
There are many pitfalls in using system to ascertain if a
command can be run — Sys.which is more suitable.
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 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).
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).
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.
system does is to launch a shell which then runs
command. On Windows the command is run directly – use
shell for an interface which runs comamnd via a
shell (by default the Windows shell cmd.exe).
This means that it cannot be assumed that redirection or piping will
work in system (redirection sometimes does, but we have seen
cases where it stopped working after a Windows security patch), and
shell must be used on Windows.
command differ, but
shQuote is a portable interface.
show.output.on.console, minimized,
invisible only do something on Windows (and are most relevant
to a GUI interface there).
wait is implemented by appending & to the command: this
is shell-dependent, but required by POSIX and so widely supported.
system2.
.Platform for platform-specific variables.
pipe to set up a pipe connection.
# 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 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.