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)NA) which indicates whether to
capture the output of the command as an Rcharacter vector.NA)
indicating whether messages written to NA) indicating whether the Rinterpreter should wait for the command to finish, or run it
asynchronously. This will be ignored (and the interpreter will
always wait) if intern = TRUE.command is redirected to the file.NA), indicates
whether to capture the output of the command and show it on the Rconsole (not used by Rterm, which shows the output in the
terminal unless wait is false).NA), indicates whether a
command window should be displayed initially as a minimized window.NA), indicates whether a
command window should be visible on the screen.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 Rerror is generated.
#ifdef windows
Under the Rgui console intern = TRUE also captures
stderr unless ignore.stderr = TRUE.
#endif
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).
#ifdef windows
Some Windows commands return out-of-range status values
(e.g., -1) and so only the bottom 16 bits of the value are used.
If intern = FALSE, wait = TRUE, show.output.on.console = TRUE the
ignore.stdout = TRUE or
ignore.stderr = TRUE) output from a command that is a
Rgui) or the window running R(Rterm).
Not all Windows executables properly respect redirection of output, or
may only do so from a console application such as Rterm and not
from Rgui: for example,
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 intern = FALSE is interface-specific, and it
is unsafe to assume that such messages will appear on a GUI console
(they do on the OS X GUI's console, but not on some others).
Rgui or Rterm is being used, and whether a
console command or GUI application is run by the command.By default nothing will be seen in either front-end until the command finishes and the output is displayed.
For console commands Rgui will open a new invisible = FALSE, a commands window will appear for the
duration of the command. For Rterm a separate commands window
will appear for console applications only if wait = FALSE and
invisible = FALSE.
GUI applications will not display in either front-end unless
invisible is false.
It is possible to interrupt a running command being waited for from
the keyboard (using the Esc key in Rgui or Ctrl-C
in Rterm) or from the Rgui menu: this should at least
return control to the Rconsole. Rwill attempt to shut down the
process cleanly, but may need to force it to terminate, with the
possibility of losing unsaved work, etc.
Do not try to run console applications that require user
input from Rgui setting intern = TRUE or
show.output.on.console = TRUE. They will not work.
system behaves.
For the benefit of programmers, the more important ones are summarized
in this section.
systemlaunches a shell which then runscommand. On
Windows the command is run directly -- useshellfor an
interface which runscommandviaa shell (by default
the Windows shell This means that it cannot be assumed that redirection or piping will
work insystem(redirection sometimes does, but we have seen
cases where it stopped working after a Windows security patch), andsystem2(orshell) must be used on Windows.
stdoutandstderrwhen not
captured depends on howRis running: Windows batch commands behave
like a Unix-alike, but from the Windows GUI they are
generally lost.system(intern = TRUE)capturesignore.stderr =
TRUE.commanddiffer, butshQuoteis a portable interface.show.output.on.console,minimized,invisibleonly do something on Windows (and are most relevant
tosystem2 for a more portable and flexible interface
which is recommended for new code.
command is parsed as a command plus arguments separated by
spaces. So if the path to the command (or a single argument such as a
file path) contains spaces, it must be quoted e.g.shQuote.
#ifdef windows
Only double quotes are allowed on Windows: see the examples. (Note: a
Windows path name cannot contain a double quote, so we do not need to
worry about escaping embedded quotes.) command must be an executable (extensions shell if you want to pass a shell command-line.
The search path for command may be system-dependent: it will
include the Rcommand 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 Rcharacter 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.
#endif
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.
shell or shell.exec for a less raw
interface.
#endif
#ifdef unix
.Platform for platform-specific variables.
pipe to set up a pipe connection.
# launch an editor, wait for it to quit
system("notepad myfile.txt")
# launch your favourite shell:
system(Sys.getenv("COMSPEC"))
## note the two sets of quotes here:
system(paste('"c:/Program Files/Mozilla Firefox/firefox.exe"',
'-url cran.r-project.org'), wait = FALSE)Run the code above in your browser using DataCamp Workspace