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.
system
launches a shell which then runscommand
. On
Windows the command is run directly -- useshell
for an
interface which runscommand
viaa 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.
stdout
andstderr
when 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
.command
differ, butshQuote
is a portable interface.show.output.on.console
,minimized
,invisible
only 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)
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