Invoke a System Command
system invokes the OS command specified by
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the system command to be invoked, as a character string.
a logical (not
a logical (not
a logical (not
if a character vector is supplied, this is copied one
string per line to a temporary file, and the standard input of
arguments that are accepted on Windows but ignored on this platform, with a warning.
This interface has become rather complicated over the years: see
system2 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.\sspaceby
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
system does not use a shell and there is a separate
shell which passes command lines to a shell.
popen is used to invoke the
command and the output collected, line by line, into an R
character vector. If
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
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.
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.
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
intern = FALSE, the return value is an error code (
for success), given the invisible attribute (so needs to be printed
explicitly). If the command could not be run for any reason, the
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
intern = FALSEis 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).
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
For the benefit of programmers, the more important ones are summarized
in this section.
The most important difference is that on a Unix-alike
systemlaunches a shell which then runs
command. On Windows the command is run directly – use
shellfor an interface which runs
commandvia a shell (by default the Windows shell
cmd.exe, which has many differences from a POSIX shell).
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.
What happens to
stderrwhen not captured depends on how R is running: Windows batch commands behave like a Unix-alike, but from the Windows GUI they are generally lost.
system(intern = TRUE)captures ‘stderr’ when run from the Windows GUI console unless
ignore.stderr = TRUE.
The behaviour on error is different in subtle ways (and has differed between R versions).
The quoting conventions for
shQuoteis a portable interface.
invisibleonly do something on Windows (and are most relevant to
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.
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# 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.
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