system2
Invoke a System Command
system2
invokes the OS command specified by command
.
Usage
system2(command, args = character(),
stdout = "", stderr = "", stdin = "", input = NULL,
env = character(), wait = TRUE,
minimized = FALSE, invisible = TRUE)
Arguments
- command
- the system command to be invoked, as a character string.
- args
- a character vector of arguments to
command
. - stdout, stderr
- where output to stdout or
stderr should be sent. Possible values are
""
, to the R console (the default),NULL
orFALSE
(discard output),TRUE
(capture the output in a character vector) or a character string naming a file. - stdin
- should input be diverted?
""
means the default, alternatively a character string naming a file. Ignored ifinput
is supplied. - 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. - env
- character vector of name=value strings to set environment variables.
- 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) ifstdout = TRUE
.
Details
Unlike system
, command
is always quoted by
shQuote
, so it must be a single command without arguments.
For details of how command
is found see system
.
On Windows, env
is only supported for commands such as
Some Unix commands (such as some implementations of ls
) change
their output if they consider it to be piped or redirected:
stdout = TRUE
uses a pipe whereas stdout =
"some_file_name"
uses redirection.
Because of the way it is implemented, on a Unix-alike stderr =
TRUE
implies stdout = TRUE
: a warning is given if this is
not what was specified.
Value
- If
stdout = TRUE
orstderr = 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. Ifcommand
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.In other cases, 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 is127
. Otherwise ifwait = TRUE
the value is the exit status returned by the command, and ifwait = FALSE
it is0
(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. #endif
Note
system2
is a more portable and flexible interface than
system
, introduced in R2.12.0. It allows redirection
of output without needing to invoke a shell on Windows, a portable way
to set environment variables for the execution of command
, and
finer control over the redirection of stdout
and stderr
.
Conversely, system
(and shell
on Windows) allows the
invocation of arbitrary command lines.
There is no guarantee that if stdout
and stderr
are both
TRUE
or the same file that the two streams will be interleaved
in order. This depends on both the buffering used by the command and
the OS.
See Also
system
.
#ifdef windows
shell
and shell.exec
.
#endif