shell runs the command specified by cmd, usually under
a shell, because that is what the POSIX standards require.shell(cmd, shell, flag = "/c", intern = FALSE, wait = TRUE,
translate = FALSE, mustWork = FALSE, ...)NULL (no shell). If missing, a suitable shell is chosen:
see ‘Details’.bash or tcsh or sh the default is changed to
"-c".intern = TRUE.TRUE, "/" in cmd is
translated to "\".TRUE failure to run the command
will give an R error, if FALSE a warning and if NA,
no R message.system.intern = TRUE, a character vector giving the output of the
command, one line per character string, or an error message if the
command could not be run. If intern = FALSE, the return value is an error code, given the
invisible attribute (so needs to be printed explicitly). If the
command could not be run for any reason, the value is -1 and
an R warning is generated. Otherwise if wait = FALSE the value
is the error code returned by the command, and if wait = TRUE
it is the zero (the conventional success value), If intern = FALSE and wait = TRUE (the defaults) the text
output from a command that is a console application will appear in
the R console (Rgui) or the window running R (Rterm).shell is specified, the environment variables
R_SHELL and COMSPEC are tried in turn: COMSPEC
should always succeed. Using shell = NULL invokes the command
cmd directly, in which case an extension of .exe is
assumed. It is possible to use batch files directly if their
extension is given: Windows (rather than R) then chooses a shell. See system for fuller details: shell is a more
user-friendly wrapper for system. To make use of Windows
file associations, use shell.exec.system, shell.exec