## What changes were proposed in this pull request?
This affects mostly running job from the driver in client mode when results are expected to be through stdout (which should be somewhat rare, but possible)
Before:
```
> a <- as.DataFrame(cars)
> b <- group_by(a, "dist")
> c <- count(b)
> sparkR.callJMethod(c$countjc, "explain", TRUE)
NULL
```
After:
```
> a <- as.DataFrame(cars)
> b <- group_by(a, "dist")
> c <- count(b)
> sparkR.callJMethod(c$countjc, "explain", TRUE)
count#11L
NULL
```
Now, `column.explain()` doesn't seem very useful (we can get more extensive output with `DataFrame.explain()`) but there are other more complex examples with calls of `println` in Scala/JVM side, that are getting dropped.
## How was this patch tested?
manual
Author: Felix Cheung <felixcheung_m@hotmail.com>
Closes#16670 from felixcheung/rjvmstdout.
## What changes were proposed in this pull request?
This PR corrects SparkR to use `shell()` instead of `system2()` on Windows.
Using `system2(...)` on Windows does not process windows file separator `\`. `shell(tralsate = TRUE, ...)` can treat this problem. So, this was changed to be chosen according to OS.
Existing tests were failed on Windows due to this problem. For example, those were failed.
```
8. Failure: sparkJars tag in SparkContext (test_includeJAR.R#34)
9. Failure: sparkJars tag in SparkContext (test_includeJAR.R#36)
```
The cases above were due to using of `system2`.
In addition, this PR also fixes some tests failed on Windows.
```
5. Failure: sparkJars sparkPackages as comma-separated strings (test_context.R#128)
6. Failure: sparkJars sparkPackages as comma-separated strings (test_context.R#131)
7. Failure: sparkJars sparkPackages as comma-separated strings (test_context.R#134)
```
The cases above were due to a weird behaviour of `normalizePath()`. On Linux, if the path does not exist, it just prints out the input but it prints out including the current path on Windows.
```r
# On Linus
path <- normalizePath("aa")
print(path)
[1] "aa"
# On Windows
path <- normalizePath("aa")
print(path)
[1] "C:\\Users\\aa"
```
## How was this patch tested?
Jenkins tests and manually tested in a Window machine as below:
Here is the [stdout](https://gist.github.com/HyukjinKwon/4bf35184f3a30f3bce987a58ec2bbbab) of testing.
Closes#7025
Author: hyukjinkwon <gurwls223@gmail.com>
Author: Hyukjin Kwon <gurwls223@gmail.com>
Author: Prakash PC <prakash.chinnu@gmail.com>
Closes#13165 from HyukjinKwon/pr/7025.
