I want to check if the output directory given to my R script is writable. But to check that, I also need to check the groups the current effective user belongs to, and I do not seem to be able to find out how to do that. The query does not google well either.

This is what I have so far

```
is.writable <- function(dir)
    {
        if(file.info(outputDir)['size'] == NA || file.info(outputDir)['isdir'] == FALSE)
            return FALSE
        mode <- file.info['mode']
        owner <- file.info['uname']
        fileGroup <- file.info['grname']
        user <- Sys.info()[["effective_user"]]

        if(bitwAnd(mode,2) != 0) # every can access
            return TRUE
        #if(bitwAnd(mode,16) != 0) # 16=0020 in octal, some group has access
        if(bitwAnd(mode,128) != 0) # 128 = 0200 in octal, the owner has write access
            return user == owner
    }

```

I used [this](https://stackoverflow.com/a/14104522/397405) to implement it. I would be able to do it on my own once I figure out how to get a vector of the groups the effective user belongs to. 

Thank you,