[Rd] [R] shQuote and cat

[Duncan Murdoch]

Vadim Organovich wrote:
> It is precizely a shell command that I am trying to generate. To be specific let's try to have R 'output' the following shell command: 'echo "\"a\""'. This is is a valid command, at least in bash:
> bash-3.2$ echo "\"a\""
> "a"
>
> Now in R:
>   
>> x <- 'echo "\"a\""'
>> cat(x, '\n')
>>     
> echo ""a""
>   
>> cat(shQuote(x), '\n')
>>     
> "echo \"\"a\"\""
>
> Whichever way you do it it is not right. Again I think cat('echo "\"a\""') should be printing *echo "\"a\""* (asterics are not a part of the output)
>   

But you were running on Windows, so the default shell is CMD, not bash.  
shQuote doesn't support bash specifically, but I imagine it uses the 
same quoting rules as sh, so you should use shQuote('"a"', type="sh").  
On my machine that gives '"a"', and echo '"a"' prints "a" as you wanted.

I don't think you want to quote the "echo" in the command, only the 
string that you want it to print, i.e. for the full command you should use

 > cat(paste("echo", shQuote('"a"', type="sh")))
echo '"a"'

Duncan Murdoch
> ________________________________________
> From: Duncan Murdoch [murdoch at stats.uwo.ca]
> Sent: Wednesday, July 23, 2008 2:38 PM
> To: Vadim Organovich
> Cc: r-devel at r-project.org
> Subject: Re: [Rd] [R] shQuote and cat
>
> On 7/23/2008 2:53 PM, Vadim Organovich wrote:
>   
>> I feel like it now belongs to r-devel more than to r-help.
>>
>> My output was garbled because I sent the original message as HTML, sorry about that.
>>
>> Your output, "\"\\"a\\"\"", is what I get too. That is
>>  > cat(shQuote(shQuote(shQuote("a"))), '\n')
>> "\"\\"a\\"\""
>> , which I think should be "\"\\\"a\\\"\"".
>>
>> Actually, the R's output, "\"\\"a\\"\"", is not even a valid string literal, that is
>>     
>>> x <- "\"\\"a\\"\""
>>>       
>> fails.
>>     
>
> It's not intended to be a string literal in R, it's intended to be input
> to the Windows CMD shell.  If you want a string literal in R, don't use
> cat().  cat() shows you the naked contents of the string without any
> quoting to make it a valid string literal.
>
>   
>> Now, by cat() being the inverse of shQuote() I mean printing the same literal as it goes into shQuote, quotes included:
>>     
>>> cat(shQuote("a"), '\n')
>>>       
>> "a"
>>
>> whereas
>>     
>>> cat("a", '\n')
>>>       
>> a  ## no quotes
>>
>> If cat() is not the inverse of shQuote() in the above sense, what is?
>>     
>
> On Unix-like systems I think asking the shell to echo the output, i.e.
>
> system(paste("echo", shQuote(input)), intern=TRUE)
>
> is intended to reproduce the input.  However, the Windows CMD shell is
> different.  I don't know how to strip quotes off a string in it, e.g. I see
>
> C:\WINDOWS\system32 echo "a"
> "a"
>
> Nevertheless, the quotes *are* necessary when passing filenames to
> commands, and they'll typically be stripped off when doing that, e.g.
>
> echo hi >"a"
>
> will create a file named a, not named "a", and
>
> echo hi >"a b"
>
> will create a file with a space in the name.
>
> Duncan Murdoch
>
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