[Rd] [R] shQuote and cat

Vadim Organovich vogranovich at jumptrading.com
Wed Jul 23 22:46:07 CEST 2008

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\""

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)

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"

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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