shQuote and cat
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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