shQuote and cat
On Jul 23, 2008, at 4:46 PM, 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.
Well, you input is not right ('\"' is parsed in R into '"') - you have
to escape the backslash that you want to keep and all is well:
> x <- 'echo "\\"a\\""'
> cat(x, '\n')
echo "\"a\""
> cat(shQuote(x), '\n')
'echo "\"a\""'
> system(paste("sh -c",shQuote(x,type="sh")))
"a"
Cheers,
Simon
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"
"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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