Quote Strings for Use in OS Shells

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Description

Quote a string to be passed to an operating system shell.

Usage

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shQuote(string, type = c("sh", "csh", "cmd", "cmd2"))

Arguments

string

a character vector, usually of length one.

type

character: the type of shell quoting. Partial matching is supported. "cmd" and "cmd2" refer to the Windows shell. "cmd" is the default under Windows.

Details

The default type of quoting supported under Unix-alikes is that for the Bourne shell sh. If the string does not contain single quotes, we can just surround it with single quotes. Otherwise, the string is surrounded in double quotes, which suppresses all special meanings of metacharacters except dollar, backquote and backslash, so these (and of course double quote) are preceded by backslash. This type of quoting is also appropriate for bash, ksh and zsh.

The other type of quoting is for the C-shell (csh and tcsh). Once again, if the string does not contain single quotes, we can just surround it with single quotes. If it does contain single quotes, we can use double quotes provided it does not contain dollar or backquote (and we need to escape backslash, exclamation mark and double quote). As a last resort, we need to split the string into pieces not containing single quotes and surround each with single quotes, and the single quotes with double quotes.

In Windows, command line interpretation is done by the application as well as the shell. It may depend on the compiler used: Microsoft's rules for the C run-time are given at https://msdn.microsoft.com/library/ms880421. It may depend on the whim of the programmer of the application: check its documentation. The type = "cmd" quoting surrounds the string by double quotes and escapes internal double quotes by a backslash. As Windows path names cannot contain double quotes, this makes shQuote safe for use with many applications when used with system or system2. The Windows cmd.exe shell (used by default with shell) uses type = "cmd2" quoting: special characters are prefixed with "^". In some cases, two types of quoting should be used: first for the application, and then type = "cmd2" for cmd.exe. See the examples below.

References

Loukides, M. et al (2002) Unix Power Tools Third Edition. O'Reilly. Section 27.12.

Discussion in \Sexpr[results=rd]{tools:::Rd_expr_PR(16636)}.

See Also

Quotes for quoting R code.

sQuote for quoting English text.

Examples

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test <- "abc$def`gh`i\\j"
cat(shQuote(test), "\n")
## Not run: system(paste("echo", shQuote(test)))
test <- "don't do it!"
cat(shQuote(test), "\n")

tryit <- paste("use the", sQuote("-c"), "switch\nlike this")
cat(shQuote(tryit), "\n")
## Not run: system(paste("echo", shQuote(tryit)))
cat(shQuote(tryit, type = "csh"), "\n")

## Windows-only example, assuming cmd.exe:
perlcmd <- 'print "Hello World\\n";'
## Not run: 
shell(shQuote(paste("perl -e", 
                    shQuote(perlcmd, type = "cmd")),
              type = "cmd2"))

## End(Not run)

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