[R] escape characters in shell commands

[James Rome]

On a Windows platform I am trying to count the number of lines in a file.
In a DOS window, the following works:
C:\Users\jar>findstr /R /N "^" D:\my_dir\my_file | find /C ":"
5317
(it works with double \\ also)

But in R, I need to make this string up with the file name I get from
file.choose():
filename = file.choose()
#get the number of lines in the file
# first make a command string with the filename in it
cmd = paste('findstr /R /N "^" ', filename, ' | find /C ":"', sep=""')
nrec = as.numeric(shell(cmd))

But R puts in escape characters for the ":
> cmd
[1] "findstr /R /N \"^\" D:\\my_dir\\my_file | find /C \":\""

and shell(cmd) does not work properly. And
> nrec=shell('findstr /R /N "^"D:\\my_dir\\my_file | find /C ":"')
> nrec
[1] 0
does not work either.

What am I doing wrong?

Thanks,
Jim