R character value is null, but fails is.null() test - r

Question

Very strange error on a value being not null, but is claimed to be null, even though it evaluates to FALSE on is.null() test. See below. In this case, pid seems to be null, but the test fails, causing me all sorts of 'next step' problems in the code.
> pid <- system2('ps', args = "-ef | grep 'ssh -f' | grep -v grep | tr -s ' ' | \ cut -d ' ' -f 3", stdout = TRUE)
> pid
character(0)
> is.null(pid)
[1] FALSE
> if(!is.null(pid) && nchar(pid)) {cat('got some pid')}
Error in if (!is.null(pid) && nchar(pid)) { :
missing value where TRUE/FALSE needed
> if(!is.null(pid)) {cat('got some pid? Really?')}
got some pid? Really?
What does folks think is happening here? Here is my version information of R:
> version
_
platform x86_64-pc-linux-gnu
arch x86_64
os linux-gnu
system x86_64, linux-gnu
status
major 3
minor 2.2
year 2015
month 08
day 14
svn rev 69053
language R
version.string R version 3.2.2 (2015-08-14)
nickname Fire Safety
Full version of the OS:
Linux rserver 3.16.0-44-generic #59~14.04.1-Ubuntu SMP Tue Jul 7 15:07:27 UTC 2015 x86_64 x86_64 x86_64 GNU/Linux
In the end, I simply want this code to run:
> if (nchar(pid) > 0) {
+ cat('do something\n')
+ }
Error in if (nchar(pid) > 0) { : argument is of length zero

Answer 1

The fact that you have an empty character variable doesn't mean that it's NULL. Here's an example:
pid <- character()
> pid
character(0)
> is.null(pid)
[1] FALSE
> pid <- NULL
> pid
NULL
> is.null(pid)
[1] TRUE