Rscript: how is $ interpreted in strings passed as inline code

Question

Does anyone know how $ is interpreted within an inline call to rscript?

In the example below, I am trying to get z to be the "a" element within the x list. However, z=x$a gives me x instead of the element of the list.

> Rscript -e "x = list(a=c(1,2,3), b=c(4,5,6)); z = x$a; print(z); print(\"z$a\")"
$a
[1] 1 2 3

$b
[1] 4 5 6

[1] "z"

Notice that even in the print statement, the $ causes somewhat obscure behavior. I would have expected this to print z$a, but it prints z instead.

Two questions:

This likely has more to do with your shell than Rscript. What shell are you running? Sounds like the value is being interpreted before Rscript even sees it. — MrFlick, May 18 '15 at 18:18
Then this is a problem with bash doing variable substitution on your string. I don't use bash myself but i think you can prevent expansion with single quotes: `Rscript -e 'x = list(a=c(1,2,3), b=c(4,5,6)); z = x$a; print(z); print ("z$a")'` — MrFlick, May 18 '15 at 18:24
@MrFlick: Thanks. Do I understand it correctly that bash is substituting $a by the value of the shell variable a, which, since it is not defined, is defaulted to $a=""? — W7GVR, May 18 '15 at 19:21
Yes, that's consistent with your results. (I don't use bash as my primary shell so i'm not as familiar with what's returned for undefined variables) — MrFlick, May 18 '15 at 19:25

score 1 · Accepted Answer · answered May 18 '15 at 18:19

1

To answer you second question, you could do x[['a']]

answered May 18 '15 at 18:19

Quinn Weber

Thanks for the alternative: much cleaner than using \$ and, likely, less shell specific. – W7GVR May 19 '15 at 10:42

score 0 · Answer 2 · answered May 18 '15 at 18:12

0

It seems that using \$ instead of $ in the inline code does the trick.

>/lrlhps/apps/R/R-3.1.1/bin/Rscript -e "x = list(a=c(1,2,3), b=c(4,5,6)); z = x\$a; print(z); print(\"z\$a\")"
[1] 1 2 3
[1] "z$a"

Still, does anyone what "$" does?

answered May 18 '15 at 18:12

W7GVR

2 Answers2