Question

In Bash, I want to compare the fields of 2 different CSVs (field 2 of file1 and field 3 of file2):

diff <(cut -d, -f2 file1) <(cut -d, -f3 file2)

I tried to implement this more generally in Ruby:

def identical_files?(file1, field1, file2, field2)                                                                                                              
  %x{diff <(cut -d, -f#{field1} #{file1}) <(cut -d, -f#{field2} #{file2})}.blank?                                   
end

Printing the output of the %x{} block, I see sh: Syntax error: "(" unexpected. Does I/O redirection not work when running shell commands within Ruby? Is this because it’s only supported by bash but not sh?

Answer 1

It doesn’t work because, as the error you’re getting indicates, Ruby shells out to sh, not Bash. And, of course, sh does not support that syntax.

You can instead call Bash explicitly:

`bash -c 'cat <(echo foo)'`  #=> "foo"