unit testing - Mock system call in ruby

Question

Know of a way to mock %[]? I'm writing tests for code that makes a few system calls, for example:

def log(file)
  %x[git log #{file}]
end

and would like to avoid actually executing system calls while testing this method. Ideally I'd like to mock %x[..] and assert that the correct shell command is passed to it.

Answer 1

%x{…} is Ruby built-in syntax that will actually call Kernel method Backtick (`). So you can redefine that method. As backtick method returns the standard output of running cmd in a subshell, your redefined method should return something similar to that ,for example, a string.

module Kernel
    def `(cmd)
        "call #{cmd}"
    end
end

puts %x(ls)
puts `ls`
# output
# call ls
# call ls