TIL about testing commands

This is a walk-with-me-while-I-do-things series, I'm going to tell you what I learned while I'm learning

If this was our program

# my_tool.rb # Removes and returns the first item from the command-line arguments command = ARGV.shift puts command 

you can test this code using RSpec with

# specs/my_tool_spec.rb RSpec.describe "my_tool" do it "says hello" do expect { system "ruby my_tool.rb hello" }. to output("hello\n"). to_stdout_from_any_process end end 

Here we have three fundamental pieces:

1. the method Kernel#system that invokes a command as if we were on the command line itself. The argument passed as a string will be the command being executed;

2. the RSpec matcher output that checks against the expectation and passes if the expectation block on which it is invoked outputs to_stdout or to_stderr the string passed as an argument;

3. the to_stdout_from_any_process that we have to use instead of to_sdtdout because the system call is actually spawning a subprocess, and RSpec wouldn't be able to access $stdout otherwise.

If we weren't testing a command line invocation, we could have written

expect { print "Same process" }. to output("Same process"). to_stdout 

More sources are here. To use RSpec for capturing the output and where you can go to the official doc.