From 62e86efb43a3c682a7ad2637a8cf031ff0e598c9 Mon Sep 17 00:00:00 2001
From: Jonas Arvidsson <jonas054@gmail.com>
Date: Wed, 30 Apr 2014 21:32:02 +0200
Subject: [PATCH] New cop UnneededPercentX

Part of #835.
---
 CHANGELOG.md                                  |  1 +
 config/enabled.yml                            |  4 +++
 lib/rubocop.rb                                |  1 +
 lib/rubocop/cop/style/unneeded_percent_x.rb   | 26 ++++++++++++++
 .../cop/style/unneeded_percent_x_spec.rb      | 34 +++++++++++++++++++
 5 files changed, 66 insertions(+)
 create mode 100644 lib/rubocop/cop/style/unneeded_percent_x.rb
 create mode 100644 spec/rubocop/cop/style/unneeded_percent_x_spec.rb

diff --git a/CHANGELOG.md b/CHANGELOG.md
index a46c5870c584..7c9b8851be36 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -8,6 +8,7 @@
 * Add new cop `EachWithObject` to prefer `each_with_object` over `inject` or `reduce`. ([@geniou][])
 * [#1010](https://github.com/bbatsov/rubocop/issues/1010): New Cop `Next` check for conditions at the end of an interation and propose to use `next` instead. ([@geniou][])
 * The `GuardClause` cop now also looks for unless and it is configurable how many lines the body of an if / unless needs to have to not be ignored. ([@geniou][])
+* [#835](https://github.com/bbatsov/rubocop/issues/835): New cop `UnneededPercentX` checks for `%x` when backquotes would do. ([@jonas054][])
 
 ### Changes
 
diff --git a/config/enabled.yml b/config/enabled.yml
index 8042f3c12791..00003764b173 100644
--- a/config/enabled.yml
+++ b/config/enabled.yml
@@ -541,6 +541,10 @@ UnneededCapitalW:
   Description: 'Checks for %W when interpolation is not needed.'
   Enabled: true
 
+UnneededPercentX:
+  Description: 'Checks for %x when `` would do.'
+  Enabled: true
+
 VariableInterpolation:
   Description: >-
                  Don't interpolate global, instance and class variables
diff --git a/lib/rubocop.rb b/lib/rubocop.rb
index d5b4e40fc84a..11dfbbe5738f 100644
--- a/lib/rubocop.rb
+++ b/lib/rubocop.rb
@@ -219,6 +219,7 @@
 require 'rubocop/cop/style/trivial_accessors'
 require 'rubocop/cop/style/unless_else'
 require 'rubocop/cop/style/unneeded_capital_w'
+require 'rubocop/cop/style/unneeded_percent_x'
 require 'rubocop/cop/style/variable_interpolation'
 require 'rubocop/cop/style/variable_name'
 require 'rubocop/cop/style/when_then'
diff --git a/lib/rubocop/cop/style/unneeded_percent_x.rb b/lib/rubocop/cop/style/unneeded_percent_x.rb
new file mode 100644
index 000000000000..51209c9f2ebe
--- /dev/null
+++ b/lib/rubocop/cop/style/unneeded_percent_x.rb
@@ -0,0 +1,26 @@
+# encoding: utf-8
+
+module Rubocop
+  module Cop
+    module Style
+      # This cop checks for usage of the %x() syntax when `` would do.
+      class UnneededPercentX < Cop
+        MSG = 'Do not use `%x` unless the command string contains backquotes.'
+
+        def on_xstr(node)
+          add_offense(node, :expression) if node.loc.expression.source !~ /`/
+        end
+
+        private
+
+        def autocorrect(node)
+          string, = *node
+          @corrections << lambda do |corrector|
+            corrector.replace(node.loc.expression,
+                              "`#{string.loc.expression.source}`")
+          end
+        end
+      end
+    end
+  end
+end
diff --git a/spec/rubocop/cop/style/unneeded_percent_x_spec.rb b/spec/rubocop/cop/style/unneeded_percent_x_spec.rb
new file mode 100644
index 000000000000..7b2daeb589dc
--- /dev/null
+++ b/spec/rubocop/cop/style/unneeded_percent_x_spec.rb
@@ -0,0 +1,34 @@
+# encoding: utf-8
+
+require 'spec_helper'
+
+describe Rubocop::Cop::Style::UnneededPercentX do
+  subject(:cop) { described_class.new }
+
+  it 'registers an offense for a %x string without backquotes' do
+    inspect_source(cop, '%x(ls)')
+    expect(cop.messages)
+      .to eq(['Do not use `%x` unless the command string contains ' \
+              'backquotes.'])
+  end
+
+  it 'accepts a %x string with backquotes' do
+    inspect_source(cop, '%x(echo `ls`)')
+    expect(cop.offenses).to be_empty
+  end
+
+  it 'accepts a `` string without inner backquotes' do
+    inspect_source(cop, '`ls`')
+    expect(cop.offenses).to be_empty
+  end
+
+  it 'accepts a `` string with inner backquotes' do
+    inspect_source(cop, '`echo \`ls\``')
+    expect(cop.offenses).to be_empty
+  end
+
+  it 'auto-corrects' do
+    new_source = autocorrect_source(cop, '%x(ls)')
+    expect(new_source).to eq('`ls`')
+  end
+end