Append to the .pry_history file instead of overwriting it.

This is more consistent with what other shells do, and allows you to
keep history from multiple consecutively-open Pry windows.

lib/pry/pry_class.rb

@@ -140,13 +140,27 @@ class Pry
   # Load Readline history if required.
   def self.load_history
     Readline::HISTORY.push(*File.readlines(history_file).map(&:chomp)) if File.exists?(history_file)
+    @loaded_history = Readline::HISTORY.to_a
   end
 
-  # Save Readline history if required.
+  # Save new lines of Readline history if required.
   def self.save_history
-    File.open(history_file, 'w') do |f|
-      f.write Readline::HISTORY.to_a.join("\n")
+    history_to_save = Readline::HISTORY.to_a
+
+    # Omit any history we read from the file.This check is needed because
+    # `hist --clear` would otherwise cause us to not save history in this
+    # session.
+    if history_to_save[0...@loaded_history.size] == @loaded_history
+      history_to_save = history_to_save[@loaded_history.size..-1]
     end
+
+    File.open(history_file, 'a') do |f|
+      f.puts history_to_save.join("\n") if history_to_save.size > 0
+    end
+
+    # Update @loaded_history so that future calls to save_history
+    # will do the right thing.
+    @loaded_history = Readline::HISTORY.to_a
   end
 
   # Get the full path of the history_path for pry.

test/test_pry_history.rb

@@ -0,0 +1,78 @@
+require 'helper'
+require 'tempfile'
+
+describe Pry do
+
+  before do
+    Readline::HISTORY.shift while Readline::HISTORY.length > 0
+    @hist = Tempfile.new(["tmp", ".pry_history"]).tap(&:close).path
+    File.open(@hist, 'w') {|f| f << "1\n2\n3\n" }
+    @old_hist = Pry.config.history.file
+    Pry.config.history.file = @hist
+    Pry.load_history
+  end
+
+  after do
+    File.unlink @hist
+    Pry.config.history.file = @old_hist
+  end
+
+  describe ".load_history" do
+    it "should read the contents of the file" do
+      Readline::HISTORY.to_a.should === ["1", "2", "3"]
+    end
+  end
+
+  describe ".save_history" do
+    it "should include a trailing newline" do
+      Readline::HISTORY << "4"
+      Pry.save_history
+      File.read(@hist).should =~ /4\n\z/
+    end
+
+    it "should not change anything if history is not changed" do
+      File.open(@hist, 'w') {|f| f << "4\n5\n6\n" }
+      Pry.save_history
+      File.read(@hist).should == "4\n5\n6\n"
+    end
+
+    it "should append new lines to the file" do
+      Readline::HISTORY << "4"
+      Pry.save_history
+      File.read(@hist).should == "1\n2\n3\n4\n"
+    end
+
+    it "should not clobber lines written by other Pry's in the meantime" do
+      Readline::HISTORY << "5"
+      File.open(@hist, 'a') {|f| f << "4\n" }
+      Pry.save_history
+
+      Readline::HISTORY.to_a.should == ["1", "2", "3", "5"]
+      File.read(@hist).should == "1\n2\n3\n4\n5\n"
+    end
+
+    it "should not delete lines from the file if this session's history was cleared" do
+      Readline::HISTORY.pop while Readline::HISTORY.size > 0
+      Pry.save_history
+      File.read(@hist).should == "1\n2\n3\n"
+    end
+
+    it "should save new lines that are added after the history was cleared" do
+      Readline::HISTORY.pop while Readline::HISTORY.size > 0
+      Readline::HISTORY << "4"
+      Pry.save_history
+      File.read(@hist).should == "1\n2\n3\n4\n"
+    end
+
+    it "should only append new lines the second time it is saved" do
+      Readline::HISTORY << "4"
+      Pry.save_history
+      File.open(@hist, 'a') {|f| f << "5\n" }
+      Readline::HISTORY << "6"
+      Pry.save_history
+
+      Readline::HISTORY.to_a.should == ["1", "2", "3", "4", "6"]
+      File.read(@hist).should == "1\n2\n3\n4\n5\n6\n"
+    end
+  end
+end