This script removes and recreates all simulators in Xcode 6.

simulator_populator

#!/usr/bin/env ruby

device_types_output = `xcrun simctl list devicetypes`
device_types = device_types_output.scan /(.*) \((.*)\)/

runtimes_output = `xcrun simctl list runtimes`
runtimes = runtimes_output.scan /(.*) \(.*\) \((com.apple[^)]+)\)$/

devices_output = `xcrun simctl list devices`
devices = devices_output.scan /\s\s\s\s(.*) \(([^)]+)\) (.*)/

devices.each do |device|
  puts "Removing device #{device[0]} (#{device[1]})"
  `xcrun simctl delete #{device[1]}`
end

device_types.each do |device_type|
  runtimes.each do |runtime|
    puts "Creating #{device_type} with #{runtime}"
    command = "xcrun simctl create '#{device_type[0]} #{runtime[0]}' #{device_type[1]} #{runtime[1]}"
    command_output = `#{command}`
    sleep 0.5
  end
end

The bash commands are probably simpler but you can try this for a easier yet more complex solution:
Per @saniul
gem install snapshot; snapshot reset_simulators

https://github.com/KrauseFx/snapshot

Per SO:
killall Xcode

sudo killall -9 com.apple.CoreSimulator.CoreSimulatorService

rm -rf ~/Library/Developer/CoreSimulator/Devices

open /Applications/Xcode.app

http://stackoverflow.com/a/31421158/142358

This worked brilliantly!

Awesome...it works great...thank you so much..... before I tried deleting old Xcode and install new Xcode..thinking some thing wrong with Xcode...that didn't solve the problem..but this script did.

Thank you guys! both solutions work great!

I can confirm the solution by @tinyfrog works. Thanks!

still good. thanks

You should use the --json parameter to get machine processable output.

xcrun simctl list devices --json

In Xcode 9 Apple has changed the format of the output of xcrun simctl list runtimes, so to get it working again you'd need to update the regexp in line 7 to:
runtimes = runtimes_output.scan /(.*) \(.*\) - (com.apple.+)$/

In 2019 (and maybe >2019) do:

device_types_output = `xcrun simctl list devicetypes`
device_types = device_types_output.scan /(.*) \((.*)\)/


runtimes_output = `xcrun simctl list runtimes`
runtimes = runtimes_output.scan /(.*) \(.*\) - (com.apple[^)]+)$/

runtimes.each do |runtime|
  puts " #{runtime}"
end

devices_output = `xcrun simctl list devices`
devices = devices_output.scan /[A-Z0-9-]{36}/

devices.each do |device|
  puts "Removing device #{device}"
  `xcrun simctl delete #{device}`
end

device_types.each do |device_type|
  runtimes.each do |runtime|
    puts "Creating #{device_type} with #{runtime}"
    command = "xcrun simctl create '#{device_type[0]} #{runtime[0]}' #{device_type[1]} #{runtime[1]}"
    puts command
    command_output = `#{command}`
    sleep 0.5
  end
end

Or just for erase data:

#!/usr/bin/env ruby

devices_output = `xcrun simctl list devices`
devices = devices_output.scan /[A-Z0-9-]{36}/

devices.each do |device|
  puts "Erase device #{device} data"
  `xcrun simctl erase #{device}`
end