ios simulator appear with UDID in xcode 6

killall Xcode

– Kills all running Xcode processes.

sudo killall -9 com.apple.CoreSimulator.CoreSimulatorService

– Kills all running CoreSimulatorService’s processes.

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

– Removes all Simulator data, from all the Simulators.

open /Applications/Xcode.app – Opens Xcode.

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The above steps from xcode 10 are invalid, please use the following steps:

xcrun simctl shutdown all
shutdown all devices

xcrun simctl list devices|grep -v "unavailable"|grep -Eo "[0-9A-F\-]{36}"
list all available devices

xcrun simctl list devices|grep -v "unavailable"|grep -Eo "[0-9A-F\-]{36}"|xargs xcrun simctl delete
delete all devices(note: this is a single line)

more detail xcrun simctl help