adb doesn't show nexus 5 device

I had a similar problem with my Nexus 4(Android version 4.4.2), it wasn't listed in adb devices.

Make sure USB debugging is enabled from device, and do the following on your PC:

1. Update Android SDK (Google USB Driver)

2. From PC Control Panel, System -> Device manager -> Right click Nexus 4 -> Update driver.

3. Set android-sdk-folder\extras\google\usb_driver as path to search, include subfolders checked.

If windows tells you that the driver is up to date, just uninstall the driver (right click on nexu4 -> uninstall driver) and start from step 2 again.

After that, open a cmd and type adb kill-server and then a adb devices, now it will include your device.

https://developer.android.com/studio/run/oem-usb.html

Follow these steps,

• Enable Developer options in your device. To enable the developer mode, Settings->About phone, tap Build number option 7 times continuously
• Go to Settings-> Developer options and Turn on USB debugging
• Make sure you reconnected the device via USB and grant permission on the dialog that appears.
• From the above steps it didn't work try this step, Go to Settings->Security and turn on Unknown Sources

What you need to do is this:

Settings | About Phone

Scroll to the bottom to build number.Tap on build number about 7 times. Each time you will get a popup message saying you are x steps away from being a developer

When you get to the final step you will get a message saying now you are a developer

Go back into settings and you will see a new setting Developer options there you will see a lot of options for developers. Enable USB debugging

Re-connect you phone to the usb, and you should see you device under adb devices.

I hope this answer helps someone else.