How to use ADB Shell when Multiple Devices are connected? Fails with "error: more than one device and emulator"

Use the -s option BEFORE the command to specify the device, for example:

adb -s 7f1c864e shell

See also http://developer.android.com/tools/help/adb.html#directingcommands

adb -d shell (or adb -e shell).

This command will help you in most of the cases, if you are too lazy to type the full ID.

From http://developer.android.com/tools/help/adb.html#commandsummary:

-d - Direct an adb command to the only attached USB device. Returns an error when more than one USB device is attached.

-e - Direct an adb command to the only running emulator. Returns an error when more than one emulator is running.

Another alternative would be to set environment variable ANDROID_SERIAL to the relevant serial, here assuming you are using Windows:

set ANDROID_SERIAL=7f1c864e
echo %ANDROID_SERIAL%
"7f1c864e"

Then you can use adb.exe shell without any issues.