can't use adb shell in recovery mode I have a china tablet named Quest. I have accidentally repeated pattern lock more than 5 times. Now I have also forgotten the email address.

So for a factory-reset of my tablet I simply booted it into recovery mode. But it shows an Android bot lying with red exclamation mark. After a spending hours I found a shortcut i.e, vol + and power to access recovery menu. But my tablet doesn't go to recovery menu even when I pressed the keys.

So I simply connected my tablet to my pc via USB and installed drivers. In adb command prompt I typed adb devices. Now my tablet is listed as 20110301 recovery. I can simply use only adb pull, adb push, and reboot commands. But I cant access adb shell to unlock my tablet: it simply says exec '/system/bin/sh' failed: No such file or directory (2).

What can I do?
You have a stock recovery. Try to boot to fastboot, using ADB: Apply this command in ADB while in recovery: adb reboot-bootloader, or: adb reboot bootloader. Try each one, if the first works, good. Check if the device is visible by fastboot devices. If it outputs nothing, try the other command. It should work now.

Now you have to flash the recovery using fastboot flashing abilities. After that, try again.