Android SDK and Path

The correct configuration in your case would look like this:

export ANDROID_HOME=/opt/android/sdk
export PATH=$PATH:$ANDROID_HOME/tools:$ANDROID_HOME/platform-tools

Note that the environment variable is called ANDROID_HOME, not ANDROID_SDK_HOME. Also, the third line in your sample was missing a $ sign (and contained an additional space). Lastly, there is no need to repeat the same path (DRY) - just use the already defined variable.

EDIT (2020-03-08):

While the ANDROID_SDK environment variable still continues to work for the most part, it has been deprecated by Google, and replaced with ANDROID_SDK_ROOT. So you might want to use the new name for a new setup.

More details: https://developer.android.com/studio/command-line/variables

I don't know what happened as I don't know much technical details behind it. but I removed the following line

export ANDROID_SDK_HOME ={PATH}:/opt/android/sdk

and restarted the terminal and ran ddms it worked.