Android adb command for host, provides a command line if you want to access the device shell of Android. Once you installed Adb as mentioned in “How to Install ADB & Fastboot on Ubuntu” you will need to connect your Android Mobile over USB and type command,
$ adb devices List of devices attached daemon not running; starting now at tcp:5037 daemon started successfully B2NGAA8831702707 device
here, if you have run adb devices command first time, it will start the adb server daemon and list the device attached over USB with device serial number and name.
Now, you can connect to this device using adb shell as,
$ adb shell
You can also connect to adb shell using serial number as,
$ adb -s B2NGAA8831702707 shell