Installing tcpdump for Android

• Download a tcpdump precompiled binary for ARM (Tcpdump-4.2.1-arm.tar.gz) from http://omappedia.org/wiki/USB_Sniffing_with_tcpdump. Decompress to obtain the tcpdump binary.
• Connect Android device to computer via USB cable, open a command terminal and execute the following commands:

adb push tcpdump /sdcard/tcpdump

adb shell

su

mount -o remount,rw /system

cat /sdcard/tcpdump > /system/bin/tcpdump

chown root:shell /system/bin/tcpdump

chmod 754 /system/bin/tcpdump

mount -o remount,ro /system

rm /sdcard/tcpdump

tcpdump -X -n -s 0 port 80

Source: https://josetrochecoder.wordpress.com/2013/11/04/installing-tcpdump-for-android/

Recovery and installing customer OS in Android phone

First download the platform-tools from Android SDK website. And then extract the tool and place your terminal to this directory. Now you are ready to run different commands. For example, running adb devices lists the connected device with the computer.
- To reboot the bootloader adb reboot bootloader
- To unloack the device fastboot oem unlock
-  To flash a new image fastboot flash boot <address of the image file>

Rooting android device
https://www.wondershare.com/android/how-to-root-nexus-5.html