How to root Android?

The process of rooting varies widely by
device, but usually includes exploiting a
security bug(s) in the firmware (i.e. in
Android) of the device, and then copying the
su binary to a location in the current
process's PATH (e.g. /system/xbin/su )
and granting it executable permissions with
the chmod command. A supervisor
application like SuperUser or SuperSU can
regulate and log elevated permission requests
from other applications. Many guides,
tutorials, and automatic processes exist for
popular Android devices facilitating a fast and
easy rooting process.
For example, shortly after the HTC Dream
( HTC G1)was released, it was quickly
discovered that anything typed using the
keyboard was being interpreted as a
command in a privileged (root) shell.
Although Google quickly released a patch to
fix this, a signed image of the old firmware
leaked, which gave users the ability to
downgrade and use the original exploit to
gain root access. Once an exploit is
discovered, a custom recovery image that
skips the digital signature check of a firmware
update package can be flashed. In turn, using
the custom recovery, a modified firmware
update can be installed that typically includes
the utilities (for example the Superuser app)
needed to run apps as root.
The Google-branded Android phones, the
Nexus One, Nexus S , Galaxy Nexus, Nexus 4
and Nexus 5 as well as their tablet
counterparts, the Nexus 7 and Nexus 10 , can
be boot-loader unlocked by simply connecting
the device to a computer while in boot-loader
mode and running the Fastboot program with
the command "fastboot oem unlock".[10]
After accepting a warning, the boot-loader is
unlocked, so a new system image can be
written directly to flash without the need for
an exploit.
In 2011, Motorola , LG Electronics and HTC
added security features to their devices at
the hardware level in an attempt to prevent
users from rooting retail Android
devices. [citation needed ] For instance, the
Motorola Droid X has a security boot-loader
that puts the phone in "recovery mode" if a
user loads unsigned firmware onto the
device, and the Samsung Galaxy S II displays
a yellow triangle indicator if the device
firmware has been modified.