Hacking_and_Penetration_Testing_with_Low_Power_Devices

Adding a graphical environment
59
When you view the output of a command in the terminal, you are actually looking at two
streams: standard out (stdout) and standard error (stderr). If you wish to split stdout and stderr,
you can redirect them separately using the syntax 1>/path/stdout-filename 2>/
path/stderr-filename. For example, the following would search for all shared library files
and save the list to a file and throw away error messages about inaccessible directories: find /
-name '*.so' 1>/sharedlibs.txt 2>/dev/null.
Immediately following our grep command, we find a double vertical pipe that is
used as a logical OR. The OR evaluates to true if the expression on either side of the
double vertical pipe returns true (success). OR uses a technique known as shortcircuiting.
If the first expression is true, the second is never executed. In our case,
if the package was found, the expression deb_pkgs¼"${deb_pkgs}${pkg} " that
appends the package to our list of packages to install is never executed.
The next code segment builds a list of packages to be installed:
unset deb_pkgs
pkg¼"lxde-core"
check_dpkg
pkg¼"slim"
check_dpkg
if [ "x${board}" ¼ "xAM33XX" ] ; then
pkg¼"xserver-xorg-video-modesetting"
check_dpkg
fi
pkg¼"xserver-xorg"
check_dpkg
pkg¼"x11-xserver-utils"
check_dpkg
The first line in this segment unset deb_pkgs clears the variable holding our list
of packages to be installed. The remaining lines in this segment set the pkg variable
and then call check_dpkg repeatedly. The only thing new is an if statement that is
used to install a video-modesetting package on the appropriate hardware (such as
the BeagleBone Black).
The if statement might look a bit funny if you aren’t used to working with
shell scripts. The general syntax of an if statement is if [ condition ] ; then followed
by a number of commands and closed with an fi (if spelled backward).
Be very careful with whitespace in if statements in shell scripts. The whitespace
before and after each of the square brackets is not optional. You might wonder about
the strange condition statement "x${board}" ¼ "xAM33XX" being used in this script.
Prefixing our variable and target string with an x is a common trick that prevents
accidental modification of our shell variable board. Incidentally, while we have
indented the code inside the if statement to make it more readable, doing so is
optional.