Tag Archives: bash shell

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Kolmogorov Complexity, Natural Language Programming and the Bash shell

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The following post superficially applies the concepts of Kolmogorov complexity of an object and natural language programming using the bash shell. Part of the inspiration for this post came from this video: http://www.youtube.com/watch?v=KyB13PD-UME

In this post we will be treating strings as objects in a similar sense of Kolmogorov complexity. Then we will apply an alias name or function name to the object which then the alias/function name can be perceived as a natural language sentence.

Take the following object:

sudo nmap -sP -n -T4 $(netstat -rn | awk ‘{print $2}’ | egrep ‘[[:digit:]]{1,3}\.[[:digit:]]{1,3}\.[[:digit:]]{1,3}\.[[:digit:]]{1,3}’ | fgrep -v “0.0.0.0” | sed -e ‘s/\([0-9]\)\{1,3\}$/1-254/g’)

To individuals who are not familiar with the bash shell or bash shell programming, this object does not make a whole lot of sense. What does it do? What does it mean? Why is this one-liner algorithm useful to some individuals?

For those of who you aren’t sure, this one-liner algorithm is used for ping sweeping your local subnet based upon the gateway’s IP address. So if your gateway is 192.168.1.1 then when the bash shell expands and processes the sub-shell variable $(netstat -rn | awk ‘{print $2}’ | egrep ‘[[:digit:]]{1,3}\.[[:digit:]]{1,3}\.[[:digit:]]{1,3}\.[[:digit:]]{1,3}’ | fgrep -v “0.0.0.0” | sed -e ‘s/\([0-9]\)\{1,3\}$/1-254/g’‘), it would result with 192.168.1.1-254. Lastly, the string would result with sudo nmap -sP -n -T4 192.168.1.1-254.

To take the time to type out this 196 character object each time you connect to a network that you’re exploring, would be extremely tedious and time consuming. Ergo to save an individual some time and keystrokes, this is where we will apply the ‘alias’ function that is built into the bash shell:

alias PingSweepLocalSubnet=”sudo nmap -sP -n -T4 $(netstat -rn | awk ‘{print $2}’ | egrep ‘[[:digit:]]{1,3}\.[[:digit:]]{1,3}\.[[:digit:]]{1,3}\.[[:digit:]]{1,3}’ | fgrep -v “0.0.0.0” | sed -e ‘s/\([0-9]\)\{1,3\}$/1-254/g’)”

Here the 20 character alias PingSweepLocalSubnet saves the end user 176 characters to type and makes more sense depending on if the user is familiar with networking terminologies. Granted this may not be, “the shortest description of this object” and the proper simplifying algorithm according to Kolmogorov complexity method, but this is where the idea of natural language programming is applied. With this object, if we use the Kolmogorov complexity concept, is nearly incompressible. I wanted this alias to be time saving and to be almost a form of natural language programming.

We must also remember that we do not necessarily need to use the ‘alias’ function from the bash shell. We can also achieve the same result by using ‘function’ from bash shell:

function PingSweepLocalSubnet()
{
sudo nmap -sP -n -T4 $(netstat -rn | awk ‘{print $2}’ | egrep ‘[[:digit:]]{1,3}\.[[:digit:]]{1,3}\.[[:digit:]]{1,3}\.[[:digit:]]{1,3}’ | fgrep -v “0.0.0.0” | sed -e ‘s/\([0-9]\)\{1,3\}$/1-254/g’);
}

In turn end users may want to use function assignments rather than aliases. Aliases are limited and are simply string substitutions. For further reading on using aliases or functions take a look at: http://linuxgazette.net/issue53/eyler.html Whether or not the end user decides to use ‘alias’ or ‘function’ is subjective to the user.

Code, Posts

Testing for existence of programs via bash

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For those of you who are into shell scripting, and want to know what in my opinion is the best way to test for whether programs exist or not, pay close attention.

In my humble opinion, the best way to test for the existence of a program, is to test the exit code of the previous command in an if loop.

Firstly, if you run a command that doesn’t exist, the exit code will always be ‘127′. If you don’t want to see the output of the command while testing for it, then just simply redirect all the output to /dev/null .

idontexist &> /dev/null
echo $?
127

Now, a friend suggested using this method for testing for programs in bash:

idontexist &> /dev/null || echo "You don't have 'idontexist'"; exit 1;

There are a few qualms with this. Firstly, depending on the program, if the program does not exit with the exit code of ‘0’, your shell script will then state ‘You don’t have ‘idontexist’ and then exit. However, you might actually have the program! Some programs exit with non-zero exit codes if you pass no arguments to them or even if you pass a ‘-h’ for a help argument to it. Also, since the echo statement is being terminated via the semicolon, ‘exit’ will then be executed. Ergo, this method looks great for just being a one liner, but if I want to test for the existence of a program, and if it doesn’t exist, and then exit with an error message displayed to the user, I highly suggest my following method:

idontexist &> /dev/null;

if [[ "$?" == "127" ]];
 then
     echo "'idontexist' appears to not be installed, check your PATH variable"; exit 1;
fi

More to come…