Bash: Conditions (if-then-else)

Hello

This time I will show you how to make a script with condition in Bash, which translated is:

If X thing you want is done, Y action is done, if it is not done then another action is done.

Simple explanation no 😀

Now, I have thought for a few minutes about some example to use in this tutorial, the following dilemma / problem / situation occurred to me:

We are on our company's network, and we want to know if X computer is connected to the network. For this we make a script what will he do ping towards that computer, and if it is on a network (that is, if it returns the ping) will tell us that YES, it is on the network, otherwise (that is, it is not on the network) it will tell us that it is NOT on the network.

Once this is done, now I will explain how to cycle with conditions 🙂

Here is the code:

ping -c 1 DIRECCION-IP
if [ $? -ne 0 ]; then
echo "No está en red"
else
echo "Sí está en red"
fi

Don't worry, I'll explain this to you in detail 😉

ping is the command we will use, and it will tell us if that PC is on the network. To tell which PC we want to check whether or not it is on the network, we must change IP ADRESS by obviously, the IP address of the PC that we want to check.

As you can see, I put «-c 1«, Which is necessary to us. When we ping a computer, this action does not stop (the ping) until we press ourselves [Ctrl] + [C], so putting «-c 1»We tell it to do only one verification (only one ping attempt) and no other, this will make it stop instantly, that is… it will check if the computer is on the network only once.

If you have doubts about this, no pain ... they say so and I will be happy to explain it to you again 😉

Now comes the cycle, because what I just explained is nothing more than a normal command / action 😀

if [ $? -ne 0 ]; then
echo "No está en red"
else
echo "Sí está en red"
fi

So that you understand this, I will explain an extremely important detail of Bash 🙂

The most knowledgeable of the subject could brand me as wrong or something like that, but hey, I write this for the novices or less experts, as long as they understand it then great 😉

It happens that as in Bash it's something like 0 y 1, that is, either you are alive or you are dead, when a command or action is executed: Either executed well No problem (1), Or there was some problem or error (0).

We carry out X action or command, and what we did may have been carried out well or badly, it may have had an error or not, and this is where the detail is 😉

If what we send to do (in this case: ping -c 1 IP-ADDRESS) did not give an error and it was successful, so it will return the value: 1 . Otherwise, and if the action (that is, the ping) was not successful, it will return value 0.

 And finally, what the above code means is this:

If value 0 was returned then

Display the text: «Not on network»

Otherwise (and NOT 0, but 1 is returned)

Display the text: «If it is in network«

This that I have just explained to you, will serve us in the future for many things, because it is really useful to be able to say that if X action gave an error, do Y action, and in case X action has not given an error, then do Z action.

I know that some might get a bit confused, so I have tried to explain it in several different ways, trying to make everyone understand it one way or another. In case anyone is left in doubt, let me know.

Now, let's make our script 😀

We must follow the steps in this tutorial: Bash: How to make an executable script

Then let's copy the following code into that file (script.sh), and at the end let's say «exit»(Without the quotes):

ping -c 1 DIRECCION-IP
if [ $? -ne 0 ]; then
echo "No está en red"
else
echo "Sí está en red"
fi

It should look like this (remember that they must have changed the IP-ADDRESS to the IP they want):

Here you can see the running script:

% CODE1%

As you can see, in the end he tells us «Yes it is in network»🙂

The important thing here is that they actually understand this about the conditions, so I leave you another code to explain it again, but from another point of view.

read "texto"
if [ "$texto" = "3" ]; then
echo "Correcto"
else
echo "Incorrecto"
fi


What this means is simple, I leave the explanation line by line:

1st line: What we write, that will be the value of the variable «text»(Without the quotes).

2nd line: Check if the content of the variable (what we just wrote) is 3.

3st line: In case of being 3, it will show us the text «Correct»(Without the quotes).

4th line: Otherwise (that is, in case we have not written 3).

5th line: It will show us the text «Incorrect»(Without the quotes).

6th line: End of condition.

As they have been able to realize, if we put threw out and then between double quotes («) a text, this will cause that text to be displayed in the terminal. That is, if we put:

echo "esto es una prueba"

It will show us the text in the terminal: this is a test

But going back to this second example, I will show you the usefulness (and execution) of this second script with something VERY simple 😀… the typical «how much is 1 + 2?«

I leave the code of the complete script:

#!/bin/bash
# -*- ENCODING: UTF-8 -*-
echo "¿Cuánto es 1 + 2?"
read "texto"
if [ "$texto" = "3" ]; then
echo "Correcto"
else
echo "Incorrecto"
fi
exit

Here is how the script works:

% CODE2%

And well ... nothing more to add.

This is something elementary, simple yes, but still I have tried to explain it as well as possible, because not everyone has the soul of a programmer, and many times we need to make scripts like these (or similar), in any case I hope this article is useful someone 🙂

Any doubt or question, complaint or suggestion please leave it here, I will gladly answer you and thus, we all learn a little more 😀

regards