Forem: Augusto Zanella Bardini

Shell Scripting for Newbies 03

Augusto Zanella Bardini — Fri, 20 Jul 2018 03:03:29 +0000

Conditionals

In the last tutorial, you've given a look at how to interact with the user and how to return simple outputs accordingly to the user interaction.

Ironically, your cat's been using the computer lately to ask for things. Of course it's not working, but you can solve this problem! Lets talk to your cat!

First of all, we'll want to ask him what does he want. Let's do it this way:

#/bin/bash

printf "What do you want, fella?\n"
read answer

TIP: printf is a better alternative to the command echo. By now, it's just the same, except from you'll need to write \n in the end of the sentences in order to go to the next line.

Now that you've got his answer in the variable answer, we'll see this NEW ~~hell no~~ AWESOME logic: The if command. It works almost as simple as it is written, but with some caveats. We already know that it's possible to test the truth of a statement with the command test, so let's just tell the computer what to do if it's true:

#/bin/bash

printf "What do you want, fella?\n"
read answer

#if the test is true, then it'll be printed the message for the cat
if test $answer = 'food'
then printf "There's meat in the fridge, dumbass!\n"
fi

Here we've literally responded to the cat to find his own food! To tell the computer where the if conditional ends, just use ~~this is not a joke~~ the command fi.
There's a simple trick to make the code even better readable. We can exchange the command test to a pair of [ ] around the statements, have a look:

#/bin/bash

printf "What do you want, fella?\n"
read answer

#if the test is true, then it'll be printed the message for the cat
if [ $answer = 'food' ]
then printf "There's meat in the fridge, dumbass!\n"
fi

Cool, but your lil' kitty might not really know what he wants. So let's just add another literal command, say hello to the else command! In case his answer is anything but food, let's just ignore it ~~poor cat~~ and give him the cuddling option.

#/bin/bash

printf "What do you want, fella?\n"
read answer

#if the test is true, then it'll be printed the message for the cat
if [ $answer = "food" ]
then 
        printf "There's meat in the fridge, dumbass!\n"
else
        #now if his last answer was anything but food, you'll make this question 
        printf "Want some cuddle, so? [yes/no]\n"
        read answer
        #if his answer is yes again, you tell him you are busy,
        #else, you tell him to top annoying!
        if [ $answer = "yes" ]
        then
                printf "Sorry, I'm busy learning shell scripting.\n"
        else
                printf "So stop annoying me!\n"

        fi 
fi

TIP: Learning how to use the if command is actually intuitive. The ~~boring~~ funny part is understanding completely the test command. You can have a deeper look at bash expressions and when you should use them clicking here.

Now this is getting bigger! Once the cat's answer is anything but food in the first question, we give him another question with ~~please stop~~
two more options! Notice that this question is only made if the first answer IS NOT food. The possible outputs of our script are shown below.

What do you want, fella?
food
There's meat in the fridge, dumbass!

What do you want, fella?
idk
Want some cuddle, so? [yes/no]
yes
Sorry, I'm busy learning shell scripting.

What do you want, fella?
communism
Want some cuddle, so? [yes/no]
no
So stop annoying me!

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Shell Scripting for Newbies 02

Augusto Zanella Bardini — Wed, 18 Jul 2018 02:46:24 +0000

Variables

With simple variables, you can...

save values using =
find those values putting $ in your echo

"One Look Is Worth A Thousand Words"

#/bin/bash

var="Yes, did it!"
echo "Have you done it? $var"

The result of running this script is:

Have you done it? Yes, did it!

Talking to the user

But let's do something useful with this concepts. Imagine you can only show the date for the user if he knows the secret password! You might want to ask him the password, so do it by using the command read.

Then, if the user type the ~~easiest ever~~ right password "dog", the date is shown, otherwise, the program is closed. Let's have a look at the code before anything.

#/bin/bash

echo "What's THE SECRET PASSWORD?"

#Puts the user's answer in the variable answer
read answer

#If answer is dog dog, print date!
test "$answer" == "dog" && date

Now, gotta understand it:

The operator == means equals to
The operator && means and

As we had already saved the user's answer in the variable $answer with the command read, when we compare the variable $answer to "dog", if they are the same, it prints the date; else, it finishes the program (because there are no more commands to execute).

Therefore, the result of the script must be both below:

With the right password:

What's THE SECRET PASSWORD?
dog
Ter Jul 17 23:32:39 -03 2018

With the wrong password:

What's THE SECRET PASSWORD?
cat

I'm not convinced!

So let's crack this code a bit more!

#/bin/bash

echo "What's THE SECRET PASSWORD?"

#Puts the user's answer in the variable answer
read answer

#REVERSE LOGIC!
#If answer is not dog, exit; Else, print date!
test "$answer" != "dog" && exit
date

Stuff is different here:

The operator != means different

Now we use the reverse logic to do the same thing. This time, the command exit can only happen if the user's answer is different from "dog". Otherwise, it will continue running the script and eventually reach the command date.

The result is exactly the same as in the previous script!

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Shell Scripting for Newbies 01

Augusto Zanella Bardini — Wed, 18 Jul 2018 01:57:43 +0000

Let's suppose you want to check your any linux system stats, you might probably use commands such as w, date, or df. That means you need to write three commands and check their results, one by one... Not the funniest hobby!

That's where shell scripting arrives: lets automatize tasks, and a little more...

Hello Mars

To start, let's just create an hellomars.sh file. Do this by typing in your terminal:

touch hellomars.sh

Then, use ~~vim~~ your finest editor to edit this file and have some ~~lack of~~ fun creating your first Hello Mars script!

#/bin/bash
echo "Hello Mars!"

Once created, give this file permission to be executed and execute the file itself by typing:

chmod +x hellomars.sh
./hellomars.sh

As you may have noticed, echo is the command to write something as output. The #/bin/bash is used to tell linux which interpreter it must use (there are plenty of them, sh, bash, dash...). The output of the script might be as follows:

Hello Mars!

One after the other

Now that you know how to write something in the screen, lets write something relevant. Putting all those first commands together, you can make another file such, let's call it mysystem.sh:

#/bin/bash
echo "Aliens using this computer:"
w
echo "What year am I?"
date
echo "Is there any computer left?"
df -h

tip: -h flag turns df a little more human readable.

By running this script, the output must be something as shown below:

Aliens using this computer:
 22:13:13 up  1:54,  1 user,  load average: 0,55, 0,56, 0,60
USER     TTY      FROM             LOGIN@   IDLE   JCPU   PCPU WHAT
bardini  tty7     :0               20:22    1:54m  2:28   0.60s /sbin/upstart --user
What year am I?
Ter Jul 17 22:13:13 -03 2018
Is there any computer left?
Filesystem      Size  Used Avail Use% Mounted on
udev            2,9G     0  2,9G   0% /dev
tmpfs           586M   17M  570M   3% /run
/dev/sda1       105G   33G   67G  33% /
tmpfs           2,9G   31M  2,9G   2% /dev/shm
tmpfs           5,0M  4,0K  5,0M   1% /run/lock

People might not understand the functions you are using in your script, so comment it using hashtags:

#/bin/bash
#This shows users in this computer
echo "Aliens using this computer:"
w

#This shows the date and time of this computer
echo "What year am I?"
date

#This shows the disk usage of the computer
echo "Is there any computer left?"
df -h

About

You can find all my scripts in my github!

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