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Learn BASH with me in 5 mins

I just started learning Linux bash from today. From my first impression of the language, I infer that it is a language with all the basic capabilities as of an infant high level language. May be I am right or wrong.Time will tell . We will keep going and keep discovering gradually. Let's start with the usual protocol of learning a language.

The HELLO WORLD program.

How to print things in shell. This is the first thing everyone wants to know while learning any language.

Anything that is not a variable is printable . And we print/echo it using the famous ECHO keyword

$ echo hello world
hello world

Printing a number.

$ echo 1
Printing a string with double quotes
$ echo "my name is arindam"
my name is arindam
Printing a string with single quotes
$ echo 'my name is Arindam'
my name is Arindam
Printing a number with quotes
$ echo '1' 1

Creating  Variables and recalling them.

So how can we store things. How to recall that stored value. How to change that value.


Note: There should be no spaces around the assignment operator (=). Also, there is no return value after the assignment statement is executed.

$ X=999

$ echo $X

$ $X
bash: 999: command not found

A simple = sign works great for assigning values but, the spaces around a important. Otherwise you will get an error.

To recall the value inside a variable, use the $ sign.
If you don't use the echo keyword and try to print the value by just a $ sign (people coming from languages like python would understand why someone would try such a thing).

Saving Strings in variable

$ X=arin

$ echo $X

$ x="hi world"

$ x=hi world
bash: world: command not found

You can store a single word with spaces around without using quotes. But if there are spaces, then you need to use quotes. Other bash breaks down "x=hi world" as two commands x=hi and world. Obviously this doesn't work.

Dynamically changing value

$ echo $X

$ echo $((X+1))

What happened here. I wanted to use the variable X and get an incremented value of the same.
You need to use a double parenthesis in these cases. Note that this won't change the value of X to the new value.

Using Bash as a calculator

$ echo $((X*2))+$X

$ echo $(($((X*2))+$X))

This probably is an overkill but , if you need to do it, this is how you can.

From the first line, you can observe that , bash evaluates each section separately, and just displays their value in the same format. This is like interpretation

The second does the job , because we asked to evaluate the equation using $((equation)).
I think this looks messy and risky but, just for example sake, it works.

Another way , is to use the keyword "expr". Whatever is mentioned after this keyword become the expression to be solved/interpreted.

$ echo $(expr 5 + 5)

$ echo $(expr 4 * $n)

Iterations and LOOPS

Every body loves loops. Iterations are a part of every language. Bash provides the omnipresent FOR  loop and WHILE loop.

$ for num in 1 2 3
> do
> echo $num
> done

So here, we looped on a list of numbers.

while [ $X -le 99 ]
    echo $X

Here, I have printed from number 1 to 99 but only odd numbers.

Accepting input

Another common feature of any language is accepting an input from the user. We have the 'read' keyword for it.

read name
echo Welcome $name


My reference:

How do deal with ranges. Bash has a syntax for that. {start..end}

for num in {1..50}
echo $num

Now , what if the end limit of your range, is inside a variable. You might think, I'll just do {1..$N}. Sorry that doesn't work. There is a better way to do this. If you know C syntax, then you must be familiar with this.


$ for ((i=1;i<=n;i++)); do echo $i; done

Ranges with Step

We want to add a step value. We can do it as {start..end..step}

$ for num in {1..10..2}; do echo $num; done

If condition with comparison operators

if [ $A -gt $B ]
        echo $(($A-$B))
    echo $(($B-$A))

There are many operators available. Below table should be referred.

For string comparisons, the operators are different.


$ if [ 'Y' == 'Y' ]; then echo YES; else echo NO; fi

$ if [ 'N' == 'Y' ]; then echo YES; else echo NO; fi

Multiple conditions inside IF

There might be a condition when you have two or more possibilities for the if or else part to be true.

read D
if [ $D == 'Y' -o $D == 'y' ]; then echo YES; else echo NO; fi

Here, the -o stands for OR. Even || works for OR operation but the syntax changes slightly.

read D
if [ $D == 'Y' ] || [ $D == 'y' ]; then echo YES; else echo NO; fi

Also note, for AND operation , -a is used. Also && can be used.


Find out if a triangle is scalene , equilateral or isosceles given sides of a triangle a, b, c.


read a
read b
read c

if [ $a -eq $b -a $a -eq $c ];then
elif [ $a -eq $b ] || [ $a -eq $c ]||[ $b -eq $c ];then 
 echo SCALENE;


Find out the average to 3 decimals of accuracy. Given an array of numbers.

Input Format
The first line contains an integer, .
lines follow, each containing a single integer.
Output Format
Display the average of the integers, rounded off to three decimal places.


read N
for ((i=1;i<=N;i++))
read temp
printf "%.3f" $(echo $s/$N | bc -l) 

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