“$” might symbolize dollars to many of you, but for bash users it’s the holy grail for retrieving stored data. Speaking of which, do you know what’s the common place to store a piece of data? A variable.
Variable
The name itself incurs that variable is something that could be varied. So is it like a chameleon that changes colors all the time? Not precisely, if you remember from the intro its a common place to store a piece of data. That’s right. Mathematically and computationally, a variable is something that stores or holds a piece of data or value.
You might have heard the term “place holder” and that’s exactly what a variable is. So how to store and retrieve information through variables?
place="World"
echo "Hello $place"
In the above code snippet, place is a variable and World is the value assigned to the variable. In the next line, the value assigned to the variable is retrieved through echo command. As aforementioned, “$” is the holy grail to retrieve stored value in bash. An important thing to note here is, “=” doesn’t mean variable name has a constant value, it just means that a value is assigned to a variable name.
Now the value of variable place could be varied.
place="Mars"
echo "Hello $place"
Now the value of variable has changed to Mars from World. Imagine a box where you could place an item and replace that item whenever you wish.
What if you want to store many pieces of data in a single box?
Array
Array is an ordered series of arrangement. Computationally, array holds multiple values, whereas a normal variable holds a single value. Array could also be defined as a special type of variable as it could hold more than one value.
A typical array consists of an array name and an index. Each index number associates with an element in the array.
Lets see how to create an array.
movies=('Primer' 'Inception' 'Enemy' 'Tenet')Two things to keep in mind. Unlike other programming languages, bash arrays are not led by commas after every element but spaces. There is no space after array name(i.e, movies=), it’s followed by “=”. If there is a space followed by the array name, shell interprets the array name as a program to execute and “=” as the first parameter.
Now lets try to retrieve all the elements of the array.
As you could see in the above screenshot, you could retrieve the array elements in two ways: one using the @ symbol between the square brackets or using * symbol.
Here are some tips, you could remember @ by memorizing it like all and * is a wild card which signifies everything. The syntax of retrieving an array could look intimidating, so always remember “$” is the holy grail for retrieving information, array names are placed in between flower brackets {}, followed by special characters or numbers in square brackets.
Lets see how to retrieve an individual element of the array.
Index numbers in bash arrays starts with 0 (similar to most of the programming language, unlike R). So index 2 returns the value of Enemy.
Let’s see what happens when negative indices are used.
So the negative index number retrieves values from reverse order.
You could also assign elements to the array explicitly in three ways: Subscript Assignment, Index Assignment and assignment by name.
Subscript Assignment
movies=([4]='Ten cloverfield lane' [5]='Timecrimes') 
Index Assignment
movies[1]='Vanilla Sky'
movies[2]='Mr.Nobody'
Assignment by names
movies[zero]='Magnolia'
What if you want to print the elements from one particular index to another?
Here you are telling the array that from all its elements(@) , print from index 1 to index 3.
Here you are telling the array that from all its elements(@), start from index 1 and print all the following elements.
What if, you want to access the index numbers?
echo "${!movies[@]}"
The “!” symbol before array name helps to retrieve indices.
What if you want to see how many elements are there in an array?
echo "${#movies[@]}"
The “#” symbol before array name helps to retrieve number of elements.
How to retrieve specific information from an element in the array?
Here the information of zeroth element(Magnolia) is retrieved, starting from index 0 till index 2(not included).
What if you want to assign a sequence of numbers? You could do it manually but here’s a easy way to do it.
seq is a shell command that prints out sequence of numbers. In the above example, seq is placed in between ` `.
Now lets try to write a simple shell script from the above learnings.
#!/usr/bin/bash
tvshows=('Mr.Robot' 'Homeland' 'The Americans' 'Death Note' 'Erased')
for a in "${!tvshows[@]}"; do
printf "${tvshows[$a]} has an index number of $a\n"
done
Let’s save the code as lp.sh. Next lets make it executable – chmod +x lp.sh.
When the program is executed, you get the above result.
Array Modification
Lets create a new array.
names=('Luther' 'Ambrose' 'Reddington' 'Mare')To change the value of index 2 you could do the following.
names[2]=’Carrie’
What if, you want to append values by adding more elements to the array?
What if, you want to add an element at a particular index position?
What if you want to delete an element?
Here, unset is a shell builtin.
What if you want to merge two arrays?
Lets create another array.
surnames=('Dexter' 'Hannibal' 'Broody' 'Dan' 'Gordan' 'Steve')
What if you want to delete an array?
unset <array_name>
Associative Array
An associative array is an array that stores string value as an index. It could be declared and used in a bash script. This feature was added in bash 4. To be sure, just check your bash version.
Declaring an associative array is easy, here’s the syntax.
declare -A <array_name>Lets declare an associative array and initialize some elements.
Like in normal array, if you want to access an element of the array, use the following syntax.
If you want to list the indices of the associative array, do the following.
Now lets try to write a simple script to print the indices(keys) and values of the associative array.
Here’s a catch, what if you want to display the indices in an alphabetic order?
To reverse it, use –reverse flag.
Well that’s it for today. Do practice arrays; because they are powerful! Show some love by sharing and commenting below the article. You can find more about me here.