Skip to main content

Using Variables and Basic Loops

1. Using Variables in Shell Scripts​

In shell scripting, variables allow you to store data, such as text or numbers, that can be referenced later in the script.

Creating Variables:​

To create a variable, simply assign a value to it. In Bash, variables are created by assigning a value with the = operator, and no spaces should be used around the = sign.

my_variable="Hello, Shell Scripting!"

Examples:​

1. To create a variable called name that stores your name:​
name="John"
2. To create a variable called number that stores a numerical value:​
number=5

Important: In Bash, variables are case-sensitive. For example, my_variable and MY_VARIABLE are two different variables.

Accessing Variables:​

To use or access the value stored in a variable, prefix the variable name with a dollar sign ($).

Examples:​

1. To print the value of the variable name:​
echo $name
2. To print the value of number:​
echo $number

Using Variables in a Script:​

Example: A script that stores a message in a variable and prints it:​
#!/bin/bash
greeting="Welcome to Shell Scripting!"
echo $greeting

2. Using Variables with User Input​

You can also get input from the user and store it in a variable. The read command allows you to accept input from the user.

Example: A script that asks the user for their name and stores it in the variable username:​
#!/bin/bash
echo "What is your name?"
read username
echo "Hello, $username!"

If the user types Alice, the output will be:

Hello, Alice!

3. Basic Loops in Shell Scripting​

Loops are used to repeat a set of commands a specific number of times or until a condition is met. Shell scripting supports several types of loops, but we will focus on two basic types: for loops and while loops.

For Loop​

The for loop is used to iterate over a set of values. The basic syntax is as follows:

for variable in value1 value2 value3
do
  command1
  command2
done
Example: A simple for loop that prints numbers from 1 to 5:​
#!/bin/bash
for i in 1 2 3 4 5
do
  echo "Number: $i"
done

For Loop with Ranges:​

You can also loop through a range of numbers using {} to define the range.

#####Example: Looping from 1 to 5:

#!/bin/bash
for i in {1..5}
do
  echo "Number: $i"
done

While Loop​

The while loop runs as long as a condition is true. The basic syntax is:

while [ condition ]
do
  command1
  command2
done
Example: A while loop that prints numbers from 1 to 5:​
#!/bin/bash
i=1
while [ $i -le 5 ]
do
  echo "Number: $i"
  ((i++))  # Increment the variable i by 1
done

Using Loops to Automate Tasks:​

Example: A script that creates multiple directories (folders) using a for loop:

#!/bin/bash
for dir in dir1 dir2 dir3 dir4
do
  mkdir $dir
  echo "Created directory: $dir"
done

This will create four directories (dir1, dir2, dir3, dir4) and print the message for each directory.

4. Nested Loops​

Sometimes, you may need to nest one loop inside another. This is known as a nested loop.

Example: A script that prints a multiplication table using nested loops:​
#!/bin/bash
for i in {1..3}
do
  for j in {1..3}
  do
    result=$((i * j))
    echo "$i x $j = $result"
  done
done

5. Breaking and Continuing in Loops​

Sometimes, you may need to break out of a loop or skip an iteration. You can use break and continue to control the flow of a loop.

  • Break Example: The break command stops the loop immediately.​
#!/bin/bash
for i in 1 2 3 4 5
do
  if [ $i -eq 3 ]
  then
    break  # Exit the loop when $i is 3
  fi
  echo "Number: $i"
done
  • Continue Example: The continue command skips the current iteration and moves to the next iteration.​
#!/bin/bash
for i in 1 2 3 4 5
do
  if [ $i -eq 3 ]
  then
    continue  # Skip this iteration when $i is 3
  fi
  echo "Number: $i"
done

Key Takeaways:​

  • Variables allow you to store and manipulate data within your script.
  • In Bash, variables are created by assigning values using the = sign, without spaces around it.
  • Loops are used to automate repetitive tasks in scripts. There are different types of loops, such as for and while.
  • break exits the loop immediately and continue skips the current iteration and moves to the next.
  • Nested loops allow you to use one loop inside another, useful for more complex tasks like multi-dimensional operations or table generation.
© 2025 Neplearn