Kervie Sazon

Posted on Feb 3

Linux Fundamentals - Part 7: Bash Scripting (If Statements and Loops)

#beginners #linux #programming #tutorial

If Statements

If statements execute code base on a condition. If the condition is true, the code block will run.

The condition is enclosed in square brackets [ ] and the statement ends with fi, which is if spelled backward, marking the end of the if block.

Example: Basic If Statement.

num=15
if [ $num -gt 10 ]; then
   echo "Number is greater than 10"
fi

If...Else Statements

If...else statements provide a way to execute one block of code if a condition is true and another block if it is false.

The else keyword introduces the alternative block, and the statement ends with fi.

Example: If...Else Statement

num=8
if [ $num -gt 10 ]; then
  echo "Number is greater than 10"
else
  echo "Number is 10 or less"
fi

Nested if Statements

Nested if statements allow you to place an if statement inside another if statement, enabling more complex logic.

Each if block must be closed with its own fi.
Example: Nested If Statement.

num=5
if [ $num -gt 0 ]; then
  if [ $num -lt 10 ]; then
    echo "Number is between 1 and 9"
  fi
fi

Loops

For Loops

For loops allow you to iterate over a list of items or a range of numbers. They are useful for repeating tasks a specific number of times.

The for keyword is followed by a variable name, a range of values, and a do keyword, which marks the start of the loop block.

Example: For Loop.

for i in {1..5}; do
  echo "Iteration $i"
done

The loop runs 5 times, with i going from 1 to 5.
So the output will be:

Iteration 1
Iteration 2
Iteration 3
Iteration 4
Iteration 5

While Loops

While loops execute a block of code as long as a specified condition is true.

They are useful for tasks that need to repeat until a certain condition changes.

The condition is enclosed in square brackets [ ], and the loop ends with done.
Example: While Loop.

count=1
while [ $count -le 5 ]; do
  echo "Count is $count"
  ((count++))
done

This while loop also runs 5 times, incrementing count each time.

The output will be:

Count is 1
Count is 2
Count is 3
Count is 4
Count is 5

Once count becomes 6, the condition [ $count -le 5 ] is false, so the loop stops.

Until Loops

Until loops are similar to while loops, but they execute until a specified condition becomes true.

The condition is enclosed in square brackets [ ], and the loop ends with done.

Example: Until Loop.

count=1
until [ $count -gt 5 ]; do
  echo "Count is $count"
  ((count++))
done

Here, it keeps looping until count is greater than 5.

So the output is:

Count is 1
Count is 2
Count is 3
Count is 4
Count is 5

When count reaches 6, the condition [ $count -gt 5 ] becomes true, and the loop stops.

Break and Continue

Break and continue statements are used to control loop execution. break exits the loop, while continue skips to the next iteration.

These statements are typically used inside conditional blocks to alter the flow of the loop.

Example: Break and Continue.

for i in {1..5}; do
  if [ $i -eq 3 ]; then
    continue
  fi
  echo "Number $i"
  if [ $i -eq 4 ]; then
    break
  fi
done

This is the explanation of the above code.
When i = 1 - prints.
When i = 2 - prints.
When i = 3 - continue skips the echo, so nothing prints.
When i = 4 - prints, then break stops the loop.
i = 5 is never reached.
So the output will be:

Number 1
Number 2
Number 4

continue skips the current iteration, and break exits the loop completely.

Nested Loops

Nested loops allow you to place one loop inside another, enabling more complex iteration patterns.

Each loop must be closed with its own done.

Example: Nested Loops.

for i in {1..3}; do
  for j in {1..2}; do
    echo "Outer loop $i, Inner loop $j"
  done
done

Let’s break it down:
i = 1 → j = 1, 2

i = 2 → j = 1, 2

i = 3 → j = 1, 2