loops

Loops in Bash are essential for automating repetitive tasks, iterating through lists, or executing commands multiple times. Bash provides three primary types of loops: for, while, and until. Each has its own use cases and syntax.

1. for Loop

The for loop in Bash is used to iterate over a list of items (such as numbers, files, or strings) and execute a block of code for each item.

Syntax:

for variable in list
do
  # Commands to execute
done

Example 1: Iterating Over a List of Items

for fruit in apple banana cherry
do
  echo "I love $fruit"
done

Output:

I love apple
I love banana
I love cherry

Example 2: Iterating Over a Range of Numbers (using {})

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

Output:

Number 1
Number 2
Number 3
Number 4
Number 5

Example 3: Iterating with Step Size

You can specify a step size when iterating over a range using the seq command or a specific step in the {} range.

for i in {1..10..2}
do
  echo "Odd number: $i"
done

Output:

Odd number: 1
Odd number: 3
Odd number: 5
Odd number: 7
Odd number: 9

Alternatively, using seq:

for i in $(seq 1 2 10)
do
  echo "Odd number: $i"
done

2. while Loop

The while loop runs as long as a given condition is true. It is useful when you don't know how many times you need to iterate, but you have a condition to check before continuing the loop.

Syntax:

while condition
do
  # Commands to execute
done

Example 1: Basic while Loop

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

Output:

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

Example 2: Looping Until a Condition is Met

You can use a while loop to keep iterating as long as a condition is true (or until it's false).

count=5
while [ $count -gt 0 ]
do
  echo "Count is $count"
  ((count--))  # Decrement count by 1
done

Output:

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

3. until Loop

The until loop works similarly to the while loop, but it continues as long as the condition is false. It’s used when you want to execute commands until a certain condition becomes true.

Syntax:

until condition
do
  # Commands to execute
done

Example 1: Basic until Loop

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

Output:

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

Example 2: Infinite until Loop (with a break)

You can also create an infinite until loop. This is often used with a break statement to stop the loop when a certain condition is met.

count=1
until [ $count -gt 5 ]
do
  echo "Count is $count"
  ((count++))
  if [ $count -eq 3 ]; then
    echo "Stopping at count 3"
    break
  fi
done

Output:

Count is 1
Count is 2
Count is 3
Stopping at count 3

4. Loop Control Statements

• break: Exits the loop prematurely.
• continue: Skips the rest of the current iteration and moves to the next one.

Example with break:

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

Output:

Number 1
Number 2
Breaking at 3

Example with continue:

for i in {1..5}
do
  if [ $i -eq 3 ]; then
    continue  # Skip the rest of the loop for i=3
  fi
  echo "Number $i"
done

Output:

Number 1
Number 2
Number 4
Number 5

5. Nested Loops

You can nest loops within each other to perform more complex tasks.

Example: Nested for Loops

for i in {1..3}
do
  for j in {1..2}
  do
    echo "i=$i, j=$j"
  done
done

Output:

i=1, j=1
i=1, j=2
i=2, j=1
i=2, j=2
i=3, j=1
i=3, j=2

Example: Nested while Loop

i=1
while [ $i -le 3 ]
do
  j=1
  while [ $j -le 2 ]
  do
    echo "i=$i, j=$j"
    ((j++))
  done
  ((i++))
done

Output:

i=1, j=1
i=1, j=2
i=2, j=1
i=2, j=2
i=3, j=1
i=3, j=2

Summary of Loops in Bash:

1. for loop: Iterates over a list of items (or range) and executes commands for each item.

   • Best for known iterations or ranges.

2. while loop: Executes commands as long as the condition is true.

   • Useful when you want to repeat something until a condition changes.

3. until loop: Executes commands until the condition becomes true.

   • Opposite of the while loop; it stops when the condition is true.

4. Loop control: Use break to exit early or continue to skip the current iteration.

By mastering these loops and their variations, you'll be able to automate a wide range of tasks in Bash effectively!

If you’ve ever used a Linux operating system used on most Virtual Private Servers, you may have heard of bash. It’s a Unix shell that reads and executes various commands.

What Is Bash?

Bash, short for Bourne-Again Shell, is a Unix shell and a command language interpreter. It reads shell commands and interacts with the operating system to execute them.

Why Use Bash Scripts?

Bash scripts can help with your workflow as they compile many lengthy commands into a single executable script file. For example, if you have multiple commands that you have to run at a specific time interval, you can compile a bash script instead of typing out the commands manually one by one. You then execute the script directly, when it’s necessary.

Pro Tip Linux has a bash shell command manual. Type man command to find descriptions of all the technical terms and input parameters.

Get Familiar With Bash Commands

Bash is available on almost all types of Unix-based operating systems and doesn’t require a separate installation. You will need a Linux command prompt, also known as the Linux terminal. On Windows you would use something like PuTTy. It’s a program that contains the shell and lets you execute bash scripts. 

1. Comments

Comments feature a description on certain lines of your script. The terminal doesn’t parse comments during execution, so they won’t affect the output.

There are two ways to add comments to a script. The first method is by typing # at the beginning of a single-line comment. # Command below prints a Hello World text echo “Hello, world!”

2. Variables

Variables are symbols that represent a character, strings of characters, or numbers. You only need to type the variable name in a command line to use the defined strings or numbers.

To assign a variable, type the variable name and the string value like here: testvar=“This is a test variable”

In this case, testvar is the variable name and This is a test variable is the string value. When assigning a variable, we recommend using a variable name that’s easy to remember and represents its value.

To read the variable value in the command line, use the $ symbol before the variable name. Take a look at the example below:

testvar=“This is a test variable”
echo $testvar

In order to let the user enter the variable contents use:

read testvar
echo $testvar

3. Functions

A function compiles a set of commands into a group. If you need to execute the command again, simply write the function instead of the whole set of commands.

There are several ways of writing functions. The first way is by starting with the function name and following it with parentheses and brackets:

function_name () {
    first command
    second command
}

Or, if you want to write it in a single line: function_name () { first command; second command; }

4. Loops

Loop bash commands are useful if you want to execute commands multiple times. There are three types of them you can run in bash – for, while, and until. The for loop runs the command for a list of items:

for item in [list]
do
    commands
done

The following example uses a for loop to print all the days of the week:

for days in Monday Tuesday Wednesday Thursday Friday Saturday Sunday
do
    echo “Day: $days”
done

On line 2, “days” automatically becomes a variable, with the values being the day names that follow. Then, in the echo command, we use the $ symbol to call the variable values.

The output of that script will be as follows:

Day: Monday
Day: Tuesday
Day: Wednesday
Day: Thursday
Day: Friday
Day: Saturday
Day: Sunday

Notice that even with just one command line in the loop script, it prints out seven echo outputs.

The next type of loop is while. The script will evaluate a condition. If the condition is true, it will keep executing the commands until the output no longer meets the defined condition.

while [condition]
    do
commands
done

5. Conditional Statements

Many programming languages, including bash, use conditional statements like if, then, and else for decision-making. They execute commands and print out outputs depending on the conditions. The if statement is followed by a conditional expression. After that, it’s followed by then and the command to define the output of the condition. The script will execute the command if the condition expressed in the if statement is true.

However, if you want to execute a different command if the condition is false, add an else statement to the script and follow it with the command.

Let’s take a look at simple if, then, and else statements. Before the statement, we will include a variable so the user can input a value:

echo “Enter a number”
read num
if [[$num -gt 10]]
then
echo “The number is greater than 10”
else
echo “The number is not greater than 10”

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OK, so that's it. The 5 building blocks of Bash in plain English. Simple, right?!