Bash For Loop

Introduction

One of the most fundamental ideas in programming languages is the loop. When you want to run a string of commands multiple times until a certain condition is met, loops come in handy.

Loops are useful for automating repeated operations in scripting languages like Bash.

In Bash scripting, there are three basic loop constructs: for loop, while loop, and until loop.

We'll go through the fundamentals of for loops in Bash in this tutorial. We will also address a few FAQs on Bash for Loop.

The Standard Bash for Loop

The for loop iterates across a list of items, carrying out the commands specified.

The Bash for loop is written as follows:

for item in [LIST]
do
  [COMMANDS]
done

The list could be a sequence of strings separated by spaces, a range of integers, the output of a command, an array, or something else entirely.

Loop over strings

The loop will go over each item in the list of strings in the example below, with the variable element set to the current item:

for element in Hydrogen Helium Lithium Beryllium
do
  echo "Element: $element"
done

The following is the output of the loop:

Output

Element: Hydrogen
Element: Helium
Element: Lithium
Element: Beryllium

Loop over a number range

By specifying a start and endpoint for the range, you can describe a range of numbers or characters using the sequence expression. The following is an example of a sequence expression:

{START..END}

An example loop that iterates through all numbers from 0 to 3 is as follows:

for i in {0..3}
do
  echo "Number: $i"
done

Output

Number: 0
Number: 1
Number: 2
Number: 3

When using ranges, you can now specify an increment starting with Bash 4. The expression is written as follows:

{START..END..INCREMENT}

Here's an example of how to increment by 5:

for i in {0..20..5}
do
  echo "Number: $i"
done

Output

Number: 0
Number: 5
Number: 10
Number: 15
Number: 20

Loop over array elements

The for loop can also be used to iterate over an array of elements.

In the example below, we're creating an array called BOOKS and iterating through its elements.

BOOKS=('In Search of Lost Time' 'Don Quixote' 'Ulysses' 'The Great Gatsby')

for book in "${BOOKS[@]}"; do
  echo "Book: $book"
done

Output

Book: In Search of Lost Time
Book: Don Quixote
Book: Ulysses
Book: The Great Gatsby

The C-style Bash for loop

The C-style for loop has the following syntax:

for ((INITIALIZATION; TEST; STEP))
do
  [COMMANDS]
done

When the loop begins, the INITIALIZATION portion is only run once. The TEST section is then assessed. The loop is terminated if it is false. If the TEST is true, the for loop's body commands are executed, and the STEP portion is modified.

The loop in the following example code starts with i = 0 and checks if  i ≤ 10 before each iteration. If true, the loop prints the current value of i and [increments the variable] i by 1 (i++), else it ends.

for ((i = 0 ; i <= 1000 ; i++)); do
  echo "Counter: $i"
done

The loop will iterate, 1001 times, resulting in the following output:

Output

Counter: 0
Counter: 1
Counter: 2
...
Counter: 998
Counter: 999
Counter: 1000

break and continue Statements

The for loop can be controlled using the break and continue statements.

break Statement

The break statement ends the current loop and transfers control to the statement that comes after it. When a given condition is met, it is frequently used to end the loop.

The if statement is used in the following example to stop the loop from running once the current iterated item equals 'Lithium.'

for element in Hydrogen Helium Lithium Beryllium; do
  if [[ "$element" == 'Lithium' ]]; then
    break
  fi
  echo "Element: $element"
done

echo 'All Done!'

Output

Element: Hydrogen
Element: Helium
All Done!

continue Statement

The continue command ends the current loop iteration and moves program control to the next loop iteration.

We're iterating through a set of numbers in the following example. The continue statement will cause execution to return to the beginning of the loop and continue with the next iteration when the current iterated item equals '2':

for i in {1..5}; do
  if [[ "$i" == '2' ]]; then
    continue
  fi
  echo "Number: $i"
done

Output

Number: 1
Number: 3
Number: 4
Number: 5

Bash for Loop Examples

Rename files with spaces in the filename

By replacing space with an underscore, the following example shows how to rename all files in the current directory that have a space in their names:

for file in *\ *; do
  mv "$file" "${file// /_}"
done

Let's take a look at the code one line at a time:

• The first line sets up a for loop that loops over a list of all files with a space in their name. The list is created by the expression *\ *.
• The second line replaces the space in each item of the list with an underscore (_) and copies the file to a new location. The portion ${file// /_} replaces a pattern within a parameter with a string using shell parameter expansion.
• The word done denotes the end of a loop segment.

Changing file extension

The following example explains how to use the Bash for loop to rename all files in the current directory that finish in .jpeg to.jpg by replacing the file extension.

for file in *.jpeg; do
    mv -- "$file" "${file%.jpeg}.jpg"
done

Let's take a look at the code one line at a time:

• The first line starts a for loop that iterates over a list of all files that end in '.jpeg.'
• The second line replaces '.jpeg' with '.jpg' for each item on the list and moves the file to a new location. ${file%.jpeg} removes the '.jpeg' element of the filename using shell parameter expansion.
• done means that the loop segment has ended.

FAQs on Bash for Loop

Conclusion

The Bash for loop is used to run a series of commands over and over again for a predetermined number of times.

If you have any queries, please leave a comment below and we’ll be happy to respond to them.