The for loop in Bash scripting is a powerful tool used to repeat commands and perform repetitive tasks efficiently. Whether you’re managing files, executing commands multiple times, or processing data, mastering the for loop can significantly simplify your scripts.
In this article, we’ll walk through several examples of for loops, starting with the basics and advancing to more complex applications.
Each example includes easy-to-understand explanations, tables, and links to help you understand Bash for loops thoroughly.
What is a For Loop in Bash?
In Bash, a for loop allows us to iterate over a series of items and perform a command for each one. Syntax for the basic for loop in Bash is:
for variable in list
do
command1
command2
...
doneThe loop will execute each command block for each item in the list, using the variable to represent each item in the sequence.
Basic For Loop Example
Let’s start with a simple example that prints numbers from 1 to 5.
Code:
for i in 1 2 3 4 5
do
echo "Number: $i"
doneOutput:
Number: 1
Number: 2
Number: 3
Number: 4
Number: 5In this example, the loop iterates through the numbers 1 to 5, printing each one with the echo command.
Using Ranges in For Loops
We can use {} to specify ranges, which is often simpler and more readable.
Code:
for i in {1..5}
do
echo "Count: $i"
doneOutput:
Count: 1
Count: 2
Count: 3
Count: 4
Count: 5This method is especially useful for long sequences since Bash will automatically fill in all numbers in the specified range.
Step Values in For Loops
Sometimes you may need to skip numbers in a loop, which is done by adding a step value {start..end..step}.
Code:
for i in {0..20..5}
do
echo "Step Value: $i"
doneOutput:
Step Value: 0
Step Value: 5
Step Value: 10
Step Value: 15
Step Value: 20The for loop increments by 5 instead of 1, making it efficient for specific tasks.
Looping Through Arrays
Bash loops are also ideal for processing arrays. Here’s an example with an array of fruits.
Code:
fruits=("apple" "banana" "cherry")
for fruit in "${fruits[@]}"
do
echo "Fruit: $fruit"
doneOutput:
Fruit: apple
Fruit: banana
Fruit: cherryThis loop uses "${fruits[@]}" to reference all elements in the array, iterating over each fruit name.
Looping Over Files in a Directory
You can use for loops to iterate through files in a directory. This is useful for file manipulation and batch processing.
Code:
for file in /path/to/directory/*
do
echo "Processing $file"
doneOutput:
Processing file1.txt
Processing file2.txt
...Replace /path/to/directory with the directory path containing the files. The loop will display each file name in that directory.
Using C-Style For Loops in Bash
Bash also supports a C-style for loop structure, which can be handy for complex iterations.
Code:
for ((i=1; i<=5; i++))
do
echo "Iteration $i"
doneOutput:
Iteration 1
Iteration 2
Iteration 3
Iteration 4
Iteration 5In this style, you can control the initialization, condition, and increment, making it more flexible for customized loops.
Combining For Loops with Conditional Statements
You can add conditional statements like if statements inside a for loop for greater control.
Code:
for i in {1..10}
do
if (( $i % 2 == 0 ))
then
echo "$i is even"
else
echo "$i is odd"
fi
doneOutput:
1 is odd
2 is even
3 is odd
4 is even
...In this example, the loop uses the if condition to determine whether each number is odd or even.
Nested For Loops
Nested loops allow for more complex operations, like processing two lists simultaneously.
Code:
for i in {1..3}
do
for j in {a..c}
do
echo "Pair: $i$j"
done
doneOutput:
Pair: 1a
Pair: 1b
Pair: 1c
Pair: 2a
Pair: 2b
Pair: 2c
Pair: 3a
Pair: 3b
Pair: 3cHere, we loop over both numbers and letters to generate all combinations.
Practical Examples of For Loops
Example 1: Counting Lines in Files
Count the number of lines in each text file in a directory.
Code:
for file in *.txt
do
lines=$(wc -l < "$file")
echo "$file has $lines lines"
doneThis loop uses wc -l to count the lines in each .txt file in the current directory.
Example 2: File Renaming
Rename all .jpg files in a directory by appending _backup to their names.
Code:
for file in *.jpg
do
mv "$file" "${file%.jpg}_backup.jpg"
doneThis loop processes each .jpg file, appending _backup to the filename.
For Loop Syntax Comparison Table
| Loop Type | Syntax | Description |
|---|---|---|
| Basic For Loop | for var in list; do commands; done | Executes commands for each item in the list |
| Range-Based Loop | for var in {start..end}; do commands; done | Loops over a numerical range |
| C-Style For Loop | for ((init; cond; increment)); do commands; done | C-style looping for more complex conditions |
| Array Loop | for var in "${array[@]}"; do commands; done | Loops over elements in an array |
| Directory Loop | for file in /path/*; do commands; done | Iterates through all files in a directory |
| Nested For Loop | for i in list1; do for j in list2; do commands; done; done | Executes loops within loops for complex tasks |
Best Practices for Using For Loops
- Use Range-Based Loops for Simplicity: When iterating over numbers,
{1..N}syntax is usually faster and easier to read. - Optimize for Large Files: For handling numerous files, combining loops with
findcan improve performance. - Avoid Hard-Coding: Make your loops dynamic by using variables for counts or ranges to improve flexibility.
- Include Error Handling: Always add checks to handle errors, especially when working with files to avoid data loss or corruption.
Resources and Further Reading
- GNU Bash Reference Manual – Comprehensive guide on Bash scripting.
- Bash For Loop Tutorial – Additional examples and tutorials on for loops.
- Stack Overflow – Community Q&A for Bash scripting solutions.
Conclusion
Bash for loops are incredibly versatile and can be applied to a range of tasks, from processing lists and arrays to handling complex conditional logic. By understanding the syntax and applying these practical examples, you can make your scripts more efficient and adaptable for any command-line automation tasks.
