Introduction to Bash ‘for’ Loops
One of the fundamental concepts in any programming language is the loop. It allows us to execute a block of code multiple times, which is especially handy for repetitive tasks. In bash scripting, one of the most commonly used loop constructs is the ‘for’ loop. This tutorial will walk you through the basics and intricacies of for loops in bash.
Basic Syntax of ‘for’ Loop
The basic structure of a for loop in bash is as follows:
for VARIABLE in ITEM_1 ITEM_2 ... ITEM_N
do
COMMAND1
done
Let’s break this down:
- VARIABLE: A placeholder that will hold each value in a list or an array sequentially.
- ITEM_1 … ITEM_N: A sequence of items that the VARIABLE will iterate through.
- The do keyword initiates the cycle that will run once for each item.
- COMMAND1: The command that will execute for each item. It can be any valid bash command or script.
- Finally, the loop iteration ends with done.
Here’s a practical example:
for fruit in apple banana cherry
do
echo "I like $fruit!"
done
This simple loop will print out a statement for each fruit item listed.
Looping Over a Range of Numbers
You can loop over a range of numbers by using the ‘seq‘ command inside the for loop or by using brace expansion.
Here is how you do it using seq:
for number in $(seq 1 10)
do
echo "Counting... $number"
done
And here is the same loop using brace expansion:
for number in {1..10}
do
echo "Counting... $number"
done
Both code snippets will count from 1 to 10, outputting each number on a new line.
Iterating Over Array Elements
If you have an array, you can iterate over its elements like so:
fruits=(apple banana cherry)
for fruit in "${fruits[@]}"
do
echo "Fruit: $fruit"
done
The notation ${fruits[@]} indicates that each element of the array fruits will be passed to the loop.
Iterating Over Files and Directories
The for loop can also be used to easily loop through files and directories.
To process each .txt file in a directory, you can use:
for file in *.txt
do
echo "Processing $file"
done
This will match all files ending with .txt in the current directory, processing each one in turn.
Nested for Loops
Nested loops are loops within loops, and bash supports them readily. Here’s an example of a nested for loop:
for outer in 1 2 3
do
for inner in a b c
do
echo "Outer: $outer; Inner: $inner"
done
done
This will create a combination of every element from the first loop with every element from the second loop.
Using the C-Style for Loop
While less common, you can use a C-style for loop in Bash, which looks like this:
for ((i = 0; i < 10; i++))
do
echo "C-style counting: $i"
done
This style is familiar to those with a background in languages like C or Java.
Best Practices and Tips
- Quote your variables to prevent unwanted word splitting, especially when dealing with strings that could potentially contain spaces.
- Prefer brace expansion or seq with caution depending on your script’s portability requirements. Brace expansion is a feature of bash, while seq is an external program that might not be installed on every system.
- When using wildcards (e.g.,
*.txt), be aware of the potential for no files to match the pattern. You might want to include additional checks before processing files. - For more complex loops, consider readability. If your loop gets too complicated, it could be a sign to refactor your script or to use functions.
Conclusion
In this tutorial, we covered the use of for loops in bash scripting, including syntax, examples, and best practices. You are now equipped to implement for loops in your own scripts effectively. Happy scripting!
