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[Keno Kivabe](https://blogs.kenokivabe.com/) # For Loop in Bash: Basics and Examples *Repeat commands using classic `for` loops.* *** Bash scripting is a powerful way to automate tasks in Linux and UNIX-like systems. One staple of any programming language is the ability to repeat actions—enter the **`for` loop**. In this article, we'll explore the basics of the `for` loop in Bash, see its syntax, usage patterns, and go through practical examples. ## What is a `for` Loop? A `for` loop allows you to execute a set of commands repeatedly, iterating through a sequence of values. This is especially helpful when you need to manipulate groups of files, process batch jobs, or automate repetitive tasks. ## Basic Syntax of `for` Loops in Bash There are two primary forms of the `for` loop in Bash: ### 1\. List-Based (Classic) Syntax ``` for VARIABLE in VALUE1 VALUE2 VALUE3; do commands done ``` - `VARIABLE`: The control variable that holds the current value in the iteration. - `VALUE1 VALUE2 VALUE3`: A list of values to iterate over. - `commands`: The block of code to execute each iteration. #### Example ``` for color in red green blue; do echo "Color: $color" done ``` **Output**: ``` Color: red Color: green Color: blue ``` ### 2\. C-like Syntax (Numeric Loop) If you want to iterate over a sequence of numbers, use this familiar construct: ``` for (( initial; condition; increment )); do commands done ``` #### Example ``` for (( i=1; i<=5; i++ )); do echo "Iteration $i" done ``` **Output**: ``` Iteration 1 Iteration 2 Iteration 3 Iteration 4 Iteration 5 ``` ## Common Use Cases ### 1\. Looping Over Files List all `.txt` files in a directory: ``` for file in *.txt; do echo "Found file: $file" done ``` ### 2\. Processing Arguments Iterate over command line arguments: ``` for arg in "$@"; do echo "Argument: $arg" done ``` ### 3\. Reading Lines from a File Read every line from a file: ``` while IFS= read -r line; do echo "Line: $line" done < filename.txt ``` Or using a `for` loop with `$(cat ...)` (Note: less robust with spaces): ``` for line in $(cat filename.txt); do echo "Word or line: $line" done ``` ### 4\. Ranges with `seq` Iterate over a number range using the `seq` utility: ``` for i in $(seq 1 5); do echo "Counting: $i" done ``` ## Tips and Best Practices - **Quote variables**: Always quote your variables to prevent word splitting and glob expansion: `"$file"`, `"$var"`. - **Be wary with spaces**: Using `for x in $(cat file.txt)` splits on whitespace; for line-by-line reading, prefer `while read`. - **Brace Expansion**: For simple numeric or alphabetic ranges, use `{}`: ``` for i in {1..5}; do echo "Number: $i" done ``` ## Conclusion The Bash `for` loop is a fundamental feature that can save you time and optimize your workflows. Whether you’re iterating over files, arguments, or numeric ranges, understanding these looping basics will enhance your scripting skills. Experiment with the examples above to level up your command-line automation\! [Back to All Articles](https://blogs.kenokivabe.com/)

*Repeat commands using classic `for` loops.* *** Bash scripting is a powerful way to automate tasks in Linux and UNIX-like systems. One staple of any programming language is the ability to repeat actions—enter the **`for` loop**. In this article, we'll explore the basics of the `for` loop in Bash, see its syntax, usage patterns, and go through practical examples. ## What is a `for` Loop? A `for` loop allows you to execute a set of commands repeatedly, iterating through a sequence of values. This is especially helpful when you need to manipulate groups of files, process batch jobs, or automate repetitive tasks. ## Basic Syntax of `for` Loops in Bash There are two primary forms of the `for` loop in Bash: ### 1\. List-Based (Classic) Syntax ``` for VARIABLE in VALUE1 VALUE2 VALUE3; do commands done ``` - `VARIABLE`: The control variable that holds the current value in the iteration. - `VALUE1 VALUE2 VALUE3`: A list of values to iterate over. - `commands`: The block of code to execute each iteration. #### Example ``` for color in red green blue; do echo "Color: $color" done ``` **Output**: ``` Color: red Color: green Color: blue ``` ### 2\. C-like Syntax (Numeric Loop) If you want to iterate over a sequence of numbers, use this familiar construct: ``` for (( initial; condition; increment )); do commands done ``` #### Example ``` for (( i=1; i<=5; i++ )); do echo "Iteration $i" done ``` **Output**: ``` Iteration 1 Iteration 2 Iteration 3 Iteration 4 Iteration 5 ``` ## Common Use Cases ### 1\. Looping Over Files List all `.txt` files in a directory: ``` for file in *.txt; do echo "Found file: $file" done ``` ### 2\. Processing Arguments Iterate over command line arguments: ``` for arg in "$@"; do echo "Argument: $arg" done ``` ### 3\. Reading Lines from a File Read every line from a file: ``` while IFS= read -r line; do echo "Line: $line" done < filename.txt ``` Or using a `for` loop with `$(cat ...)` (Note: less robust with spaces): ``` for line in $(cat filename.txt); do echo "Word or line: $line" done ``` ### 4\. Ranges with `seq` Iterate over a number range using the `seq` utility: ``` for i in $(seq 1 5); do echo "Counting: $i" done ``` ## Tips and Best Practices - **Quote variables**: Always quote your variables to prevent word splitting and glob expansion: `"$file"`, `"$var"`. - **Be wary with spaces**: Using `for x in $(cat file.txt)` splits on whitespace; for line-by-line reading, prefer `while read`. - **Brace Expansion**: For simple numeric or alphabetic ranges, use `{}`: ``` for i in {1..5}; do echo "Number: $i" done ``` ## Conclusion The Bash `for` loop is a fundamental feature that can save you time and optimize your workflows. Whether you’re iterating over files, arguments, or numeric ranges, understanding these looping basics will enhance your scripting skills. Experiment with the examples above to level up your command-line automation\!