Bash Scripting: For Loops

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What is a For Loop?

A for loop is a control flow statement that allows code to be executed repeatedly. It is used when you want to perform a task for every item in a set. This set could be a list of numbers, strings, file names, or any other type of data that bash can handle.

The basic structure of a for loop in bash is as follows:

for variable in set
do
    command1
    command2
    ...
    commandN
done

In this structure, variable is a placeholder that represents each item in the set as the loop iterates over it. The commands between do and done are executed for each item.

Basic For Loop Example

Let's start with a simple example. Suppose we want to print the numbers from 1 to 10. Here's how we can do it using a for loop:

for current_number in {1..10}
do
    echo "Current number is: $current_number"
    sleep 1
done

In this script, current_number is the variable that represents each item in the set {1..10}. The echo command prints the current number, and sleep 1 pauses the script for one second before the next iteration. The loop ends after it has iterated through all the numbers in the set.

Practical Use of a For Loop: Compressing Log Files

Now, let's look at a more practical example. Suppose we have a directory full of log files, and we want to compress each of them using the tar command. Here's how we can do it using a for loop:

for file in /var/log/*.log
do
    tar -czvf "$file.tar.gz" "$file"
done

In this script, file is the variable that represents each log file in the /var/log directory. The tar -czvf "$file.tar.gz" "$file" command compresses each log file and creates a .tar.gz file for it. The loop ends after it has iterated through all the log files in the directory.

Variable Naming in For Loops

It's important to note that the variable used in the for loop can be named anything. It's common to see single-letter variable names in for loop examples online, but the variable name doesn't actually matter. For example, the following script is equivalent to the previous one:

for f in /var/log/*.log
do
    tar -czvf "$f.tar.gz" "$f"
done

In this script, f is the variable that represents each log file. Even though it's a different name, it serves the same purpose as file in the previous script.

Conclusion

For loops are a powerful tool in bash scripting. They allow you to automate tasks that need to be performed on every item in a set. Whether you're printing numbers, compressing log files, or doing something else entirely, for loops can make your scripts more efficient and easier to read.

• Bash Scripting
• Bash Scripting: Introduction
• Variables
• Basic Math
• Control Statements
• While Loops
• Functions
• Case Statements
• Arguments

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Version 1.0

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