A for loop in Bash is a control structure that allows you to repeat a set of commands for each item in a list, over a range, or based on other criteria. It’s commonly used for tasks like iterating over files in a directory, processing elements in an array, or executing a block of code multiple times.

Basic Syntax

The basic syntax of a for loop in Bash is as follows:

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for VARIABLE in LIST
do
    # Commands to be executed
done

  • VARIABLE is a placeholder that takes the value of each item in LIST one by one.
  • LIST can be a list of values, a range of numbers, or even a command that generates a list.

  • The do and done keywords define the start and end of the loop block.

    Example 1: Loop Over a List of Values

    Here’s an example of a for loop that iterates over a list of words:

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    for word in apple banana cherry
    do
        echo "The fruit is: $word"
    done

    Output:

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    The fruit is: apple
    The fruit is: banana
    The fruit is: cherry

    Example 2: Loop Over a Range of Numbers

    You can also loop over a sequence of numbers:

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    for i in {1..5}
    do
        echo "Number: $i"
    done

    Output:

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    Number: 1
    Number: 2
    Number: 3
    Number: 4
    Number: 5

    Example 3: Loop Over Files in a Directory

    A common use case is looping over files in a directory:

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    for file in /path/to/directory/*
    do
        echo "Processing $file"
    done

    This loop will iterate over each file in the specified directory and execute the commands inside the loop for each file.
    For example, read all files from /home/john directory.

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    for file in /home/john/*
    do
        echo "Processing $file"
    done

    Output:

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    Processing /home/john/Desktop
    Processing /home/john/Documents
    Processing /home/john/Downloads
    Processing /home/john/Music
    Processing /home/john/Pictures
    Processing /home/john/Public
    Processing /home/john/Templates
    Processing /home/john/Videos
    Processing /home/john/snap
    Processing /home/john/tut

    Example 4: C-Style For Loop

    Bash also supports a C-style for loop, similar to loops in languages like C or Java:

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    for (( i=1; i<=5; i++ ))
    do
        echo "Iteration $i"
    done

    Output:

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    Iteration 1
    Iteration 2
    Iteration 3
    Iteration 4
    Iteration 5

    Summary

  • for loops are versatile and can be used to iterate over lists, ranges, or output from commands.

  • They are useful for automating repetitive tasks, processing files, and more.

This should give you a solid understanding of how to use for loops in Bash scripts.