Bash Script for Loop Explained with Examples

If you're a Linux or Unix user, chances are you've used Bash at least once or twice. Bash is a command-line shell that lets you interact with your operating system in a more direct and powerful way than using a graphical user interface. One of most powerful features of Bash is for loop, which lets you automate repetitive tasks by iterating over a list of values. In this article, we'll explore how Bash for loop works and provide some examples of how you can use it in your own scripts.

What is a For Loop?

A for loop is a control structure that lets you execute a block of code repeatedly for a fixed number of times, or over a list of values. basic syntax of a for loop is as follows −

for variable in list
do
   commands
done

Here, variable is a placeholder that takes on value of each item in list in turn, and commands is a block of Bash commands that are executed each time loop runs. loop continues until every item in list has been processed.

Iterating Over a List of Values

The simplest use case for a for loop is to iterate over a list of values. Let's say you have a list of file names in a directory, and you want to print out each name on a separate line. Here's how you would do it −

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

In this example, we use wildcard character * to specify that we want to loop over all files in directory. echo command is used to print out each file name on a separate line. Notice that we use $ symbol to reference file variable when we call echo.

Iterating Over a Range of Numbers

Another common use case for a for loop is to iterate over a range of numbers. Let's say you want to print out numbers 1 to 10 on separate lines. Here's how you would do it −

Example

for i in {1..10}
do
   echo $i
done

In this example, we use curly braces {} to specify range of numbers we want to loop over. echo command is used to print out each number on a separate line.

Output

1
2
3
4
5
6
7
8
9
10

Iterating Over an Array

In Bash, you can also create arrays to store multiple values in a single variable. Here's an example of how you can use a for loop to iterate over an array of names −

Example

names=("Alice" "Bob" "Charlie" "Dave")

for name in "${names[@]}"
do
   echo "Hello, $name!"
done

Output

Hello, Alice!
Hello, Bob!
Hello, Charlie!
Hello, Dave!

In this example, we define an array called names that contains four values. "${names[@]}" syntax is used to expand array into a list of values that for loop can iterate over. echo command is used to print out a greeting for each name.

Nested For Loops

Sometimes you may need to use a for loop inside another for loop. This is called a nested for loop. Let's say you have a list of file extensions and a list of directories, and you want to loop over each combination of extension and directory. Here's how you would do it −

Example

extensions=("txt" "pdf" "docx")
directories=("/path/to/dir1" "/path/to/dir2" "/path/to/dir3")

for ext in "${extensions[@]}"
do
   for dir in "${directories[@]}"
   do
      echo "Files with extension .$ext in $dir:"
         ls "$dir"/*.$
      .$ext
   done
done

In this example, we define two arrays called `extensions` and `directories`. We use a nested for loop to iterate over each combination of extension and directory. `echo` command is used to print out a message indicating extension and directory being processed, and `ls` command is used to list all files in directory with current extension.

Skipping an Iteration with Continue

Sometimes you may want to skip an iteration of for loop based on a certain condition. This can be done using `continue` keyword. Let's say you have a list of file names, and you want to print out all names except for files that start with letter "a". Here's how you would do it −

for file in /path/to/directory/*
do
if [[ "$file" == /path/to/directory/a* ]]; then
   continue
fi
   echo $file
done

In this example, we use an `if` statement to check if file name starts with letter "a". If it does, we use `continue` keyword to skip rest of code in loop and move on to next iteration. If file name does not start with "a", we use `echo` command to print out file name.

Advanced For Loop Techniques

In addition to basic syntax of for loop, Bash also provides a number of advanced techniques that you can use to make your for loops more powerful and flexible. Here are some examples −

Using Output of a Command as a List

You can use output of a command as list to iterate over in a for loop. Let's say you want to print out all directories in your home directory that have more than 10 files in them. Here's how you would do it −

for dir in $(find ~/ -type d -exec sh -c 'ls -1 "{}" | wc -l' \; -print | awk -F: '$1>=10 {print $2}')
do
   echo $dir
done

In this example, we use find command to locate all directories in our home directory. We then use a combination of sh, ls, wc, and awk commands to count number of files in each directory and filter out directories that have fewer than 10 files. resulting list of directories is then used as input to for loop.

Using a C-style For Loop

Bash also provides a way to use a C-style for loop syntax, which can be useful for more complex looping scenarios. Here's an example −

Example

for ((i=0; i<10; i++))
do
   echo $i
done

Output

0
1
2
3
4
5
6
7
8
9

In this example, we use ((...)) syntax to define a C-style for loop that counts from 0 to 9. i++ expression is used to increment value of i by 1 each time loop runs.

Using a While Loop with a Read Command

Finally, you can use a while loop with read command to iterate over a list of values entered by user. Here's an example −

while read line
do
   echo "You entered: $line"
done

In this example, we use read command to read in a line of text entered by user. while loop then continues to run until user enters an end-of-file character (usually Ctrl+D). echo command is used to print out each line of text entered by user.

Tips For Using Bash For Loop Effectively

While Bash for loop is a powerful tool, there are some tips and tricks you can use to make your loops even more effective. Here are some things to keep in mind when using Bash for loop −

Use Descriptive Variable Names

When defining variable that holds each item in list, use a name that describes item's purpose. For example, instead of using x or i, use a name like filename or counter.

Use Quotes Around Variable References

When referencing variable that holds each item in list, always use quotes around reference to ensure that script works correctly even if item contains spaces or other special characters.

Use a Counter Variable For Complex Iterations

When iterating over multiple lists or performing complex calculations in a loop, use a separate counter variable to keep track of current iteration number. This can make loop easier to read and debug.

Avoid Infinite Loops

Always ensure that your loop has a clear termination condition to avoid an infinite loop. For example, if you are iterating over a range of numbers, ensure that loop terminates when it reaches end of range.

Use Conditional Statements When Necessary

If you need to skip certain iterations or terminate loop early based on a certain condition, use conditional statements like if and break to control flow of loop.

By following these tips, you can write more efficient and effective Bash for loops that accomplish your tasks quickly and accurately.

Conclusion

The Bash for loop is a powerful tool for automating repetitive tasks in Linux and Unix environments. By using a for loop, you can easily iterate over a list of values, a range of numbers, or an array. You can also use nested for loops to iterate over multiple lists simultaneously. Additionally, you can use `continue` keyword to skip iterations based on a certain condition. With examples provided in this article, you should be able to start using Bash for loop in your own scripts and make your work more efficient and streamlined.