Looping over files and directories in Bash Scripting

Introduction

Looping over files and directories helps you do the same task on many files automatically. It saves time and avoids mistakes from doing things one by one.

You want to rename all photos in a folder.

You need to check the size of every file in a directory.

You want to convert all text files to another format.

You want to delete all temporary files in a folder.

You want to list all files and do something with each.

Syntax

Bash Scripting

for file in /path/to/directory/*; do
  command "$file"
done

The * means all files and folders inside the directory.

Use quotes around $file to handle names with spaces.

Examples

This prints the full path of every file and folder inside the Documents folder.

Bash Scripting

for file in ~/Documents/*; do
  echo "$file"
done

This shows the content of every text file in the /tmp folder.

Bash Scripting

for file in /tmp/*.txt; do
  cat "$file"
done

This lists only directories inside /var/log by using */ which matches folders.

Bash Scripting

for dir in /var/log/*/; do
  echo "Directory: $dir"
done

Sample Program

This script goes through everything in the current folder. It prints if the item is a file or a directory.

Bash Scripting

#!/bin/bash

# Loop over all files in the current directory
for file in *; do
  if [ -f "$file" ]; then
    echo "File: $file"
  elif [ -d "$file" ]; then
    echo "Directory: $file"
  fi
done

Important Notes

Use -f to check if it is a file and -d to check if it is a directory.

Be careful with spaces in file names; always quote variables like "$file".

Using */ matches only directories inside a folder.

Summary

Loops let you repeat commands on many files or folders easily.

Use for with a pattern like * to select files.

Check if an item is a file or directory with -f or -d.

Practice

1. What does the following bash loop do?
for file in *; do echo "$file"; done

easy

A. It runs the echo command once without any output.

B. It deletes all files in the current folder.

C. It lists only directories in the current folder.

D. It prints the names of all files and directories in the current folder.

Solution

1. Step 1: Understand the loop variable and pattern

   The loop uses * which matches all files and directories in the current folder.

2. Step 2: Analyze the command inside the loop

   Each matched item is stored in file and printed with echo.

3. Final Answer:

   It prints the names of all files and directories in the current folder. -> Option D

4. Quick Check:

   Loop over * prints all names [OK]

Hint: Remember * matches all files and folders [OK]

Common Mistakes:

• Thinking * matches only files, not directories
• Assuming the loop deletes files
• Believing echo runs only once

2. Which of the following is the correct syntax to loop over all directories only in the current folder?

easy

A. for dir in *; do if [ -f "$dir" ]; then echo "$dir"; fi; done

B. for dir in *; do echo "$dir"; done

C. for dir in */; do echo "$dir"; done

D. for dir in /*; do echo "$dir"; done

Solution

1. Step 1: Understand the pattern for directories

   The pattern */ matches only directories in the current folder because directories end with a slash.

2. Step 2: Check the loop and echo command

   The loop iterates over directories and prints their names correctly.

3. Final Answer:

   for dir in */; do echo "$dir"; done -> Option C

4. Quick Check:

   */ matches directories only [OK]

Hint: Use */ pattern to match directories only [OK]

Common Mistakes:

• Using * matches files and directories, not only directories
• Using -f to check for directories instead of -d
• Using absolute path /* instead of current folder

3. What will be the output of this script if the current folder contains files: file1.txt, file2.txt and a directory docs/?

for item in *; do
  if [ -f "$item" ]; then
    echo "File: $item"
  else
    echo "Not a file: $item"
  fi
done

medium

A. File: file1.txt
File: file2.txt
Not a file: docs

B. File: file1.txt
File: file2.txt
File: docs

C. Not a file: file1.txt
Not a file: file2.txt
Not a file: docs

D. File: file1.txt
Not a file: file2.txt
Not a file: docs

Solution

1. Step 1: Identify file and directory checks

   The script uses [ -f "$item" ] to check if the item is a file.

2. Step 2: Apply checks to each item

   file1.txt and file2.txt are files, so print "File: ..."; docs is a directory, so print "Not a file: docs".

3. Final Answer:

   File: file1.txt File: file2.txt Not a file: docs -> Option A

4. Quick Check:

   -f true for files only [OK]

Hint: Use -f to test files, else it's not a file [OK]

Common Mistakes:

• Assuming directories pass -f test
• Confusing -f with -d
• Expecting directory names with trailing slash

4. Find the error in this script that tries to print only directories:

for item in *; do
  if [ -f "$item" ]; then
    echo "$item"
  fi
done

medium

A. The echo command is missing quotes around $item.

B. It uses -f to check files instead of -d for directories.

C. The loop pattern * does not match any files.

D. The script will print directories correctly.

Solution

1. Step 1: Understand the test used

   The script uses [ -f "$item" ] which checks if the item is a file, not a directory.

2. Step 2: Identify correct test for directories

   To check directories, [ -d "$item" ] should be used instead.

3. Final Answer:

   It uses -f to check files instead of -d for directories. -> Option B

4. Quick Check:

   Use -d to test directories [OK]

Hint: Use -d to check directories, not -f [OK]

Common Mistakes:

• Confusing -f and -d tests
• Assuming * matches only directories
• Ignoring quotes around variables

5. You want to count how many files (not directories) are in the current folder using a bash loop. Which script correctly does this?

hard

A. count=0
for item in *; do
if [ -f "$item" ]; then
count=$((count + 1))
fi
done
echo $count

B. count=0
for item in */; do
count=$((count + 1))
done
echo $count

C. count=0
for item in *; do
if [ -d "$item" ]; then
count=$((count + 1))
fi
done
echo $count

D. count=0
for item in *; do
count=$((count + 1))
done
echo $count

Solution

1. Step 1: Understand the goal

   We want to count files only, so we must check each item with [ -f "$item" ].

2. Step 2: Check each option

   count=0 for item in *; do if [ -f "$item" ]; then count=$((count + 1)) fi done echo $count correctly loops over all items, increments count only if item is a file, then prints count.

3. Final Answer:

   count=0 for item in *; do if [ -f "$item" ]; then count=$((count + 1)) fi done echo $count -> Option A

4. Quick Check:

   Use -f test and increment count [OK]

Hint: Increment count only if -f test passes [OK]

Common Mistakes:

• Counting directories instead of files
• Counting all items without checks
• Using */ pattern which matches directories only