Looping over files and directories in Bash Scripting - Cheat Sheet & Quick Revision

Recall & Review

beginner

What command is commonly used in bash to loop over all files in a directory?

The for loop is commonly used with a wildcard pattern like for file in *; do ... done to loop over all files in the current directory.

beginner

How do you loop over only directories in bash?

Use a pattern that matches directories, for example: for dir in */; do ... done. The trailing slash / ensures only directories are matched.

beginner

What does the following script do?

for file in *.txt; do echo "$file"; done

It loops over all files ending with .txt in the current directory and prints each filename on its own line.

intermediate

Why should you quote variables like "$file" inside loops?

Quoting variables prevents word splitting and preserves spaces or special characters in filenames, avoiding errors during processing.

intermediate

How can you loop over files recursively in bash?

You can use the find command with a loop, for example:
find . -type f -name "*.txt" -exec bash -c 'echo "$0"' {} \; or use a while read loop with find and read.

Which pattern matches only directories in a bash for loop?

A */

B *

C *.txt

D .*

The pattern */ matches only directories because the trailing slash indicates directories.

What is the purpose of quoting variables like "$file" inside loops?

A To prevent word splitting and handle spaces in filenames

B To make the script run faster

C To convert filenames to uppercase

D To ignore hidden files

Quoting variables prevents word splitting and preserves spaces or special characters in filenames.

How do you loop over all .txt files in the current directory in bash?

A while read file; do ... done < *.txt

B for file in *; do if file ends with .txt; done

C loop *.txt

D for file in *.txt; do ... done

The for file in *.txt; do ... done syntax loops over all files ending with .txt.

Which command helps to loop over files recursively in bash?

A ls -R

B find

C grep

D cat

The find command is used to search files recursively and can be combined with loops.

What does the wildcard * match in bash?

A Only files starting with a dot

B Only files with extensions

C All files and directories except hidden ones

D Only directories

The wildcard * matches all files and directories except hidden ones (those starting with a dot).

Explain how to loop over all files in a directory using bash scripting.

Think about using a for loop with * and why quoting matters.

Describe how to loop over directories only and why the pattern differs from looping over files.

Directories end with a slash in patterns.

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