Bash Script Loop Through Text File Of Filenames

Bash Script: Looping Through a Text File of Filenames

This article details how to efficiently process a list of filenames stored in a text file using a bash script. This is a common task in scripting, useful for automating file manipulation, processing, or analysis. We'll explore different looping methods and best practices for robustness and error handling. Understanding this technique is crucial for anyone working with bulk file operations in a Linux or macOS environment.

What We'll Cover:

• Reading filenames from a text file.
• Using while loops for efficient processing.
• Incorporating error handling to manage unexpected situations.
• Advanced techniques for file type filtering and more complex operations.

Reading Filenames from a Text File

The foundation of our script lies in correctly reading the filenames from the input text file. Let's assume your filenames are listed one per line in a file named filenames.txt.

A simple and effective method is using the while loop with read:

while IFS= read -r filename; do
  # Process each filename here
  echo "Processing: $filename"
done < filenames.txt

IFS= read -r filename reads each line into the filename variable. IFS= prevents word splitting, and -r prevents backslash escapes from being interpreted. The < filenames.txt redirects the file's content into the loop.

Processing Each Filename

Inside the while loop, you can perform any operation on each filename. For example, to display the contents of each file:

while IFS= read -r filename; do
  echo "Processing: $filename"
  if [ -f "$filename" ]; then  # Check if the file exists
    cat "$filename"
  else
    echo "Error: File '$filename' not found."
  fi
done < filenames.txt

This enhanced script includes a crucial check using [ -f "$filename" ] to verify the file's existence before attempting to process it. This prevents errors and improves script robustness.

Advanced Techniques and Error Handling

Let's expand on this by incorporating more sophisticated error handling and file type filtering:

#!/bin/bash

# Check if the filenames file exists
if [ ! -f "filenames.txt" ]; then
  echo "Error: filenames.txt not found."
  exit 1
fi

while IFS= read -r filename; do
  # Check if the file exists and is a regular file
  if [ -f "$filename" ]; then
    # Check if the file is a specific type (e.g., .txt)
    if [[ "$filename" == *.txt ]]; then
      echo "Processing .txt file: $filename"
      #Process only .txt files here, for example:
      grep "keyword" "$filename" > "${filename%.txt}.out" #save grep output to a new file

    else
      echo "Skipping non-.txt file: $filename"
    fi
  else
    echo "Error: File '$filename' not found."
  fi
done < filenames.txt

echo "Script completed."
exit 0

This improved script:

• Checks for the input file: The script now verifies if filenames.txt exists before proceeding.
• Filters by file type: It only processes files ending with .txt, skipping others. You can easily adjust this to match other file extensions.
• Adds more specific processing: It uses grep to search for a keyword "keyword" within each .txt file and saves the output to a new file. Remember to replace "keyword" with your actual search term.
• Improved error messages: Provides more informative error messages to aid debugging.
• Uses exit codes: exit 0 indicates successful completion, while exit 1 signals an error.

This example showcases a powerful and adaptable framework. You can replace the grep command with any other command-line tool or custom function to perform the desired operations on your files. Remember to adjust file paths and commands to fit your specific needs. This approach promotes clean, maintainable, and robust bash scripting for file processing.