Add File Extension to Files from Bash

In an operating system, file extensions usually help identify file types and ensure proper handling by applications. When working with files in a Unix-like operating system including Linux, you may encounter situations where files sometimes lack extensions, making it difficult to identify their types or open them with the appropriate applications.

Fortunately, Bash, the default shell on many Linux distributions, provides powerful tools to simplify the process of adding file extensions. This tutorial will guide you through various options that you can use to add file extensions to files using Bash.

Adding File Extension using the mv Command

The simplest way to add an extension to a single file is using the mv (move) command ?

$ mv filename filename.extension

For example,

$ mv document document.txt

Using a for Loop

Using a for loop, you can add extensions to multiple files in a directory ?

for file in *; do
   mv "$file" "$file.extension"
done

For example, to add .txt extension to all files in a directory, you can use ?

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

Using the find Command

The find command in Linux offers exec flag to execute any commands on items found in the search. To add .txt extension to all files in subdirectories under current directory which do not have an extension, using find command with exec, you can try ?

$ find . -type f ! -name "*.*" -exec mv {} {}.txt \;

Adding Extensions to Specific Files

If you want to add extensions to files that match a particular pattern, you can modify the for loop earlier with search pattern as ?

for file in prefix*; do
   mv "$file" "$file.extension"
done

Handling Spaces in Filenames

Some files can have spaces in their names and if you want to remove all the spaces in the filename post appending the extension, you need to modify the mv command argument accordingly. So, if your filename is "some file name", it will be renamed to "some_file_name.txt"

for file in *; do
   mv "$file" "${file// /_}.extension"
done

Checking for Existing Extensions

You can add file extensions only to files which are missing them. As such, to avoid adding extensions to files that already have them, use mv command with for loop as given below ?

for file in *; do
   if [[ $file != *.* ]]; then
      mv "$file" "$file.extension"
   fi
done

Adding Extensions Based on File Type

One common possible use case where you may need to add the file extensions is to add the respective extension based on its actual file type, assuming the original extensions are somehow lost or not already available.

We can determine the file type using the type command and then we can use the case statement to instruct renaming of file types according to file's content.

for file in *; do
   type=$(file -b "$file")
   case "$type" in
      "JPEG image"*)
         mv "$file" "$file.jpg"
         ;;
      "PNG image"*)
         mv "$file" "$file.png"
         ;;
      "ASCII text"*)
         mv "$file" "$file.txt"
         ;;
   esac
done

Best Practices

Always use quotes ? You should wrap filenames in quotes to handle spaces and special characters.

Test first ? When working with multiple files, you can use echo command to preview changes before executing them as actual mv command. Here's an example ?

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

Make backups ? You should create backups before batch renaming multiple files. To create backup of a directory using cp command, try something like ?

$ cp -r directory directory_backup

Conclusion

Adding file extensions in Linux using Bash can be accomplished through various methods, from simple one-file operations to complex batch processing. You can choose the appropriate method based on your specific needs and always remember to back up your files before performing batch operations.

Remember that these commands are powerful and can potentially rename many files at once, so use them carefully and test with a small subset of files first. This guide covers the most common scenarios, but Bash's flexibility allows for many other approaches depending on your specific requirements.