How to Compare Two Files Using Diff Command in Linux [3 Effective Methods]

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Last updated: June 25, 2023

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To compare two files using diff command in Linux, try these three methods:

  1. Use the Diff command to analyze differences between files, focusing on specific line modifications.
  2. Analyze files at a finer level by comparing changes word by word, ignoring whitespace differences.
  3. Divide files into chunks and compare them individually, presenting changes in a contextual and readable format.

When using the Diff command for file comparison, be mindful of common errors such as incorrect file order, formatting differences, comparing directories instead of files, ignoring relevant differences, and not accounting for contextual changes. Avoiding these errors ensures accurate and comprehensive file comparisons in the Linux Terminal.

Read the article below to learn different methods to compare two files using diff commands and common errors with possible solutions.

When working with files in the Linux Terminal, the ability to compare them is crucial. Whether you’re tracking changes in code, analyzing log files, or simply verifying data integrity, the Diff command is an indispensable tool. By leveraging Diff, you can effortlessly identify and understand the variations between two files, empowering you to make informed decisions. In this pillar blog post, I will explore different methods to compare two files using diff command in Linux and common errors that can occur during the process.

How to Compare Two Files Using Diff Command

To compare two files using Diff command, you have three methods at your disposal. The line-by-line method allows for a granular comparison, word-by-word enables a detailed analysis, and the block-by-block approach provides a broader perspective.

1. Comparing Files Line by Line

In the line-by-line file comparison method, you can use the Diff command to analyze the differences between two files. This method is best used when you want a granular comparison of files line by line, enabling you to pinpoint specific modifications. Follow these steps:

  1. Open your Terminal window by pressing Ctrl+Alt+T.
opening terminal 12
  1. Enter the following command:
<strong>diff file1.txt file2.txt</strong>
  1. The output will be:
comparing two files in ubuntu
  1. 1,3c1,2

This line indicates that lines 1 to 3 in file1.txt have been changed and are now represented by lines 1 to 2 in file2.txt.

<strong>< editied file</strong>
<strong>< OS</strong>
<strong>< Ubuntu<strong>

These lines were present in file1.txt (lines 1 to 3), but they have been removed or modified in file2.txt.

This line separates the changes made in file1.txt from those made in file2.txt.

</strong>Linux Ubuntu</strong>

These lines were added in file2.txt and are not present in file1.txt.

  1. 8a8,10

This line indicates that lines 8 to 10 in file2.txt have been added and are not present in file1.txt.

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These lines were added in file2.txt and are not present in file1.txt.

2. Comparing Files Word by Word

Comparing files word by word provides a more granular analysis, allowing you to track changes at a finer level. This method is particularly useful for text-based files where line-level comparison might not suffice. Here are the steps:

  1. Open your Terminal and enter the following command:
<strong>diff -w file1.txt file2.txt</strong>
  1. The -w flag ignores whitespace differences, allowing you to focus on word-level changes.
comparing files word by word
  1. 1d0

This line indicates that line 1 in data.txt has been deleted and is not present in essay.txt.

<strong>< editied file</strong>

This line was present in data.txt (line 1) but has been removed in essay.txt.

  1. 3c2

This line indicates that line 3 in data.txt has been changed and is now represented by line 2 in essay.txt.

<strong>< Ubuntu</strong>

This line was present in data.txt (line 3) but has been removed or modified in essay.txt.

This line separates the changes made in data.txt from those made in essay.txt.

<strong>dsada Ubuntu</strong>

This line was added in essay.txt and is not present in data.txt.

  1. 8a8,10

This line indicates that lines 8 to 10 in essay.txt have been added and are not present in data.txt.

3. Comparing Files Block by Block

Block comparison involves dividing files into chunks, comparing them individually, and presenting the differences. This method offers a broader perspective on file changes.

To compare files using a block-based approach:

  1. Open your Terminal and enter the following command:
<strong>diff -u file1.txt file2.txt</strong>
  1. The -u flag generates unified diff output, which presents the changes in a more readable and contextual format.
comparing files block by block
  1. These lines represent the changes between the compared files. Lines starting with a hyphen (-) indicate lines present in the original file (data.txt) but not in the modified file (essay.txt). Lines starting with a plus sign (+) indicate lines present in the modified file but not in the original file.

Common Errors When Using the Diff Command for File Comparison

The Diff command is a powerful tool for comparing files in the Linux Terminal, but it’s important to be aware of potential errors that can arise. Avoid these errors to ensure accurate and effective comparisons between two files. Here are five common errors to keep in mind:

  • 🚫 Incorrect File Order: One common mistake is providing the files in the wrong order when executing the Diff command. Remember that the order matters: diff file1.txt file2.txt will yield a different result from diff file2.txt file1.txt. Always double-check the order of your files to ensure accurate comparisons. When files are mistakenly provided in the wrong order, the resulting Diff-output will reflect differences as if the order had been reversed. This can lead to confusion and inaccurate analysis of the file changes. To prevent this error, carefully verify the order of the files before running the Diff command.
  • File Formatting Differences: File formatting differences can cause misleading or unexpected results. Be cautious when comparing files with different line endings (e.g., Unix vs. Windows) or encoding formats. These differences can introduce false positives or negatives in the Diff output, leading to incorrect assessments of file variations. To avoid this error, ensure that the compared files have consistent formatting. Use tools like dos2unix or unix2dos to convert line endings if necessary. Additionally, verify that the encoding of the files is consistent to prevent inaccuracies in the Diff results.
  • 📂 Comparing Directories Instead of Files: The Diff command is designed to compare files, not directories. You may encounter an error or incorrect results if you mistakenly attempt to compare directories. The Diff command expects specific files as inputs, not entire directories. Consider using specialized tools like diff -r or rsync to compare directories. These tools are designed to handle directory comparisons and provide more appropriate and accurate results. You can avoid this error and achieve meaningful file comparisons by using the correct tool for the intended purpose.
  • 📝 Ignoring Relevant Differences: Using flags like -w (ignore whitespace) or -I (exclude specific lines) can be helpful in filtering out irrelevant differences. However, exercise caution when employing these flags, as overusing or misusing them might cause relevant differences to be overlooked. Before excluding certain elements from the comparison, thoroughly analyze and understand the nature of the compared files. Ensure that the ignored elements are genuinely irrelevant to the comparison to avoid missing important changes. Careful consideration will help you decide which differences to ignore, ensuring accurate and comprehensive file comparisons.
  • ⚠️ Not Accounting for Contextual Changes: While the Diff command excels at comparing files based on line-by-line or word-by-word changes, it may not capture contextual changes accurately. For instance, changes in paragraph structure, indentation, or formatting may not be adequately reflected in the Diff output. To mitigate this error, consider the nature of the files being compared and the level of detail required. If the context of changes is crucial, supplementary tools or techniques such as visual diff tools or structured text comparison utilities can provide a more comprehensive analysis. By accounting for contextual changes, you can obtain a more accurate representation of the differences between files.

In Conclusion

I have discussed various methods to compare two files using Diff command in the Linux Terminal. I have explored line-by-line, word-by-word, and block-based comparisons, each offering unique advantages and considerations. I have also discussed the five most common errors when using the diff command.

To further enhance your knowledge, consider exploring related articles such as Advanced Diff Techniques for Complex File Comparisons, Automating File Comparison with Diff and Shell Scripts, and Version Control and Diff. These resources will expand your understanding of file comparison techniques and provide valuable insights into leveraging the Diff command in Linux.

Frequently Asked Questions 

How can I exclude certain lines from comparison?

You can exclude specific lines from the comparison by utilizing the -I flag followed by a regular expression. For example, if you want to exclude lines starting with a hash symbol (#), you can use the command diff -I ^#.* file1.txt file2.txt. This regular expression pattern ^#.* matches any line that starts with a hash symbol, ensuring that those lines are excluded from the comparison. By leveraging the -I flag and a suitable regular expression, you can customize the comparison to exclude specific lines irrelevant to your analysis.

Can I use Diff to compare directories instead of files?

No, the Diff command is primarily used to compare individual files, not directories. If you attempt to compare directories using the Diff command directly, you will encounter an error or incorrect results. To compare directories, there are specialized tools available such as diff -r or rsync. These tools are specifically designed to handle directory comparisons. By utilizing the appropriate tools, you can accurately compare the contents of directories, including the files within them and any subdirectories.

How do I apply Diff recursively to compare subdirectories?

To apply the Diff command recursively and compare subdirectories, you can use the -r flag. For example, the command diff -r directory1 directory2 compares all files in directory1 and directory2, including their respective subdirectories. The recursive comparison ensures that all files and subdirectories within the specified directories are analyzed for differences. This can be useful when you want to compare directories and their contents comprehensively. By incorporating the -r flag into the Diff command, you can effectively compare subdirectories and gain insights into the differences at a deeper level.



Ojash is a skilled Linux expert and tech writer with over a decade of experience. He has extensive knowledge of Linux's file system, command-line interface, and software installations. Ojash is also an expert in shell scripting and automation, with experience in Bash, Python, and Perl. He has published numerous articles on Linux in various online publications, making him a valuable resource for both seasoned Linux users and beginners. Ojash is also an active member of the Linux community and participates in Linux forums.



Akshat is a software engineer, product designer and the co-founder of Scrutify. He's an experienced Linux professional and the senior editor of this blog. He is also an open-source contributor to many projects on Github and has written several technical guides on Linux. Apart from that, he’s also actively sharing his ideas and tutorials on Medium and Attirer. As the editor of this blog, Akshat brings his wealth of knowledge and experience to provide readers with valuable insights and advice on a wide range of Linux-related topics.

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