Binary File Compare: A Developer’s Guide to Using Meld and CLI Tools

Comparing binary files can be a daunting task, especially when you need to pinpoint the exact differences between firmware versions, executables, or any data stored in a non-textual format. Fortunately, tools like meld and command-line utilities offer powerful solutions to visualize and analyze binary file differences effectively. This guide will walk you through various methods to compare binary files, hex files, and Intel hex firmware files, primarily focusing on using meld for a user-friendly graphical comparison.

Why Compare Binary Files?

Binary file comparison is crucial in numerous scenarios, particularly in software development and embedded systems engineering. Imagine you are:

• Debugging Firmware Updates: Identifying subtle changes between different firmware versions for microcontrollers or embedded devices. This is essential to understand the impact of updates and troubleshoot potential issues.
• Analyzing Software Builds: Comparing executables compiled from different code versions or with varying compiler settings to detect unintended modifications or optimization effects.
• Verifying Data Integrity: Ensuring that binary data files, such as backups or configurations, are identical across different storage locations or after transmission.
• Reverse Engineering and Security Analysis: Diffing binary files can help in understanding the functionality of software or identifying security vulnerabilities by observing changes in different versions.

Traditional text-based diff tools are inadequate for binary files because they treat them as opaque sequences of bytes. We need tools that can interpret binary data in a more human-readable format, which is where meld and the xxd utility come into play.

Using Meld for Binary File Comparison

Meld is a visual diff and merge tool that excels at comparing text files, directories, and importantly, binary files when combined with the right approach. Here’s how to leverage meld for effective binary file comparison:

Comparing Binary Files Directly with Meld and xxd

The key to comparing binary files in meld is to first convert them into a human-readable hexadecimal representation using the xxd command-line tool. xxd creates a hexdump of a binary file, displaying the hexadecimal bytes alongside their ASCII representation where applicable. This allows meld to perform a line-by-line and character-by-character comparison of the hexadecimal output, effectively highlighting the binary differences.

To compare two binary files, file1.bin and file2.bin, with meld, use the following command:

meld <(xxd file1.bin) <(xxd file2.bin)

This command utilizes process substitution <(...) to feed the output of xxd file1.bin and xxd file2.bin directly to meld as if they were temporary files. Meld will then display a side-by-side comparison, visually highlighting the differences in hexadecimal and ASCII representations. The scroll bar also provides visual cues for changed lines throughout the files, as seen in the example below.

Navigating and Using Meld:

• Change Navigation: Use Alt + Down to jump to the next difference and Alt + Up for the previous difference. Alternatively, hover your mouse in the center gutter between the files and scroll to navigate between changes.
• Editing: You can directly edit either the left or right file within meld and save your changes.
• Searching: Use Ctrl + F to find specific text within the displayed hexadecimal representation. Note that search might be limited by line wraps; for more advanced searching, consider vbindiff (discussed later).

This method is particularly useful for identifying small, specific changes within binary files, such as modified strings, addresses, or configuration parameters.

Comparing Hex Files with Meld

If you already have your binary data in standard .hex files (ASCII hex files created by xxd), you can directly compare them using meld:

meld file1.hex file2.hex

This works because .hex files generated by xxd are essentially text files containing hexadecimal representations, which meld can readily compare.

Comparing Intel Hex Firmware Files with Meld and objcopy

Intel Hex files, commonly used for microcontroller firmware, present a slightly different challenge. They lack the side-by-side ASCII representation found in xxd‘s output. To effectively compare Intel Hex files in meld, we first convert them to raw binary .bin files using the objcopy utility, part of the GNU Binutils.

objcopy --input-target=ihex --output-target=binary my_firmware1.hex my_firmware1.bin
objcopy --input-target=ihex --output-target=binary my_firmware2.hex my_firmware2.bin

Caution: Ensure you specify the output filename (my_firmware1.bin) explicitly. Omitting it will cause objcopy to overwrite your original .hex file with the binary conversion, which is generally undesirable.

Once you have the binary .bin files, you can use the meld and xxd combination as described earlier:

meld <(xxd my_firmware1.bin) <(xxd my_firmware2.bin)

For convenience, you can combine these steps into a one-liner:

objcopy --input-target=ihex --output-target=binary my_firmware1.hex 1.bin && objcopy --input-target=ihex --output-target=binary my_firmware2.hex 2.bin && meld <(xxd 1.bin) <(xxd 2.bin)

When working with specific compiler toolchains, like the Microchip XC32 compiler, use the toolchain’s version of objcopy (e.g., xc32-objcopy) to ensure compatibility.

Streamlined Hex File Comparison with hex2xxdhex function

For frequently comparing hex files, especially firmware files, a utility function can significantly streamline the process. The hex2xxdhex function, available in eRCaGuy_dotfiles, automates the conversion of .hex files to .xxd.hex (full hexdump) and .xxd_short.hex (hexdump with zero-lines removed) formats.

By adding this function to your .bashrc or .zshrc file, you can simplify hex file comparisons:

hex2xxdhex path/to/myfile1.hex path/to/myfile2.hex
meld path/to/myfile1.xxd_short.hex path/to/myfile2.xxd_short.hex

The .xxd_short.hex version is particularly useful for large hex files with sparse data sections, as it removes lines containing only zeros, making comparisons in meld faster and more focused on the actual data differences.

Command-Line Alternatives for Binary File Comparison

While meld provides a powerful GUI-based solution, command-line tools are essential for scripting, remote server access via SSH, or situations where a graphical interface is not available.

diff for CLI Binary Comparison

The diff utility, commonly used for comparing text files, can also be used with xxd for binary file comparison in the command line:

diff -u --color=always <(xxd file1.bin) <(xxd file2.bin) | less -RFX

This command generates a unified diff output of the xxd representations of the binary files, with color highlighting for differences, and pipes it to less for easy viewing.

vbindiff for Interactive CLI Binary Comparison

For a more interactive command-line experience, vbindiff offers a curses-based visual binary diff tool. It provides a side-by-side display of binary files in hexadecimal, allowing you to navigate and examine differences directly in the terminal.

vbindiff file1.bin file2.bin

vbindiff is especially useful when you need to perform binary comparisons directly within a terminal session without relying on a GUI.

Conclusion

Comparing binary files effectively is a critical skill for developers and engineers. meld, combined with xxd and objcopy, provides a robust and user-friendly graphical solution for visualizing binary differences. For command-line environments, diff and vbindiff offer valuable alternatives. By choosing the right tool and method, you can confidently analyze and understand the nuances of binary data, whether you’re debugging firmware, analyzing software builds, or ensuring data integrity.