diff3 - macOS


diff3 is a command-line utility used in macOS for comparing three files line by line. It is mainly used to merge changes made to a file that has evolved into two different versions by applying the differences between one version and a common base version to the third version. This tool is crucial in version control environments, resolving conflicts, and enabling effective collaboration between multiple editors or developers.


The basic syntax of the diff3 command is:

diff3 [options] mine older yours


  • mine: This file is typically the current version you or your team has modified.
  • older: This file is the common base version from which both newer versions were derived.
  • yours: Another modified version which has diverged from the same base as your current version.


Here are some commonly used options with diff3:

  • -m: Merge mode. This option makes diff3 perform a three-way merge between the files.
  • -e, --ed: Output is an ed script to convert “older” to “yours”.
  • -x, --overlap-only: Like -e, but outputs only when non-overlapping changes have occurred.
  • -X: Works like -x but outputs for overlapping changes as well.
  • -A: Outputs all changes, bracketing conflicts with <<<<<<, =======, and >>>>>>.

Each option modifies how the comparisons are handled or how the output is formatted, providing flexibility depending on the user’s needs.


1. Basic Three-way Comparison:

diff3 fileA fileB fileC

Compares three files and shows the differences among them.

2. Merge Files and Mark Overlaps:

diff3 -m fileA fileBase fileB

Merges the three files, marking overlaps where the changes conflict.

3. Generating an ed script:

diff3 -e fileBase fileA fileB > update_script.ed

Generates a script that can be applied to fileBase to reconcile changes found in fileA and fileB.

Common Issues

  • Encoding Differences: Ensure all files use the same character encoding to avoid unexpected results.
  • Large File Sizes: Performance can degrade with large files. Consider splitting files or using more efficient tools for very large datasets.


diff3 can be integrated with scripts or combined with tools like patch or version control systems such as Git. For example, to apply changes after generating an ed script:

ed fileBase < update_script.ed

This would update the fileBase with changes from both versions.

  • diff: Compare two files line by line.
  • patch: Apply a diff file to an original.
  • cmp: Compare two files byte by byte.

For more detailed information, you can refer to the diff3 man page in your terminal:

man diff3

This command’s flexibility makes it an indispensable tool in multi-version file handling and collaborative environments where files are frequently edited.