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What is the difference between diff and diff3?
| Tool | Inputs | Best for | Typical result |
|---|---|---|---|
diff |
Two files | Inspecting line-by-line differences between two versions | A report of changed lines; choose the format you want |
diff3 |
Three files | Reconciling two edited copies against a shared earlier version | A comparison of both edits, or merged output that marks conflicts |
The key distinction is the relationship between the inputs. Two files are enough for diff. For diff3, the second input is the common ancestor, and the first and third are the two revised copies. GNU documents the order as diff3 mine older yours. GNU Diffutils 3.12 manual
When should you use diff?
Choose diff when you want to inspect two files, or corresponding files in two directories, and find their line-by-line differences. For a familiar patch-style view with surrounding unchanged lines, run:
diff -u old-file new-file
The -u option selects unified output. GNU diff shows three context lines by default in this format; use -U to specify a different context count, such as diff -U 5 old-file new-file.
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Use this workflow when there is no third, known common original to include, or when your immediate goal is simply to see what differs between two versions.
When should you use diff3?
Use diff3 when two copies were edited independently from the same earlier file. Supply the two revised files around the shared original:
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diff3 mine older yours
Here, mine and yours are the modified copies; older is their shared earlier version. This three-way comparison helps distinguish changes made in either copy from material that remained in the common ancestor.
To view the three-way comparison
Run the command without merge mode to inspect how the files differ in relation to the common original. This is useful when you want to review the edits before deciding how to reconcile them.
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To produce merged output
Add -m (also written --merge) to write merged output:
diff3 -m mine older yours
The output may contain conflict markers where the edits overlap or cannot be combined automatically. Treat those sections as unresolved work: inspect the competing changes, decide what the final content should be, and remove or resolve the markers before using the merged file.
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How should you interpret diff3’s exit status?
GNU diff3 uses distinct statuses that matter when you run it in a script:
0: the merge completed without reported conflicts.1: conflicts were found. Review the merged output; this is not the same as an execution failure.2: trouble occurred, such as an error that prevented the operation from completing normally.
A script should handle status 1 separately from status 2, so that a merge needing human review is not mistaken for a command error.
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Which tool should you choose?
- Choose
diffto compare two files and inspect changed lines. - Choose
diff -uwhen you want a unified, patch-style presentation. - Choose
diff3when you have two revised copies and their common earlier version. - Choose
diff3 -mwhen you want merged output, then review and resolve any conflict sections.
These recommendations refer to GNU Diffutils 3.12 and its documented command-line behavior; other implementations or versions may differ. GNU Diffutils manual
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