git apply Command: Apply Patches in CI

git apply takes a unified diff and applies it to files, without creating a commit.

When CI patches a file (for example to inject a generated change or apply a vendor fix), git apply applies a diff cleanly and can verify it first. Unlike am, it does not make commits.

Common flags

• --check - verify the patch applies cleanly without changing anything
• --index - apply to both the working tree and the index (so it is staged)
• --3way - fall back to a three-way merge when context does not match
• -p<n> - strip n leading path components from filenames in the patch
• --reverse / -R - undo a previously applied patch
• --reject - apply what it can and write .rej files for the rest

Example

# Verify, then apply and stage a patch in CI
git apply --check fix.patch
git apply --index fix.patch

In CI

Run git apply --check first so the job fails fast with a clear message if a patch is stale, rather than half-applying. Use --3way for resilience when the base has drifted, and --index when the change should be staged for an automated commit.

Key takeaways

• git apply --check validates a patch before touching any files, enabling fail-fast.
• --3way makes patches more robust to a drifted base via merge fallback.
• Use --index to stage the change for a follow-up automated commit.

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