Three-way merge in Git - Step-by-Step Execution

Process Flow - Three-way merge

Start: Common Base Commit

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Branch A changes

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Three-way merge compares

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Merge result with combined changes

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Commit merge

The three-way merge compares two changed versions with their common base to combine changes into one merged result.

Execution Sample

Git

git checkout branchA
# make changes and commit

git checkout branchB
# make changes and commit

git merge branchA

This sequence merges changes from branchA into branchB using a three-way merge.

Process Table

Step	Action	Base Version	Branch A Version	Branch B Version	Merge Result	Notes
1	Identify common base commit	Base file content	Base file content	Base file content	N/A	All start from same base
2	Compare base to Branch A	Base file content	Changed line in A	Base file content	N/A	Detect changes made in Branch A
3	Compare base to Branch B	Base file content	Base file content	Changed line in B	N/A	Detect changes made in Branch B
4	Merge changes	Base file content	Changed line in A	Changed line in B	Combined changes or conflict markers	If changes overlap, conflict occurs
5	Resolve conflicts if any	N/A	N/A	N/A	Final merged content	User edits conflicts manually
6	Commit merge	N/A	N/A	N/A	Merge commit created	Merge is complete

💡 Merge commit created or conflicts resolved and committed

Status Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	After Step 5	Final
Base file content	Original text	Original text	Original text	Original text	Original text	Original text
Branch A file content	Original text	Changed line in A	Changed line in A	Changed line in A	Changed line in A or resolved	Final merged content
Branch B file content	Original text	Original text	Changed line in B	Changed line in B	Changed line in B or resolved	Final merged content
Merge result	N/A	N/A	N/A	Combined changes or conflict markers	Final merged content	Final merged content

Key Moments - 3 Insights

Why does git need a common base commit for merging?

Git uses the common base commit to understand what changed in each branch separately. This helps it combine changes correctly or detect conflicts, as shown in execution_table rows 1-3.

What happens if both branches change the same line differently?

Git marks a conflict in the merge result (row 4). The user must manually resolve it (row 5) before committing the merge (row 6).

Can a merge happen without conflicts?

Yes, if changes are on different lines or parts, git automatically combines them (row 4) and creates a merge commit (row 6).

Visual Quiz - 3 Questions

Look at the execution_table at step 4. What does the 'Merge Result' show if both branches changed the same line differently?

A Conflict markers showing both changes

B Only Branch A changes

C Combined changes without any markers

D Only Branch B changes

💡 Hint

Refer to execution_table row 4 under 'Merge Result' and 'Notes' columns.

At which step does the user manually resolve conflicts?

A Step 3

B Step 4

C Step 5

D Step 6

💡 Hint

Check execution_table row 5 'Action' and 'Notes' columns.

If Branch A made no changes, what would the merge result be at step 4?

A Conflict markers appear

B Merge result equals Branch B version

C Merge result equals Base version

D Merge fails

💡 Hint

Look at variable_tracker for Branch A and Branch B content changes after step 4.

Concept Snapshot

Three-way merge in git:
- Uses common base commit plus two branch versions
- Compares changes from base to each branch
- Combines non-conflicting changes automatically
- Marks conflicts if same lines changed differently
- User resolves conflicts manually
- Final merge commit records combined history

Full Transcript

A three-way merge in git starts by identifying the common base commit shared by two branches. Then git compares the base version with each branch's version to find changes. If changes do not overlap, git automatically combines them into a merged result. If both branches changed the same lines differently, git marks conflicts in the merged file. The user must manually resolve these conflicts before committing the merge. The final merge commit records the combined changes and history from both branches.