git write-tree

Counterpart to commit-tree. Together they implement git commit.

🎯 What & why

git write-tree serialises the current index into one or more tree objects and prints the SHA of the top-level tree on stdout. It is the second half of what git commit does, paired with commit-tree.

🧠 Mental model

Index in, tree SHA out. The same index always yields the same tree SHA; write-tree is pure with respect to .git/index. It writes new tree objects into the object store but does not touch refs, HEAD, or the worktree.

🛠️ Synopsis

git write-tree [--missing-ok] [--prefix=<dir>/]
  # Stdout: 40-char SHA of the resulting tree.

🎚️ Switches & options

Flag	What it does
`--missing-ok`	Allow the index to reference blob/tree SHAs that are not present locally (partial clones).
`--prefix=<dir>/`	Write only the subtree rooted at <dir>; the printed SHA is for that subtree, not the whole index.

💡 Use cases

Build a commit by hand: write-tree, then commit-tree, then update-ref.
Compute a tree SHA for the current index without committing, e.g. to compare against another tree.
Snapshot just one directory of the index using --prefix, useful in subtree workflows.
Verify that the index is well-formed; write-tree fails loudly if entries are inconsistent.

🧪 Examples

Write the index as a tree

git write-tree

Build a commit purely with plumbing

tree=$(git write-tree); commit=$(echo 'msg' | git commit-tree $tree -p HEAD); git update-ref HEAD $commit

Tree SHA for one subdirectory

git write-tree --prefix=docs/

Tolerate missing objects (partial clone)

git write-tree --missing-ok

🎓 Recommendations

Use write-tree as a sanity check after heavy update-index work; if it fails, your index is broken.
Cache the printed SHA; if the index has not changed, calling write-tree again is wasted work but always safe.
Pair with commit-tree and update-ref to scriptably produce commits without invoking porcelain.

🪤 Common pitfalls

It writes only the index, not the worktree; uncommitted changes on disk are invisible to write-tree.
--prefix expects a trailing slash and refers to a path inside the index, not the worktree.
Without --missing-ok, a single missing object aborts the whole operation, which surprises partial-clone users.

🔗 Related modules

📝 Quiz

Hit each option, then Check answers. Score is recorded; Next is always open.