Q5 of 40 · Git

What does `git add` actually do?

GitJuniorgitstaging-areaindexfundamentalsgit-add

Short answer

Short answer: git add copies the current state of a file from the working directory into the staging area (the index). It doesn't write anything permanent — it builds the snapshot that git commit will capture. git add -p lets you stage specific hunks of a file, giving fine-grained control over each commit's content.

Detail

The staging area (also called the index) is a file in .git/index that records the exact state of each file that the next git commit will snapshot. git add updates that index to match the working-directory version of the specified files.

Key behaviours:

  • git add file.txt — stage exactly the current version of file.txt
  • git add -p — interactive hunk selection: stage part of a file while leaving the rest unstaged. Essential for clean commits when you've made multiple unrelated changes to one file.
  • git add . — stage all changes in the current directory tree (commonly misused as a "stage everything" shortcut)

Why Git has a staging area: it lets you build the perfect commit even when your working directory contains work-in-progress. You can have a test file with 3 different changes and stage only the fix-related hunk, keeping the other changes out of that commit.

// EXAMPLE

# Stage one specific file
git add tests/UserApiTest.java

# Stage all modified and deleted files (NOT untracked)
git add -u

# Stage everything — use with care (may include unintended files)
git add .

# Interactive staging — choose which hunks to include
git add -p tests/UserApiTest.java
# Shows each diff hunk; respond y (stage), n (skip), s (split), e (edit)

# Verify what's staged vs unstaged
git diff --staged   # staged: what the next commit will include
git diff            # unstaged: changes NOT yet staged

// WHAT INTERVIEWERS LOOK FOR

Knowing git add operates on the index (staging area), not directly on commits, and explaining git add -p as the tool for clean, granular commits. The distinction between staging and committing is a foundational concept.

// COMMON PITFALL

Using git add . without checking what it stages — this is how test credential files, generated outputs, or debug logs accidentally end up in commits. Always git status before git add . in an unfamiliar directory.

// Related questions

Explain the difference between the working directory, staging area, and commit.

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What does `git diff` show by default?

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How do you check the status of your repository and the current branch?

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