Comparing Changes with Git Diff

Git Diff

• We can use the git diff command to view changes between commits, branches, files, our working directory, and more!
• We often use git diff alongside commands like git status and git log, to get a better picture of a repository and how it has changed over time.
• Without additional options, git diff lists all the changes in our working directory that are NOT staged for the next commit.
• git diff --staged will list the changes between the staging area and our last commit. You can think of it as "Show me what will be included in my commit if I run git commit right now"
• git diff HEAD lists all changes in the working tree since your last commit. It compares the HEAD and the working directory and shows both staged and unstaged changes.
• git diff branch1..branch2 will list the changes between the tips of branch1 and branch2. The order matters as the first branch is referred to as File A and second as File B.
• To compare two commits, provide git diff with the commit hashes of the commits in question in the form of git diff commit1..commit2

• We can also view the changes within a specific file by providing git diff with a filename.
  

  git diff HEAD [filename]
  git diff --staged [filename]
  

Reading Git Diffs

• For each comparison, Git explains which files it is comparing. Usually this is two versions of the same file.

• Git also declares one file as "A"(old) and the other as "B"(new).

  Markers

  • File A and File B are each assigned a symbol.
    • Changes in File A are indicated with a minus sign (-)
    • Changes in File B are indicated with a plus sign (+)

  Chunks

  • A diff won't show the entire contents of a file, but instead only shows portions or "chunks" that were modified.
  • A chunk also includes some unchanged lines before and after a change to provide some context
  • Each chunk starts with a chunk header, found between @@ and @@.
  • There are two sets of numbers in the chunk. One set belongs to File A and the other belongs to File B, that is what the sign indicates
  • The number with the sign indicates from which line no, the lines are extracted from and the second number indicates how many lines have been extracted.
  • From file a, 4 lines are extracted starting from line 3.
  • From file b, 5 lines are extracted starting from line 3

Changes

• Every line that changed between the two files is marked with either a + or - symbol and lines without any sign exist in both files
• Simply the - can be interpreted as removed and + as added.
• lines that begin with - come from file A
• lines that begin with + come from file B

    diff --git a/rainbow.txt b/rainbow.txt
    index 72d1d5a..f2c8117 100644
    --- a/rainbow.txt
    +++ b/rainbow.txt
    @@ -3,4 +3,5 @@ orange
    yellow
    green
    blue
    -purple
    +indigo
    +violet