Version control with Git

Git keeps the history of a project: every saved version of every file, who changed it, and why. You can see what changed, go back to a version that worked, and work with other people without overwriting each other. Almost every software project uses it.

Three places

• the working folder: the files you edit;
• the staging area: the changes you have chosen for the next snapshot;
• the repository: the saved snapshots, called commits, kept in a hidden .git folder.

Your first commits

git init                        # start a repository in this folder
git status                      # what has changed? (use it all the time)
git add main.c stack.c stack.h  # put these changes in the staging area
git commit -m "Add a stack"     # save a snapshot with a message
git log --oneline               # list the commits, newest first

Before your first commit, tell Git who you are: git config --global user.name "Your Name" and the same with user.email.

Seeing what changed

#include <stdio.h>
#include <string.h>

int main(void) {
    const char *before[] = {"total = 0", "for each mark", "total += mark", "print total / n"};
    const char *after[] = {"total = 0", "for each mark", "total += mark", "print (double) total / n"};
    int n = 4;
    for (int i = 0; i < n; i++) {
        if (strcmp(before[i], after[i]) == 0) {
            printf("  %s\n", before[i]);
        } else {
            printf("- %s\n", before[i]);
            printf("+ %s\n", after[i]);
        }
    }
    return 0;
}

git diff shows the changes you have not staged yet, line by line: a line starting with - was removed, + was added, and the others are unchanged context. This program compares two versions of a short file the same simple way. (Git's real diff also finds inserted and deleted lines, not only changed ones.)

Branches

A branch is a separate line of work. Make one for each feature or fix, so main always works:

git switch -c fix-average   # make a branch and move to it
# ... edit, add, commit ...
git switch main             # back to main
git merge fix-average       # bring the fix into main

If both branches changed the same lines, Git stops with a merge conflict and marks the file with <<<<<<<, ======= and >>>>>>>. Edit the file to the version you want, remove the markers, then git add and git commit.

Sharing with GitHub

GitHub (or GitLab) keeps a copy of your repository online. git clone URL copies a repository to your computer, git push sends your commits up, and git pull brings other people's commits down.

Mark lesson complete