Preparation

Please make sure that you do not clone repositories inside an already tracked folder:

$ git status

If you are inside an existing Git repository, step out of it. You need to find a different location since we will clone a new repository.

If you see this message, this is good in this case:

fatal: not a git repository (or any of the parent directories): .git

Our toolbox for history inspection

First the instructor demonstrates few commands on a real life example repository https://github.com/networkx/networkx (mentioned in the amazing site The Programming Historian). Later we will practice these in groups in an archaeology exercise (below).

1. git grep to search through the repository

With git grep you can find all lines in a repository which contain some string or regular expression. This is useful to find out where in the code some variable is used or some error message printed:

$ git grep sometext

In the networkx repository you can try:

$ git clone https://github.com/networkx/networkx
$ cd networkx
$ git grep -i fixme

2. git log -S to search through the history of changes

While git grep searches the current state of the repository, it is possible to search also through all changes for “sometext”:

$ git log -S sometext

In the networkx repository you can try:

$ git log -S test_weakly_connected_component

3. git show to inspect commits

We have seen this one before already. Using git show we can inspect an individual commit if we know its hash:

$ git show somehash

For instance:

$ git show 759d589bdfa61aff99e0535938f14f67b01c83f7

4. git annotate to annotate code with commit metadata

Try it out on a file - with git annotate you can see line by line who and when the line was modified last. It also prints the precise hash of the last change which modified each line. Incredibly useful for reproducibility.

$ git annotate somefile

Example:

$ git annotate networkx/convert_matrix.py

If you annotate in a terminal and the file is longer than the screen, Git by default uses the program less to scroll the output. Use /sometext <ENTER> to find “sometext” and you can cycle through the results with n (next) and N (last). You can also use page up/down to scroll. You can quit with q.

Discussion

Discuss how these two affect the annotation:

• wrapping long lines of text/code into shorter lines
• autoformatting tools such as black

5. git checkout -b to inspect code in the past

We can create branches pointing to a commit in the past. This is the recommended mechanism to inspect old code:

$ git checkout -b branchname somehash

Example:

  # create branch called "older-code" from hash 347e6292419b
$ git checkout -b older-code 347e6292419bd0e4bff077fe971f983932d7a0e9

  # now you can navigate and inspect the code as it was back then
  # ...

  # after we are done we can switch back to "master"
$ git checkout master

  # if we like we can delete the "older-code" branch
$ git branch -d older-code

On newer Git versions this is the preferred command:

$ git switch --create branchname somehash