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Git Tips

When using Git, there are a few tips that are particularly useful when working on the Chromium codebase, especially due to its size.

[TOC]

Remember the basic git convention:

git COMMAND [FLAGS] [ARGUMENTS]

Various git commands have underlying executable with a hyphenated name, such as git-grep, but these can also be called via the git wrapper script as git grep (and man should work either way too).

Git references

The following resources can provide background on how Git works:

  • Git-SVN Crash Course -- this crash course is useful for Subversion users switching to Git.
  • Think Like (a) Git -- does a great job of explaining the main purpose of Git operations.
  • Git User's Manual -- a great resource to learn more about ho to use Git properly.
  • A Visual Git Reference -- a resource that explains various Git operations for visual learners.
  • Git Cheat Sheet -- now that you understand Git, here's a cheat sheet to quickly remind you of all the commands you need.

Optimizing (Speeding up) Git for a Large Repository

Git has numerous options, among which some are intended to optimize for large repositories. feature.manyFiles is a convenient option that turns on the group of options that optimize for large repositories. Run the following inside the Chromium git repository:

git config feature.manyFiles true

Configuring the output of "git log"

By default, the date that "git log" displays is the "author date." In Chromium, this generally corresponds to the date that the committed patch was last uploaded. In most cases, however, the date that is of interest is the date that the patch was committed in the tree. To configure "git log" to instead display the latter date for your Chromium checkout, execute the following command:

git config format.pretty 'format:%C(auto,yellow)commit %H%C(auto)%d%nAuthor:    %an <%ae>%nCommitted: %cd%n%n%w(0,4,4)%B%-%n'

If you want to change all your repos (e.g., because you have multiple Chromium checkouts and don't care about having the default for other repos), add "--global" after "config" in the above command.

Committing changes

For a simple workflow (always commit all changed files, don't keep local revisions), the following script handles check; you may wish to call it gci (git commit) or similar.

Amending a single revision is generally easier for various reasons, notably for rebasing and for checking that CLs have been committed. However, if you don't use local revisions (a local branch with multiple revisions), you should make sure to upload revisions periodically to code review if you ever need to go to an old version of a CL.

#!/bin/bash
# Commit all, amending if not initial commit.
if git status | grep -q "Your branch is ahead of 'origin/main' by 1 commit."
then
  git commit --all --amend
else
  git commit --all  # initial, not amendment
fi

Listing and changing branches

git branch  # list branches
git checkout -  # change to last branch

To quickly list the 5 most recent branches, add the following to .gitconfig in the [alias] section:

last5 = "!git for-each-ref --sort=committerdate refs/heads/ \
    --format='%(committerdate:short) %(refname:short)' | tail -5 | cut -c 12-"

A nicely color-coded list, sorted in descending order by date, can be made by the following bash function:

git-list-branches-by-date() {
  local current_branch=$(git rev-parse --symbolic-full-name --abbrev-ref HEAD)
  local normal_text=$(echo -ne '\E[0m')
  local yellow_text=$(echo -ne '\E[0;33m')
  local yellow_bg=$(echo -ne '\E[7;33m')
  git for-each-ref --sort=-committerdate \
      --format=$'  %(refname:short)  \
          \t%(committerdate:short)\t%(authorname)\t%(objectname:short)' \
          refs/heads \
      | column -t -s $'\t' -n \
      | sed -E "s:^  (${current_branch}) :* ${yellow_bg}\1${normal_text} :" \
      | sed -E "s:^  ([^ ]+):  ${yellow_text}\1${normal_text}:"
}

Searching

Use git-grep instead of grep and git-ls-files instead of find, as these search only files in the index or tracked files in the work tree, rather than all files in the work tree.

Note that git-ls-files is rather simpler than find, so you'll often need to use xargs instead of -exec if you want to process matching files.

Global changes

To make global changes across the source tree, it's often easiest to use sed with git-ls-files, using -i for in-place changing (this is generally safe, as we don't use symlinks much, but there are few places that do). Remember that you don't need to use xargs, since sed can take multiple input files. E.g., to strip trailing whitespace from C++ and header files:

sed -i -E 's/\s+$//' $(git ls-files '*.cpp' '*.h')

You may also find git-grep useful for limiting the scope of your changes, using -l for listing files.

sed -i -E '...' $(git grep -lw Foo '*.cpp' '*.h')

Remember that you can restrict sed actions to matching (or non-matching) lines. For example, to skip lines with a line comment, use the following:

'\,//, ! s/foo/bar/g'

Diffs

git diff --shortstat

Displays summary statistics, such as:

2104 files changed, 9309 insertions(+), 9309 deletions(-)