General usage for “cleaning” your working directory:

$ git clean [-d] [-f] [-i] [-n] [-q] [-e <pattern>] [-x | -X] [--] <path>...

The single config setting for this command:

• clean.requireForce

Prerequisites: You should understand gitignore files.

Cleans the working tree by recursively removing files that are not
under version control, starting from the current directory.

Normally, only files unknown to Git are removed, but if the -x
option is specified, ignored files are also removed. This can, for
example, be useful to remove all build products.

If any optional <path>... arguments are given, only those paths are
affected.

Note how both ignored files and new, staged files are not cleaned by default. Currently, there is some dissension regarding what is meant by files “known to Git.”

General execution options:

-i, --interactive
    Show what would be done and clean files interactively.

-n, --dry-run
    Don’t actually remove anything, just show what would be done.

-q, --quiet
    Be quiet, only report errors, but not the files that are
    successfully removed.

Forcing the issue:

-f, --force
    If the Git configuration variable clean.requireForce is not set
    to false, git clean will refuse to delete files or directories
    unless given -f, -n or -i. Git will refuse to delete
    directories with .git sub directory or file unless a second -f
    is given.

Removing directories as well:

-d
    Remove untracked directories in addition to untracked files. If
    an untracked directory is managed by a different Git
    repository, it is not removed by default. Use -f option twice
    if you really want to remove such a directory.

The explanation below oddly does not mention the use of the configuration setting core.excludesFile.

-e <pattern>, --exclude=<pattern>
    In addition to those found in .gitignore (per directory) and
    $GIT_DIR/info/exclude, also consider these patterns to be in
    the set of the ignore rules in effect.

-x
    Don’t use the standard ignore rules read from .gitignore
    (per directory) and $GIT_DIR/info/exclude, but do still use
    the ignore rules given with -e options. This allows removing
    all untracked files, including build products. This can be
    used (possibly in conjunction with git reset) to create a
    pristine working directory to test a clean build.

-X
    Remove only files ignored by Git. This may be useful to
    rebuild everything from scratch, but keep manually created
    files.

Create a shell alias:

alias gpristine='git reset --hard && git clean -dfx'