Is there a way to get the git root directory in one command?

Yes:

git rev-parse --show-toplevel

If you want to replicate the Mercurial command more directly, you can create an alias:

git config --global alias.root 'rev-parse --show-toplevel'

and now git root will function just as hg root.

Note: In a submodule this will display the root directory of the submodule and not the parent repository. If you are using Git >=2.13 or above, there is a way that submodules can show the superproject's root directory. If your git is older than that, see this other answer.

The man page for git-config (under Alias) says:

If the alias expansion is prefixed with an exclamation point, it will be treated as a shell command. [...] Note that shell commands will be executed from the top-level directory of a repository, which may not necessarily be the current directory.

So, on UNIX you can do:

git config --global --add alias.root '!pwd'

How about "git rev-parse --git-dir" ?

F:\prog\git\test\copyMerge\dirWithConflicts>git rev-parse --git-dir
F:/prog/git/test/copyMerge/.git

The --git-dir option seems to work.

From git rev-parse manual page:

--git-dir

    Show $GIT_DIR if defined else show the path to the .git directory.

You can see it in action in this git setup-sh script.

If you are in a submodule folder, with Git >=2.13, use:

git rev-parse --show-superproject-working-tree

If you are using git rev-parse --show-toplevel, make sure it is with Git 2.25+ (Q1 2020).

If you're already in the top-level or not in a git repository cd $(git rev-parse --show-cdup) will take you home (just cd). cd ./$(git rev-parse --show-cdup) is one way of fixing that.

This shell alias works whether you are in a git subdir, or at the top level:

alias gr='[ ! -z `git rev-parse --show-toplevel` ] && cd `git rev-parse --show-toplevel || pwd`'

updated to use modern syntax instead of backticks:

alias gr='[ ! -z $(git rev-parse --show-toplevel) ] && cd $(git rev-parse --show-toplevel || pwd)'

Since Git 2.13.0, it supports a new option to show the path of the root project, which works even when being used from inside a submodule:

git rev-parse --show-superproject-working-tree

I wanted to expand upon Daniel Brockman's excellent comment.

Defining git config --global alias.exec '!exec ' allows you to do things like git exec make because, as man git-config states:

If the alias expansion is prefixed with an exclamation point, it will be treated as a shell command. [...] Note that shell commands will be executed from the top-level directory of a repository, which may not necessarily be the current directory.

It's also handy to know that $GIT_PREFIX will be the path to the current directory relative to the top-level directory of a repository. But, knowing it is only half the battle™. Shell variable expansion makes it rather hard to use. So I suggest using bash -c like so:

git exec bash -c 'ls -l $GIT_PREFIX'

other commands include:

git exec pwd
git exec make

In case anyone needs a POSIX compliant way of doing this, without needing git executable:

git-root:

#$1: Path to child directory
git_root_recurse_parent() {
    # Check if cwd is a git root directory
    if [ -d .git/objects -a -d .git/refs -a -f .git/HEAD ] ; then
        pwd
        return 0
    fi

    # Check if recursion should end (typically if cwd is /)
    if [ "${1}" = "$(pwd)" ] ; then
        return 1
    fi

    # Check parent directory in the same way
    local cwd=$(pwd)
    cd ..
    git_root_recurse_parent "${cwd}"
}

git_root_recurse_parent

If you just want the functionality as part of a script, remove the shebang, and replace the last git_root_recurse_parent line with:

git_root() {
    (git_root_recurse_parent)
}