How to know if there is a git rebase in progress?

Question

When I start a git rebase -i, I can issue commands like git rebase --continue, or git rebase --abort. Those commands only work if a rebase is in progress.

How can I know if there is a rebase in progress?

(I would greatly appreciate some details on how rebase works internally; what does git do to a repo that gives it the "rebase in progress" status,?)

Answer 1

For one thing, there is a ORIG_HEAD in place during a rebase (but that is not limited to the rebase command)

But you can also look at the git-rebase.sh script itself (which is as "internal" as you can get ;) ).
Lines like those can give you another clue:

dotest="$GIT_DIR"/rebase-merge
test -d "$dotest" -o -d "$GIT_DIR"/rebase-apply || die "No rebase in progress?"

Answer 2

You can also check how such detection is done in __git_ps1 function in contrib/completion/git-completion.bash, which can be used for git-aware bash prompt:

                if [ -f "$g/rebase-merge/interactive" ]; then
                        r="|REBASE-i"
                        b="$(cat "$g/rebase-merge/head-name")"
                elif [ -d "$g/rebase-merge" ]; then
                        r="|REBASE-m"
                        b="$(cat "$g/rebase-merge/head-name")"
                else
                        if [ -d "$g/rebase-apply" ]; then
                                if [ -f "$g/rebase-apply/rebasing" ]; then
                                        r="|REBASE"
                                elif [ -f "$g/rebase-apply/applying" ]; then
                                        r="|AM"
                                else
                                        r="|AM/REBASE"
                                fi