Automation: Git and Ansible / Stage 1 of 6

The history of configuration changes

Not running Starting usually ~20 s

Step 5 of 6 · lab · 20 min

The pricing service is down

After a night-time restart the pricing service does not start. Its settings live under git, and several people have edited them. Bring back the working settings without losing yesterday's needed change and without rewriting the history.

Situation

The lab-pricing service gives the shop front its prices and the list of warehouses. After a night-time restart it did not come back, and the shop front says "prices unavailable". The service settings live in /etc/pricing. That directory is a git repository: whoever edits the settings commits the change right there.

Yesterday Oleg added the Kazan warehouse to the settings. The shop front already shows it to customers, so this change is needed.

Task

The lab-pricing service is running and answers: curl http://127.0.0.1:8180/health.
pricing.ini holds the settings the service was running with before the restart, and the Kazan warehouse is in them.
The history is not rewritten: the fix is a new commit on the main branch, and your colleagues' commits stay on it. In the end git status is clean.
Do not change the service code or its unit.

The /etc/pricing directory belongs to root, so run git in it with sudo: sudo git -C /etc/pricing log. There is no internet on the machine.

Lab · on your own

No commands are given in this lab: type them in the terminal yourself. Goals are checked on the machine after every command.

Goals

0 of 4

The lab-pricing service is running and answers http://127.0.0.1:8180/health — not done yet
The pricing.ini settings are the ones the service ran with before the restart — not done yet
The Kazan warehouse from yesterday's change is still in the settings — not done yet
The history is not rewritten — the fix is a new commit on main, git status is clean — not done yet

Ctrl+Enter

Starts the machine node1 · Ubuntu 24.04

The machine is starting

Hints

0 of 3

Report a problem

Lab goals

0 of 4

The lab-pricing service is running and answers http://127.0.0.1:8180/health — not done yet
The pricing.ini settings are the ones the service ran with before the restart — not done yet
The Kazan warehouse from yesterday's change is still in the settings — not done yet
The history is not rewritten — the fix is a new commit on main, git status is clean — not done yet

node1 Ubuntu 24.04

Lab goals

0 of 4

The lab-pricing service is running and answers http://127.0.0.1:8180/health — not done yet
The pricing.ini settings are the ones the service ran with before the restart — not done yet
The Kazan warehouse from yesterday's change is still in the settings — not done yet
The history is not rewritten — the fix is a new commit on main, git status is clean — not done yet

Files and directories · picked for this lab

git: change history, undo and a remote repository

Cheat sheet

$_ — insert into the terminal

git status what changed, which branch
git diff uncommitted changes
git add -A ; git commit -m "msg" commit
git log --oneline -10 short history
git log -p -- site.conf a file's history with changes
git blame site.conf who changed each line, and when
git restore site.conf throw away an uncommitted change
git revert --no-edit <commit> undo a commit with a new commit
git fetch ; git status what is new in the remote
git pull get and merge
git push send your commits
git stash ; git stash pop shelve a change and bring it back

Below are the main git subcommands; the full list is in man git, git help <command>.

History: log -p, show, blame

The /etc/demo repository with three commits by two authors:

text

student@node1:/etc/demo$ sudo git log --format='%h %an %ad %s' \
> --date=short
c9ab4a3 Boris Ops 2026-10-09 site: tune listen
4c69b7d Boris Ops 2026-10-05 site: move root to /srv/www
f7de9e0 Anna Admin 2026-10-01 site: initial config

git log -p adds each commit's change; with -1 only the last one, with -- <file> only the commits touching that file:

text

student@node1:/etc/demo$ sudo git log -p -1 -- site.conf
commit c9ab4a3032a1cf6f1f4b1fee565958cfaf2f67c4
Author: Boris Ops <boris@lab.local>
Date:   Fri Oct 9 09:15:00 2026 +0000

    site: tune listen

diff --git a/site.conf b/site.conf
index d988f4e..f671dff 100644
--- a/site.conf
+++ b/site.conf
@@ -1,4 +1,4 @@
 server {
-    listen 80;
+    listen 8O;
     root /srv/www;
 }

- is the line before the commit, + after it: 80 became 8O with the letter O. git show <commit> prints one commit the same way, with --stat only the list of files. git blame labels each line of a file with the commit that last changed it:

text

student@node1:/etc/demo$ sudo git blame site.conf
^f7de9e0 (Anna Admin 2026-10-01 10:00:00 +0000 1) server {
c9ab4a30 (Boris Ops  2026-10-09 09:15:00 +0000 2)     listen 8O;
4c69b7d7 (Boris Ops  2026-10-05 14:30:00 +0000 3)     root /srv/www;
^f7de9e0 (Anna Admin 2026-10-01 10:00:00 +0000 4) }

A ^ before the hash marks the repository's first commit. An uncommitted change shows in git status (modified: site.conf) and git diff.

Undo a change and a commit: restore, revert, reset

What to undo	Command	History
an uncommitted change to a file	`git restore site.conf`	unchanged
one commit	`git revert --no-edit <commit>`	a new commit on top
a file, back to an old commit's version	`git restore --source=<commit> site.conf`	unchanged, the change is in the working file
the branch, back to an old commit	`git reset --hard <commit>`	the commits after it leave the branch

revert makes a commit that is the reverse of the chosen one — the history stays, and it shows who undid what and when:

text

student@node1:/etc/demo$ sudo git revert --no-edit HEAD
[main 107f37c] Revert "site: tune listen"
 Date: Wed Oct 21 11:54:18 2026 +0000
 1 file changed, 1 insertion(+), 1 deletion(-)

The message of such a commit says This reverts commit c9ab4a30…. You can undo an old commit too (git revert --no-edit HEAD~2): git removes only its change and keeps the later ones. To look at a file from an old commit without changing anything — git show HEAD~3:site.conf.

reset --hard rewrites the branch: the commits after the chosen one leave it, and uncommitted changes are gone for good. git reflog remembers the commits that left:

text

student@node1:/etc/demo$ sudo git reflog | head -3
107f37c HEAD@{0}: reset: moving to HEAD~1
d01f2cc HEAD@{1}: revert: Revert "site: move root to /srv/www"
107f37c HEAD@{2}: revert: Revert "site: tune listen"

git reset --hard 'HEAD@{1}' takes the branch back to where it was before the reset. In a shared repository that others pull from, history is not rewritten — there you undo with revert.

A root-owned repository: sudo git

Files in /etc belong to root, and so does a repository in /etc/nginx. git commands there go through sudo, like editing the config. sudo git takes the author name and email from root's settings — your git config --global is not visible to it. So set them in the repository itself:

bash

sudo git -C /etc/nginx config user.name 'Lab Admin'
sudo git -C /etc/nginx config user.email 'admin@lab.local'

Without this sudo git commit and sudo git revert answer Author identity unknown — git. git without sudo in someone else's repository refuses to work — git.

A remote repository: fetch, pull, push

A bare repository is history only, with no working files: commits are pushed to it from different machines. A working copy on a server is a clone, and its remote is called origin:

bash

git init --bare -b main /srv/git/site.git   # the shared repository
git clone /srv/git/site.git ~/site           # a working copy
git remote -v                                # where origin points

git status compares the branch with what git knew about origin at the last exchange. It sees a colleague's new commit after git fetch:

text

student@node1:~/w1$ git fetch
From /srv/git/site
   b1ad7d1..8396b40  main       -> origin/main
student@node1:~/w1$ git status | head -2
On branch main
Your branch is behind 'origin/main' by 1 commit, and can be
  fast-forwarded.

git log --oneline HEAD..origin/main shows which commits will come, git diff HEAD origin/main what they will change. git pull is fetch plus a merge; if you have no commits of your own, the branch simply moves forward (Fast-forward).

git push sends your commits. If origin has commits you do not have, push refuses:

text

 ! [rejected]        main -> main (fetch first)
error: failed to push some refs to '/srv/git/site.git'

git pull first, then git push again. If both you and origin have new commits, the branches have diverged, and git pull asks how to reconcile them — git.

Shelve a change: stash

git stash moves uncommitted changes to a separate store and brings the files back to the last commit — so you can get other people's commits when pull refuses because of a local change:

text

student@node1:~/w1$ git stash
Saved working directory and index state WIP on main: b1ad7d1 site: v1
student@node1:~/w1$ git pull
Updating b1ad7d1..8396b40
Fast-forward
 index.html | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
student@node1:~/w1$ git stash pop
Auto-merging index.html
CONFLICT (content): Merge conflict in index.html
...
The stash entry is kept in case you need it again.

git stash list shows what is shelved (stash@{0}: WIP on main: …), git stash pop brings the change back and removes it from the store. Here the returned change touched the same line that came with pull — a conflict (git), and the entry stayed in the store. Remove it after resolving — git stash drop.

Conflict

A conflict means both sides changed the same lines, and git does not know which version to keep. It writes both into the file:

text

<<<<<<< Updated upstream
v2 colleague
=======
v1 local edit
>>>>>>> Stashed changes

Between <<<<<<< and ======= is the version already in the branch, between ======= and >>>>>>> the one coming in. After stash pop the markers say Updated upstream and Stashed changes, after pull — HEAD and the commit hash. git status shows such files as both modified.

To resolve:

Open the file, keep what is needed, remove all three markers.
git add <file> — the conflict is resolved.
After pull — git commit (the merge message is ready); after stash pop no commit is needed, the change is simply back in the working files.

Changed your mind about merging — git merge --abort: the files go back to how they were before pull.

Branches

bash

git branch                        # list branches
git switch -c feature             # create and switch
git switch main                   # switch to it
git merge feature                 # merge a branch into the current one
git branch -d feature             # delete a merged branch

detected dubious ownership

text

student@node1:~$ git -C /etc/demo status
fatal: detected dubious ownership in repository at '/etc/demo'
To add an exception for this directory, call:

	git config --global --add safe.directory /etc/demo

The repository belongs to another user (root here), and git will not work in it under your name: someone else's settings in .git/config could run someone else's commands. A root-owned repository needs sudo git. A safe.directory exception only lets you read: student still cannot write to root's .git — fatal: Unable to create '/etc/demo/.git/index.lock': Permission denied.

Author identity unknown

text

Author identity unknown

*** Please tell me who you are.
...
fatal: unable to auto-detect email address (got 'root@node1.(none)')

The user making the commit has no name and email. The brackets show who it is: root here — because of sudo. Set them in the repository (git config user.name, git config user.email) — git.

Your local changes … would be overwritten by merge

text

error: Your local changes to the following files would be overwritten
  by merge:
	index.html
Please commit your changes or stash them before you merge.
Aborting

The working copy has an uncommitted change to a file the incoming commits change. Either commit it and pull, or shelve it with git stash, pull, and bring it back with git stash pop — git.

Need to specify how to reconcile divergent branches

text

hint: You have divergent branches and need to specify how to
  reconcile them.
...
fatal: Need to specify how to reconcile divergent branches.

Both you and origin have commits the other side lacks (git status: have diverged). Merge them with a merge commit — git pull --no-rebase; in a terminal git opens an editor with a ready message, --no-edit takes it without the editor. To choose for the future — git config pull.rebase false. After the merge — git push.

rejected … (fetch first)

origin has someone else's commits. git pull, resolve a conflict if there is one, and git push again — git.

refusing to update checked out branch

A push into an ordinary repository with working files: git does not change the branch checked out there, or its files would drift from the history. A shared repository is made bare — git init --bare.

Permission denied (publickey) on push

The SSH key is not set up or not on the server — see ssh-keys. Check git remote -v: an SSH or an HTTPS address.

Accidentally ran reset --hard

git reflog remembers where HEAD was — you can go back: git reset --hard <hash> (git).

`git reset --hard` and `git clean -fd`

reset --hard discards uncommitted changes to tracked files with no confirmation; git reflog only rescues what was committed — uncommitted edits lost to reset --hard cannot be recovered. git clean -fd removes untracked files and directories (including ones you forgot to add to git) — also with no trash can. Before clean, run git clean -nd (a dry-run that only shows what would be removed).

`git push --force`

Overwrites the remote branch's history: commits your colleagues already pulled "disappear" from the branch — they end up with diverging copies, and merging turns into a mess. For a shared branch, git push --force-with-lease is safer (it refuses if commits you didn't know about have appeared on the server).

The history of configuration changes

Situation

Task

Signing in is needed to start the machine

History: log -p, show, blame

Undo a change and a commit: restore, revert, reset

A root-owned repository: sudo git

A remote repository: fetch, pull, push

Shelve a change: stash

Conflict

Branches

detected dubious ownership

Author identity unknown

Your local changes … would be overwritten by merge

Need to specify how to reconcile divergent branches

rejected … (fetch first)

refusing to update checked out branch

Permission denied (publickey) on push

Accidentally ran reset --hard

git reset --hard and git clean -fd

git push --force

`git reset --hard` and `git clean -fd`

`git push --force`