Reflog and recovery

Problem

What becomes confusing, fragile, or impossible without understanding reflog and recovery? This lesson answers that through explanation, a worked example, two runnable exercises, and a reference solution. No teacher-supplied worksheet is required.

Before integrated development environments, the operating system, terminal, files, and text streams were the environment. Their small composable interfaces still underpin modern tooling.

Concept explanation

Reflog and recovery belongs to “Git as a content-addressed database”. reflog and recovery is inspected by introducing one realistic failure and proving root cause before repair.

For reflog and recovery, trace concrete input, state transition, output, and failure through an operating-system boundary made visible through bytes, files, processes, and exit status.

Programs are processes. They read bytes, transform state, write bytes, open files and sockets, then report success or failure through observable boundaries. Apply that model to supplied normal, boundary, and failure cases; each case below names its input and expected evidence.

Example bank

Compare normal, boundary, failure, and cross-layer cases. Predict each observation before revealing the explanation.

Worked example

Start from supplied lab.sh. Focus: Capture healthy reflog and recovery baseline from: Commit one file, then inspect blob, tree, commit, HEAD, and index with plumbing commands.

1. Run: chmod +x lab.sh && ./lab.sh
2. Save baseline evidence. Use wc, od, stat, ps, lsof, curl, or Git plumbing as appropriate; save raw output before interpretation.
3. Boundary case: Record reflog and recovery behavior at: Amend content with same filename and compare object IDs.
4. Failure case: Diagnose this exact reflog and recovery failure before editing: Detach HEAD, make a commit, move away, then recover it from reflog.

RESULT
Evidence identifies first divergence, repair changes only proven cause, and rerun restores: Changed bytes create new blob/tree/commit IDs; reflog retains recoverable commit after ref moves. Starter-level baseline: Command exits 0, prints a byte count, then prints hexadecimal and character views of the same file.

Starter material

Create lab.sh, paste this exact content, then run the command below.

#!/usr/bin/env sh
set -eu
lab="${TMPDIR:-/tmp}/reforging-lab"
mkdir -p "$lab"
printf '%s\n' 'baseline: reflog and recovery' > "$lab/input.txt"
printf 'bytes: ' && wc -c < "$lab/input.txt"
od -An -tx1 -c "$lab/input.txt"

STOP / CLEANUPNo background process. Keep generated evidence files for your notebook.

Independent exercise

Prove root cause: reflog and recovery

1. Create second case from blank file: Record reflog and recovery behavior at: Amend content with same filename and compare object IDs.
2. Then create controlled failure: Diagnose this exact reflog and recovery failure before editing: Detach HEAD, make a commit, move away, then recover it from reflog.
3. Use Ghostty · tmux · hx · shell · curl · Git to prove behavior, then repair controlled failure.
4. Compare result against supplied acceptance checks and reference approach before marking complete.

Expected result

• Evidence identifies first divergence, repair changes only proven cause, and rerun restores: Changed bytes create new blob/tree/commit IDs; reflog retains recoverable commit after ref moves.
• Command exits 0, prints a byte count, then prints hexadecimal and character views of the same file.
• Controlled reflog and recovery failure produces captured evidence; repair restores stated invariant without hiding error.

Hints

Solution

Attempt both exercises before opening reference approach.