SSH keys and agents

Problem

What becomes confusing, fragile, or impossible without understanding SSH keys and agents? 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

SSH keys and agents belongs to “The network beneath the URL”. SSH keys and agents is inspected by introducing one realistic failure and proving root cause before repair.

For SSH keys and agents, 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 SSH keys and agents baseline from: Resolve example.com, connect with curl -v, and save response headers and body separately.

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 SSH keys and agents behavior at: Repeat with redirects disabled, then enabled.
4. Failure case: Diagnose this exact SSH keys and agents failure before editing: Request an unused local port with a two-second timeout.

RESULT
Evidence identifies first divergence, repair changes only proven cause, and rerun restores: Trace identifies resolved address, TLS/HTTP exchange, status, headers, body size, and connection failure. 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: SSH keys and agents' > "$lab/input.txt"
printf 'bytes: ' && wc -c < "$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: SSH keys and agents

1. Create second case from blank file: Record SSH keys and agents behavior at: Repeat with redirects disabled, then enabled.
2. Then create controlled failure: Diagnose this exact SSH keys and agents failure before editing: Request an unused local port with a two-second timeout.
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: Trace identifies resolved address, TLS/HTTP exchange, status, headers, body size, and connection failure.
• Command exits 0, prints a byte count, then prints hexadecimal and character views of the same file.
• Controlled SSH keys and agents failure produces captured evidence; repair restores stated invariant without hiding error.

Hints

Solution

Attempt both exercises before opening reference approach.