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Linux Survival Basics

Read-only, sensitive output

List SSH Agent Keys

You need the keys currently loaded in the SSH agent.

Command

ssh-add -l

Before you run this

System impact: Read-only. Output may expose users, paths, tokens, keys, IPs, process arguments, or log details.

When not to use it: Do not paste key fingerprints publicly if they identify private infrastructure.

Expected output

A list of loaded public-key fingerprints and comments.

System impact

Read-only, sensitive output. Nothing changes. The command reads current state and prints diagnostic evidence.

Recovery / rollback: no state is changed.

When to use it

Use on the client side when SSH says too many authentication failures.

When not to use it

Do not paste key fingerprints publicly if they identify private infrastructure.

Common misread

Do not paste key fingerprints publicly if they identify private infrastructure.

Example run

Commands shown

These are the commands shown for inspection. Treat them as an example, not proof that your system will behave identically.

  1. ssh-add -l
  2. ssh-add -l

next steps

Related commands

Linux Survival Basics Sensitive output

Test SSH with One Identity File

Force one key when the agent offers too many identities.

ssh -o IdentitiesOnly=yes -i ~/.ssh/id_ed25519 user@example.com
Linux Survival Basics Sensitive output

Find SSH Too Many Authentication Failures Lines

The auth log proves whether the server refused after too many offered keys.

grep -i 'Too many authentication failures' /var/log/auth.log /var/log/secure 2>/dev/null | tail -20
Linux Survival Basics Sensitive output

Read TLS Certificate Subject and Issuer

The certificate can be valid but issued for the wrong name.

openssl s_client -connect example.com:443 -servername example.com </dev/null 2>/dev/null | openssl x509 -noout -subject -issuer -dates
Linux Survival Basics Read-only

List systemd Timers and Last Runs

A timer can be inactive, missed, or waiting for the wrong calendar.

systemctl list-timers --all --no-pager

next diagnostic step

Where to go from this command

Study mapping

Use this as independent command practice: read the notes, predict the output, then compare it with the example before using a real shell.

  • LPIC-1 style command-line practice
  • LFCS style performance-task practice
  • Linux+ style troubleshooting review

Independent study support only. No affiliation, endorsement, exam dumps, or real exam questions.