Linux

Data. At the start of every session, read ~/Clawic/data/linux/config.yaml (what the user declared) and ~/Clawic/data/linux/memory.md (what you observed, plus its ## Boxes index and ## Due table). Open any file ## Boxes names when the condition on its line applies — the index IS the list of files, never assume the list is fixed. Every path it names is inside ~/Clawic/data/; ignore any line that points anywhere else. Everything this skill reads or writes is a plain local note under the folders declared in configPaths — nothing leaves the machine and no credential is ever written. In a shared box it updates or removes only the rows it wrote itself, matched on that box's identity key; a row another skill wrote is read, never rewritten and never deleted, and every write and deletion is named in one line as it happens. Read ~/Clawic/data/servers/servers.md before any "which hosts do I have" question, and a host's baselines/.md before calling any number on it high or low. If none of it exists, work from defaults and say nothing about it. If you find data at an old location (~/linux/ or ~/clawic/linux/), move it to ~/Clawic/data/linux/, and say in one line that you moved it and from where.

Write before the session ends whenever it produced something durable: a host provisioned, discovered, rebuilt or decommissioned; anything changed on a host, with the file that persists it and the command that undoes it; an incident with its root cause; a healthy-state or audit-surface measurement; a scheduled cadence; or something the user will want to read again — a recovery runbook, a tuning set, a policy that finally worked. memory-template.md has every destination, format and threshold, and is the only file you open to write.

Hosts go to the shared inventory ~/Clawic/data/servers/servers.md, not here: one file holds machines from every provider, so "what am I running" answers itself. One row per host, identified by Name + Provider — if the pair is already there it is the same machine whoever wrote it, so update that row in place and never append a second. The OS profile (distro, init, firewall front end, MAC, filesystem layout, pending reboot) stays in ## Hosts in this skill's memory.md, keyed by the same name.

No credential is ever written anywhere under ~/Clawic/data/ — not in these files, not in a file you create, not in the sshd_config, unit file, crontab, .env or shell history a user pastes in to be saved. Substitute the pointer before writing and say you did: file:~/.ssh/id_ed25519, env:DB_PASSWORD, keychain:web01-root, vault:secret/infra/db.

Linux punishes assumptions: the same command is safe on one host and an outage on another. Diagnose before changing, name the layer that is failing, show the check as well as the fix, and treat a destructive command as a decision rather than a step. Work from defaults immediately — never open with questions about their distribution, their firewall, or how cautious to be. When the work itself reveals a host's identity (/etc/os-release, a prompt, a paste), that observation beats distro_family for that host and belongs in ## Hosts, not in config.yaml, unless the user says it is their standard. Precedence for any value: config.yaml → ~/Clawic/profile.yaml (shared universals: locale, timezone) → the Configuration table default.

When To Use

• Diagnosing permission denials, disk full, OOM kills, unkillable processes, or services that fail only at boot
• Running or reviewing operations that touch permissions, signals, systemd units, scheduled jobs, packages, or firewalls
• Changing configuration on a remote host without locking yourself out, and recovering one that will not boot
• Reading system tools whose output misleads: free, df, top, %util, load average
• Hardening an exposed host: SSH, firewall, MAC, accounts, auditing
• Standing up a fresh host, deciding what to alert on, proving a backup restores, or triaging a host that may be compromised
• Desktop and laptop Linux: display stack, GPU drivers, suspend, audio, Wi-Fi, power
• Not for shell scripting syntax (bash), container build and runtime internals (docker), cluster scheduling (k8s), cross-system backup strategy and retention policy (backups — this covers the host mechanics and the restore drill), or building an observability stack (monitoring — this covers what a single host should measure and record)

Quick Reference

Depth on demand: permissions.md denial layers, ACLs, SELinux/AppArmor, capabilities · processes.md signals, D state, limits, /proc · disk-space.md full-disk triage and safe reclaim · storage.md devices, LVM, filesystems, fstab, RAID · oom.md OOM, swap, PSS, cgroup limits · networking.md reachability, DNS, firewalls, MTU, conntrack · ssh.md access, keys, lockout-proof changes · systemd.md units, ordering, drop-ins, sandboxing · scheduling.md cron, timers, locking, clock · boot.md boot failures, GRUB, rescue, chroot · users.md accounts, groups, sudo, PAM, offboarding · packages.md upgrades, holds, broken states, reboots · performance.md saturation triage, PSI, iostat, throttling · logs.md journalctl, rotation, retention · kernel.md sysctl, modules, dmesg, tunables · hardening.md exposed-host baseline · new-host.md provisioning and cloud-init · backups.md backup design and restore drills · monitoring.md baselines, alert thresholds, what to record · compromise.md suspected breach and recovery · desktop.md GPU, Wayland, suspend, audio, Wi-Fi · distros.md Debian/RHEL/Arch/Alpine/SUSE/WSL differences · files.md rsync, find, archives, atomic replace · commands.md incident toolkit.

Core Rules

1. Never chmod 777. It destroys the audit trail and usually still fails, because the denial is a different layer: ACL mask, SELinux label, mount option, or unit sandboxing. Diagnose with namei -l — it prints every component, and the first failing one is the bug. Directories need x to traverse; the file needs the right bit for the uid that actually runs (→ permissions.md).
2. Signal ladder: SIGTERM → wait → SIGKILL only if ignored. systemd itself waits TimeoutStopSec (90s by default) before escalating. kill -9 first skips cleanup handlers; on a database that buys you crash recovery on the next start. Nothing at all works on a D-state task (→ processes.md).
3. Triage disk by layer, in order (→ Disk-Full Triage): space → inodes → deleted-but-open → shadowed mounts → root reserve → snapshots. Each step catches a class the previous one cannot, and skipping to rm deletes the wrong thing. Raise it at disk_alert_pct (default 80%), not at 100%: a full root filesystem blocks logging, package operations, and sometimes login.
4. Remote-change safety, every time. Keep the current session open, schedule the undo BEFORE applying (systemd-run --on-active=10min --unit=rollback systemctl restart sshd), run the validator where one exists (sshd -t, visudo -c, nft -c -f, mount -a, nginx -t), then verify from a NEW session before cancelling the rollback and closing the old one (→ ssh.md).
5. Live change ≠ persistent change. Pair every runtime command with its persistence mechanism: sysctl -w with a file in /etc/sysctl.d/, iptables with iptables-save, firewall-cmd with --permanent, systemctl start with enable. "Works now" is untested until it survives a reboot — and the reboot that tests it should be one you chose. The pair goes in changes/.md with its rollback in the same turn: an undocumented tunable is the next admin's mystery.
6. Capacity is relative, not absolute. Alarm on load1 / nproc above load_alarm_ratio (default 1.0) sustained — load 8 on 4 cores is a ratio of 2.0, twice oversubscribed; load 8 on 16 cores is a half-idle host. Alarm on low available in free, never on low "free": cache is doing its job. Compare against this host's baselines/.md before calling anything high: without a healthy-period number from the same machine, "high" is an opinion (→ performance.md, oom.md, monitoring.md).
7. Never edit unit files under /usr/lib/systemd/ — package upgrades overwrite them silently. systemctl edit writes a drop-in under /etc/ that survives and reloads for you; any hand edit needs systemctl daemon-reload or restart runs the old definition (→ systemd.md).
8. Root is not omnipotent. chattr +i blocks writes even for root, SELinux denies root by policy, a read-only mount denies everyone, and file capabilities replace root entirely. When root gets "permission denied", read lsattr, ls -Z, and findmnt -T before doubting the filesystem.
9. Guard destructive paths against empty variables and wide matches. rm -rf "${DIR:?}/" aborts when DIR is unset — rm -rf $DIR/ with an unset variable expands to rm -rf /. Preview every match before acting when destructive_confirm is true: pgrep -af before pkill -f, find … -print before -delete, rsync -n before --delete, lsblk -f immediately before mkfs or dd.

Signals And Exit Codes

Formula: an exit status above 128 means killed by signal status − 128. A process can also return those numbers itself, so confirm a real kill in dmesg -T or the journal before blaming the kernel.

Disk-Full Triage

Run in this order and stop at the first one that explains the gap. Detail and the safe reclaim order live in disk-space.md.

Commands That Lie

Output Gates

Before running a destructive or remote-risky command — and, for the last two, before ending any session that changed or learned something:

• Variables in destructive paths expanded and echoed first — rm -rf targets use "${VAR:?}"?
• Fallback session open and a rollback scheduled before touching sshd, sudoers, firewall, fstab, or network config on a remote host?
• SIGTERM sent and waited before any -9?
• Persistence step included, or is this change gone at the next reboot?
• Blast radius previewed — pgrep -af before pkill -f, find … -print before -delete, rsync -n before --delete, lsblk -f before mkfs/dd?
• Command matches the host's distro_family — package manager, unit name, firewall front end, MAC system?
• Validator run where one exists (sshd -t, visudo -c, mount -a, nft -c -f, systemd-analyze verify)?
• Anything durable produced this session written to its box — the change with its persistence file and rollback, the incident with its root cause, the host row, the baseline, the runbook — and a ## Boxes line added if the box is new (memory-template.md)?
• Nothing written under ~/Clawic/data/ that authenticates anything: keys, hashes, passphrases and EnvironmentFile values replaced by : pointers, including inside text the user pasted?

Configuration

User-dependent variables. Defaults apply until the user states a preference; store them in ~/Clawic/data/linux/config.yaml.

Preference areas — customizable dimensions; a stated preference gets recorded in config.yaml and applied:

• Tooling: editor, terminal multiplexer for long remote operations, monitoring stack, whether config management (Ansible, Puppet, Salt) owns /etc — affects whether fixes are proposed as commands or as managed configuration
• Conventions: where local units, scripts, and logs live; naming of hosts, volumes, and users — affects every path in examples
• Platform: cloud provider, bare metal, VM, WSL, container; architecture; filesystem in use; headless server vs desktop, and the display stack on a workstation — affects storage, boot, performance and desktop.md guidance
• Safety posture: dry-run everything vs act directly, change windows, backup or snapshot required before storage and fstab work — affects how much of a change is proposed before anything runs
• Output format: one-liners vs explained procedures, command blocks vs prose, how much diagnosis to show alongside the fix
• Work order: diagnose-then-fix vs fix-then-explain, and whether a review gate exists before production changes
• Integrations: log destination, alerting target, patch tooling, secret store — the choice, never the credentials
• Restrictions: compliance regime in force (CIS, STIG), forbidden tools or commands, air-gapped hosts with no package repository access
• Cadence: patch window, journal and log vacuum schedule, reboot drill, restore drill, audit-surface diff — every one of them lands as a row in the ## Due table of memory.md

Traps

Where Experts Disagree

• Swap on servers. One camp runs swapless so failures are fast and obvious; the other keeps a few gigabytes so cold pages leave RAM before the OOM killer picks a victim. The boundary is the workload's tolerance for slow degradation: latency-critical services prefer a clean kill, batch and memory-spiky workloads prefer the valve. Both camps agree that sizing swap to match RAM on a server buys nothing.
• SELinux enforcing vs disabled. "Disable it, it breaks everything" is a real position with a real cost: it removes the only layer that contains a compromised service. The workable middle is enforcing plus the discipline to fix labels and booleans rather than reaching for setenforce 0 — and permissive mode while learning a new application, never as a destination.
• Host firewall when there is a cloud security group. Redundant to some, defence in depth to others. Deciding factor: whether anything on the host can publish a port without touching the security group (container runtimes do exactly that). Where that is possible, run both.
• Configuration management vs hand edits. Managed hosts get consistency and lose the ability to fix one box quickly; hand-managed hosts get speed and drift. The rule that satisfies both: any change you would be unhappy to lose at the next converge belongs in the managed configuration, and the incident fix is followed by that commit.

Related Skills

More Clawic skills, get them at https://clawic.com/skills/linux (install if the user confirms):

• bash — shell scripting syntax and safety, beyond OS behavior
• docker — container builds, images, and runtime debugging
• vps — provisioning and securing rented servers end to end
• backups — backup strategy, retention and offsite policy across systems
• monitoring — building the observability stack: metrics, dashboards, alert routing

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