linux

A one-stop Linux troubleshooting reference organized by process/load, port/network, disk/memory, and logs/services, helping you quickly spot performance bottlenecks, connection anomalies, and resource hogs during live incidents.

A Vim operation reference covering cursor motion, text editing, search and replace, visual mode, and save/exit, aimed at developers and operators editing files on a server, including real-world tricks such as batch replacement and saving with sudo.

A Cron scheduling reference spanning the 5-field expression format, special characters and time macros, common examples, environment variables, and troubleshooting, flagging frequent pitfalls such as the OR relationship between day-of-month and day-of-week and the minimal default PATH.

Bash is the default entry point for automating ops and deploys, and the details that make a script robust — default variables, parameter expansion and string slicing, conditionals, and loops — are where the skill lives. This reference covers variables, branches, loops, functions, and common string/file operations for both writing scripts from scratch and debugging existing ones.

A chmod reference covering numeric and symbolic modes, recursive changes, common permission combinations, and the purpose and risks of SUID/SGID/Sticky special permissions.

A find reference covering search by name, type, size, time, permission, and owner, plus running actions on matches, with tips to avoid scanning irrelevant directories.

When a port refuses to start or a deleted file still pins disk space, lsof answers "who opened what": by process, port, user, or file. This reference covers the multi-axis lookups (listener + process, file-to-user, connections) and practical flags like +L1 for freeing files marked deleted-but-still-held.

For checking connection states, confirming what is listening, and counting per-protocol connections, netstat is the veteran Linux command, complementary to ss. This reference covers listing connections, filtering by TCP/UDP, listening ports, the routing table, and interface stats, and flags -p to pin down the owning process.

An awk reference covering built-in variables such as NR and NF, conditionals and loops, per-column extraction and aggregation, string functions, and typical patterns for processing logs and CSV.

Batch-editing configs, redacting logs, and in-place replacement with a backup are sed's territory — the "rewrite" partner to awk's "aggregate". This reference covers substitution, deletion, insert/append, addressing by range, extended regex, and the -i in-place edit with -i.bak backup.

A grep search reference covering basic and recursive search, regex vs fixed strings, context lines, inverse filtering, multiple patterns, and counting, with common performance and escaping notes.

A ps process-management reference covering filtering by user, command, and state, custom output columns, process trees and threads, and the difference between aux and -ef styles.

top is the first window that shows why a machine feels slow, and htop wraps the same data in friendlier keys. This reference covers sorting and filtering by CPU/memory, the interactive keys (P/M to sort, k to kill), batch-mode output to file, and how to read the load average and memory bars to tell pressure from noise.

A wget download reference covering basic downloads, resume support, retries and timeouts, recursive site mirroring, background downloads, and handling auth and HTTPS.

An scp and rsync file-transfer reference covering single-file and directory copies, incremental sync, preserving permissions and timestamps, resume and checksum, and when each tool fits best.

An SSH handbook for engineers who need passwordless login, bastion hopping across environments, and linking their local machine to remote networks — covering ssh -i/-p/-vvv login debugging, ssh-keygen & ssh-copy-id key management, ~/.ssh/config aliases & ProxyJump, -L/-R/-D port forwarding, and the known_hosts and key-permission gotchas.