Unix

Permissions and the other bits you type into a shell.

chmod calculatorPermission checkboxes ⇄ octal ⇄ rwx, with setuid, setgid and sticky bitsrwxr-xr-x = 755 umask calculatorSee which file and directory permissions a umask produces, or find the umask for the permissions you wantumask 022 → files 644, dirs 755 ls -l and stat explainerPaste a line of ls -l or the output of stat and get every column explaineddrwxr-sr-x. 5 www-data … → directory, 2755, SELinux Exit code referenceWhat exit status 1, 2, 126, 127, 137, 139, 143 and 255 mean, and the 128+signal rule137 = killed by SIGKILL (OOM)Linux signal referenceSIGTERM vs SIGKILL, every signal number, default action and what sends itkill -15 = SIGTERM, kill -9 = SIGKILL Cron ⇄ systemd timer converterTurn a crontab line into a complete timer and service unit, or an OnCalendar expression back into cron30 6 * * 1-5 → Mon..Fri *-*-* 06:30:00 Diff toolCompare two texts line by line, see changed characters highlighted, get a unified diff- old line / + new line Regex testerLive matching with groups, replace preview, and a plain-English breakdown of the pattern/(\d{4})-(\d{2})/g → 2 groups, 3 matches nginx location testerPaste a config and a URL: which server and location win, and what rewrite, try_files and proxy_pass do with it/api/users → location /api/ → proxy http://127.0.0.1:3000/users EXPLAIN plan visualizerPaste PostgreSQL or MySQL EXPLAIN ANALYZE output: a tree with time per node, estimate vs actual, buffers, and what to fixEXPLAIN ANALYZE → 64% in Hash Join, 2 batches → raise work_mem SQL formatterPretty-print or minify SQL: PostgreSQL, MySQL, SQLite, SQL Server. Keyword case, indent, comma styleselect a,b from t where … → indented, one clause per line