Basic commands

How to quickly find a command's options: man, --help, apropos and tldr; how to read a man page.

A script is a text file of shell commands: the #! line picks the interpreter, arguments arrive in $1, $2 and "$@".

An exit code is a number from 0 to 255 that a command reports: 0 means success, anything else a failure or a "no".

A terminal line is a command name, options (-l, --all) and arguments; the shell splits it into words at spaces.

Shell substitutions — * ? [] {} — and how they differ from regular expressions.

The shell sends a command's input and output to a file or another command — >, >>, 2>&1, /dev/null, the | pipe.

Environment variables: env, export, $PATH, ~/.bashrc and profiles; where "command not found" comes from.

Language, encoding and formats — LANG/LC_*, generating locales, UTF-8 problems.

Moving around the filesystem: cd, cd -, cd ~, cd .., pushd/popd, CDPATH.

A two-panel file manager for the terminal: navigation, copying, an editor, SFTP.

shellcheck reads a bash or sh script without running it and finds bugs: unquoted variables, unchecked cd.

Find out which binary will run, whether it's a builtin or an alias, where it lives, and its man page.

Short names for long commands; temporary and permanent via ~/.bashrc; why alias is invisible in scripts.

test and [ ] check a file, a string or a number and answer with a code: 0 is yes, 1 is no; [[ ]] is the bash version.

Printing strings and variables; -n and -e, escaping, when to prefer printf.

Compare load average to the core count (nproc), not an absolute threshold — the three numbers mean nothing without it.

Turns stdin lines into a command's arguments; without -0, file names with spaces and newlines break the parsing.

Formatted output in the shell — more reliable than echo, formats and special characters.

The bare minimum for editing configs; Esc always returns to Normal mode, where letters are commands, not text.

A friendly console editor — save, quit, search, for editing config files.

Keeps a terminal session alive on the server after an SSH drop; sessions live in memory and don't survive a reboot.

Keeps a terminal session alive on the server after an SSH drop; sessions live in memory and don't survive a reboot.

Repeats a command every N seconds on one screen; without quotes, a pipeline goes to the shell, not the command.

Run a command once at a given time — unlike cron's recurring schedule.

flock holds a file lock while a command runs, so a second copy of the script waits for it or exits at once.

The date command: current time, output formats, timestamps for files and logs, date arithmetic.

A command interpreter for interactive input, builtins, and shell scripts.

Reads or temporarily sets the host name; selected options report FQDNs and addresses.

The Bash builtin prints the logical or physical path of the current directory.

Reports kernel identity, node name, and machine architecture, not the distribution release.

whoami prints the user name tied to the process's effective user ID — the account you act as now.