01 Help and Basics

Learning objectives

• Know how to query command help.
• Inspect and reuse command history safely.
• Clear a terminal and print text and variables.
• Determine whether a command comes from an alias, function, builtin, or external program, and preserve its exit status.

Command overview

Command	Purpose	Meaning	Basic usage	Example
man	Read a command manual	Short for manual; shows a complete command description	man command_name	man ls
--help	Read brief help	Many GNU and third-party external commands support it; it is not a uniform POSIX requirement	command --help	cp --help
history	View command history	Lists commands executed by the current shell	history or history count	history 20
clear	Clear the screen	Clears the current terminal display	clear	clear
printf	Print formatted output	More predictable behavior than echo	printf 'format' arguments	printf '%s\n' "$HOME"
command -v	Query command resolution	Returns the location or type clue the Shell will use	command -v name	command -v python3
type	Inspect Bash command type	Shows an alias, function, builtin, or file	type -a name	type -a cd

Common scenarios

Find out how to use a command:

man tar
tar --help

Show recently executed commands:

history 30

Print text and an environment-variable fact:

printf '%s\n' 'hello linux'
printf 'path_entries_are_set=%s\n' "${PATH:+yes}"

Clear the terminal screen:

clear

Notes for beginners

• In a man page, press / to search and q to quit.
• Many GNU and third-party external commands use --help for a quick option list, but POSIX does not standardize it. For Bash builtins, prefer help name or the Shell manual; on BSD/macOS, follow the local man page.
• history only displays history; it does not automatically run it. Confirm safety before rerunning a historical command.
• History can include URLs, arguments, and accidentally entered secrets. Do not put tokens directly on a command line or publicly paste complete history.
• Query only variables that are safe to disclose. Bare env or printing a full PATH/proxy configuration can expose local information.
• $? preserves only the most recent foreground pipeline’s status. Copy it into a variable immediately before another command runs.

Minimal exercise

type -a -- cd
command -v -- grep
grep -q -- 'needle' /dev/null
grep_rc=$?
printf 'grep_rc=%s\n' "$grep_rc"

Explain what kind of command cd and grep are, and use GNU grep documentation to explain exit status 1. Complete Linux environment and Shell basics first for environment, quoting, and expansion.

References

• GNU Bash Builtins
• POSIX command