Linux Administration
Linux Help and Documentation
In this lesson
Linux help and documentation refers to the built-in and online reference systems available to every administrator directly from the command line. Linux ships with one of the most comprehensive offline documentation systems of any operating system — knowing how to navigate it efficiently means an administrator can find accurate, version-specific answers without leaving the terminal, even on an air-gapped server with no internet access.
The man Page System
The man (manual) page system is the primary reference for every command, system call, configuration file, and library on Linux. Every installed package ships its man pages alongside its binaries. Man pages are the authoritative, version-matched documentation — always more reliable than a web search result that may refer to a different version.
# Open the man page for a command
man ls
# Open the man page for a config file
man sshd_config
# Open a specific section (see section table below)
man 5 passwd
# Search within a man page — press / then type your search term
# Press n for next match, N for previous, q to quitLS(1) User Commands LS(1)
NAME
ls - list directory contents
SYNOPSIS
ls [OPTION]... [FILE]...
DESCRIPTION
List information about the FILEs (the current directory
by default). Sort entries alphabetically if none of
-cftuvSUX nor --sort is specified.
-a, --all
do not ignore entries starting with .
-l
use a long listing format
...
What just happened? The man page opened in the less pager. The LS(1) header tells you the command name and its section number. Use arrow keys to scroll, / to search within the page, and q to quit.
Fig 1 — The eight man page sections and their content types
Analogy: A man page is the official instruction manual that shipped inside the box with the tool. It is always specific to the exact version installed on your system — more trustworthy than any web tutorial written for a different version.
apropos and whatis
When the exact command name is unknown, two tools search the man page database by keyword. Both read from the same prebuilt index — run sudo mandb to rebuild it if results are missing.
apropos
Searches man page names and descriptions for a keyword. Returns a list of every man page that mentions the term. Equivalent to man -k.
apropos "disk usage"
whatis
Shows the one-line description of a specific command from its man page. Equivalent to man -f. Best for quickly confirming what a known command does.
whatis ls
# Search for man pages related to "network"
apropos network
# Search for man pages related to "compress"
apropos compress
# Get a one-line description of a command
whatis tar
whatis chmod
whatis sshd
# Equivalent shorthand forms
man -k "disk" # same as apropos
man -f ls # same as whatis
# Rebuild the man page index (needed after installing new packages)
sudo mandb# apropos compress bzip2 (1) - a block-sorting file compressor gzip (1) - compress or expand files lz4 (1) - lz4, unlz4, lz4cat - compress or uncompress .lz4 files xz (1) - compress or decompress .xz and .lzma files zip (1) - package and compress (archive) files zstd (1) - zstd, unzstd - compress or decompress .zst files # whatis tar tar (1) - an archiving utility
What just happened? apropos compress searched the man page database and returned every tool that mentions compression in its description — giving a full picture of the available options. Each result shows the command name and its section number in parentheses.
--help and Built-in Help
For a fast overview of a command's flags without opening a full man page, two quick options are available on nearly every Linux command.
Prints a condensed usage summary — all flags and arguments with brief descriptions. Output goes to stdout and can be piped through less or grep.
ls --help | less
Shell built-in commands like cd, echo, and read have no man pages. Use the help command instead.
help cd
Tells you whether a command is a shell built-in, an alias, a function, or an external binary — which determines whether to use help or man.
type cd
type ls
# Quick flag reference for any external command
ls --help
grep --help
tar --help | grep "extract"
# Help for Bash built-in commands
help cd
help echo
help read
# Determine whether a command is built-in or external
type cd # cd is a shell builtin
type ls # ls is /usr/bin/ls
type ll # ll is aliased to 'ls -la'info Pages
The info system is an alternative documentation format created by the GNU project. Info pages are more detailed than man pages for GNU tools like grep, ls, and bash — they are structured as hyperlinked nodes rather than a single scrollable page.
# Open the info page for a command
info ls
info grep
info bash
# Navigate info pages:
# Arrow keys / Enter — follow a link (shown as * Menu: entries)
# n — next node
# p — previous node
# u — up one level
# q — quit/usr/share/doc and Package Documentation
Beyond man and info pages, every installed package drops additional documentation into /usr/share/doc/packagename/. This directory typically contains README files, changelogs, example configuration files, and in many cases complete usage guides — all available offline.
# List available documentation for a package
ls /usr/share/doc/nginx/
# Read the README
cat /usr/share/doc/openssh-server/README
# View example config files (often the best starting point)
ls /usr/share/doc/nginx/examples/
# Find all documentation directories for installed packages
ls /usr/share/doc/ | lessOnline Documentation Resources
When offline documentation is insufficient — for complex topics, distro-specific behaviour, or community knowledge — the following resources are the most authoritative references available.
The most comprehensive community Linux wiki. Articles are detailed, up-to-date, and applicable to all distributions despite the Arch branding. The first place to check for complex configuration topics.
Red Hat's official documentation. Written for enterprise use, exceptionally thorough, and freely accessible. The authoritative reference for RHEL, Rocky Linux, and AlmaLinux administration.
Canonical's official server guide. Covers networking, storage, services, and security from an Ubuntu perspective. Maintained with each LTS release.
The Linux man-pages project online. Useful when working from a machine without the relevant man pages installed, or for sharing links to specific command documentation.
The official Linux kernel documentation. Relevant for kernel parameters, subsystem behaviour, and low-level system internals. Necessary reading for kernel tuning work covered in Lessons 36 and 37.
Random Web Searches Are Not a Substitute for man Pages
Blog posts and Stack Overflow answers are often written for a different version of a tool, a different distribution, or a different use case. On a production server, always cross-reference with the local man page using man commandname to confirm that the flags and behaviour match your installed version. A flag that existed in version 2.x may have been renamed or removed in version 3.x.
Lesson Checklist
man N command
apropos to discover commands by keyword and whatis for one-line descriptions
--help vs help vs man depending on whether a command is external or a shell built-in
Teacher's Note
In every RHCSA and LFCS certification exam, candidates are permitted to use man pages during the exam. Practising fast navigation — opening a page, jumping to a section, searching for a flag — is as important as memorising commands. Spend time getting comfortable with / search inside man pages.
Practice Questions
1. You need to find all flags for the find command that relate to file permissions. Write the sequence of man page commands and in-page keystrokes you would use to locate this information efficiently.
2. A colleague is connected to an air-gapped production server with no internet access and cannot remember which command compresses files with the .xz format. Write the exact command they should run to discover this from the server itself.
3. Explain why man passwd and man 5 passwd return different results, and state what each version documents.
Lesson Quiz
1. The command apropos is most accurately described as which of the following?
2. Man page section 8 contains documentation for which category of content?
3. A Bash built-in command such as cd has no man page entry. The correct way to get help for it is which of the following?
Up Next
Lesson 10 — Linux Best Practices for Beginners
Safe habits, common mistakes, and the mindset that separates careful administrators from reckless ones