Commands

man

man [section num] [command]
manual in a particular section
man -k
search all manuals with keyword
man -k . -s <section-NR>
list all manual in a particular section

info

whatis

apropos

where & which

Sections

1 Executable programs or shell commands

2 System calls (functions provided by the kernel)

3 Library calls (functions within program libraries)

strftime timezone in <time.h>
fopen fread fwrite fseek fclose

4 Special files (usually found in /dev)

5 File formats and conventions eg /etc/passwd

passwd
tzfile
utmp
fstab
os-release
some saved in specified data structure
有些因为是以特别的数据结构存储的,直接用文本编辑打开会乱码,如utmp。

6 Games

fortune, ponysay, ponythink

7 Miscellaneous (including macro packages and conventions), e.g. man(7), groff(7)

8 System administration commands (usually only for root)

9 Kernel routines [Non standard]

Structures

man man as a example (we can first man man):
NAME
SYNOPSIS
DESCRIPTION
EXAMPLES
OVERVIEW
DEFAULTS
OPTIONS
EXIT STATUS
ENVIRONMENT
FILES
SEE ALSO
HISTORY

man less as a example (man uses less as default pager. What a pager does is allow you to view content in Terminal page by page, or line by line. ):
NAME
SYNOPSIS
DESCRIPTION
COMMANDS
OPTIONS
LINE EDITING
KEY BINDINGS
INPUT PREPROCESSOR
NATIONAL CHARACTER SETS
PROMPTS
SECURITY
COMPATIBILITY WITH MORE
ENVIRONMENT VARIABLES
SEE ALSO
COPYRIGHT
AUTHOR

man ps as a example (another example):
NAME
SYNOPSIS
DESCRIPTION
EXAMPLES
SIMPLE PROCESS SELECTION
PROCESS SELECTION BY LIST
OUTPUT FORMAT CONTROL
OUTPUT MODIFIERS
THREAD DISPLAY
OTHER INFORMATION
NOTES
PROCESS FLAGS
PROCESS STATE CODES
OBSOLETE SORT KEYS
AIX FORMAT DESCRIPTORS
STANDARD FORMAT SPECIFIERS
ENVIRONMENT VARIABLES
PERSONALITY
SEE ALSO
STANDARDS
AUTHOR