[el7_blog]
/dev/urandom

finding help, and using man

Using --help
The quickest way to get an overview of how to use a command is by running the command with the --help option.


[root@el7_blog.local]# man --help
man, version 1.6f

usage: man [-adfhktwW] [section] [-M path] [-P pager] [-S list]
        [-m system] [-p string] name ...

  a : find all matching entries
  c : do not use cat file
  d : print gobs of debugging information
  D : as for -d, but also display the pages
  f : same as whatis(1)
  h : print this help message
  k : same as apropos(1)
  K : search for a string in all pages
  t : use troff to format pages for printing
  w : print location of man page(s) that would be displayed
      (if no name given: print directories that would be searched)
  W : as for -w, but display filenames only

  C file   : use `file' as configuration file
  M path   : set search path for manual pages to `path'
  P pager  : use program `pager' to display pages
  S list   : colon separated section list
  m system : search for alternate system's man pages
  p string : string tells which preprocessors to run
               e - [n]eqn(1)   p - pic(1)    t - tbl(1)
               g - grap(1)     r - refer(1)  v - vgrind(1)

The historical Linux Programmer’s Manual
man man-pages

Man pages are categorized in different sections. The most relevant sections for system administrators are as follows:

  • 1: Executable programs or shell commands
  • 5: File formats and conventions
  • 8: System administration commands


Section Content Type
1 User Commands
2 System Calls
3 Library Functions
4 Special Files
5 File Formats
6 Games
7 Conventions, Standards, and Miscellaneous
8 System Administration and Privileged Commands
9 Linux Kernel API

To distinguish identical topic names in different sections, man page references include the section number in parentheses after the topic. passwd(1) describes the command to change passwords, while passwd(5) explains the /etc/passwd file format for storing local user accounts.

man passwd displays passwd(1) by default
man 5 passwd displays passwd(5)

/STRING Searches forward (down) for STRING in the man page
When performing searches, STRING allows regular expression syntax.

A keyword search of man pages is performed using # man -k KEYWORD, which displays a list of keyword-matching man page topics with section numbers.
The man -K option performs a full-text page search, not just titles and descriptions like the # man -k option.

Another useful man option is -f. The command man -f displays a short description of the item found in the man database.


[root@el7_blog.local]# man -f passwd
passwd               (1)  - update user's authentication tokens
passwd               (5)  - password file
passwd [sslpasswd]   (1ssl)  - compute password hashes


Update the mandb database with the mandb command as root without any arguments.