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Help and Documentation

Help on external commands

External commands are binary executables or scripts generally found in /bin, /sbin, /usr/bin ou /usr/sbin :

root@debian8:~# type ifconfig
ifconfig is /sbin/ifconfig

Help for an external command can generally be obtained by using the –help option with the command in question :

root@debian8:~# du --help | more
Usage: du [OPTION]... [FILE]...
or: du [OPTION]... --files0-from=F
Summarize disk usage of each FILE, recursively for directories.
Mandatory arguments to long options are mandatory for short options too.
-0, --null end each output line with NUL, not newline
-a, --all write counts for all files, not just directories
--apparent-size print apparent sizes, rather than disk usage; although
the apparent size is usually smaller, it may be
larger due to holes in ('sparse') files, internal
fragmentation, indirect blocks, and the like
-B, --block-size=SIZE scale sizes by SIZE before printing them; e.g.,
'-BM' prints sizes in units of 1,048,576 bytes;
see SIZE format below
-b, --bytes equivalent to '--apparent-size --block-size=1'
-c, --total produce a grand total
-D, --dereference-args dereference only symlinks that are listed on the
command line
-d, --max-depth=N print the total for a directory (or file, with --all)
only if it is N or fewer levels below the command
line argument; --max-depth=0 is the same as
--summarize
--files0-from=F summarize disk usage of the
--More--

Use with certain commands, the –help option is not valid:

root@debian8:~# type --help
-su: type: --: invalid option
type: usage: type [-afptP] name [name ...]

Help on built-in commands

Commands such as type, cd or umask are internal to the shell:

root@debian8:~# type type
type is a shell builtin

One of the internal Bash commands is help. Used with no argument, this command shows a list of all the internal commands:

root@debian8:~# help | more
GNU bash, version 4.3.30(1)-release (x86_64-pc-linux-gnu)
These shell commands are defined internally. Type `help' to see this list.
Type `help name' to find out more about the function `name'.
Use `info bash' to find out more about the shell in general.
Use `man -k' or `info' to find out more about commands not in this list.
A star (*) next to a name means that the command is disabled.
job_spec [&] history [-c] [-d offset] [n] or history -a>
(( expression )) if COMMANDS; then COMMANDS; [ elif COMMAND>
. filename [arguments] jobs [-lnprs] [jobspec ...] or jobs -x com>
: kill [-s sigspec | -n signum | -sigspec] p>
[ arg... ] let arg [arg ...]
[[ expression ]] local [option] name[=value] ...
alias [-p] [name[=value] ... ] logout [n]
bg [job_spec ...] mapfile [-n count] [-O origin] [-s count] >
bind [-lpsvPSVX] [-m keymap] [-f filename] > popd [-n] [+N | -N]
break [n] printf [-v var] format [arguments]
builtin [shell-builtin [arg ...]] pushd [-n] [+N | -N | dir]
caller [expr] pwd [-LP]
case WORD in [PATTERN [| PATTERN]...) COMMA> read [-ers] [-a array] [-d delim] [-i text>
cd [-L|[-P [-e]] [-@]] [dir] readarray [-n count] [-O origin] [-s count>
command [-pVv] command [arg ...] readonly [-aAf] [name[=value] ...] or read>
--More--

To get help concerning a given internal command, help is invoked with the given command as an argument:

root@debian8:~# help type
type: type [-afptP] name [name ...]
Display information about command type.
For each NAME, indicate how it would be interpreted if used as a
command name.
Options:
-a	display all locations containing an executable named NAME;
includes aliases, builtins, and functions, if and only if
the `-p' option is not also used
-f	suppress shell function lookup
-P	force a PATH search for each NAME, even if it is an alias,
builtin, or function, and returns the name of the disk file
that would be executed
-p	returns either the name of the disk file that would be executed,
or nothing if `type -t NAME' would not return `file'.
-t	output a single word which is one of `alias', `keyword',
`function', `builtin', `file' or `', if NAME is an alias, shell
reserved word, shell function, shell builtin, disk file, or not
found, respectively
Arguments:
NAME	Command name to be interpreted.
Exit Status:
Returns success if all of the NAMEs are found; fails if any are not found.

The man command

The man command is used to consult the manual of the command passed as an argument. For example man passwd produces the following output:

PASSWD(1) User Commands PASSWD(1)
NAME
passwd - change user password
SYNOPSIS
passwd [options] [LOGIN]
DESCRIPTION
The passwd command changes passwords for user accounts. A normal user may only change the password for his/her own account, while the superuser may change
the password for any account. passwd also changes the account or associated password validity period.
Password Changes
The user is first prompted for his/her old password, if one is present. This password is then encrypted and compared against the stored password. The user
has only one chance to enter the correct password. The superuser is permitted to bypass this step so that forgotten passwords may be changed.
After the password has been entered, password aging information is checked to see if the user is permitted to change the password at this time. If not,
passwd refuses to change the password and exits.
The user is then prompted twice for a replacement password. The second entry is compared against the first and both are required to match in order for the
password to be changed.
Then, the password is tested for complexity. As a general guideline, passwords should consist of 6 to 8 characters including one or more characters from
each of the following sets:
· lower case alphabetics
· digits 0 thru 9
· punctuation marks
Care must be taken not to include the system default erase or kill characters. passwd will reject any password which is not suitably complex.
Hints for user passwords
The security of a password depends upon the strength of the encryption algorithm and the size of the key space. The legacy UNIX System encryption method is
based on the NBS DES algorithm. More recent methods are now recommended (see ENCRYPT_METHOD). The size of the key space depends upon the randomness of the
password which is selected.
Compromises in password security normally result from careless password selection or handling. For this reason, you should not select a password which
appears in a dictionary or which must be written down. The password should also not be a proper name, your license number, birth date, or street address.
Any of these may be used as guesses to violate system security.
You can find advices on how to choose a strong password on http://en.wikipedia.org/wiki/Password_strength
OPTIONS
Manual page passwd(1) line 1 (press h for help or q to quit)

Each manual page can contain several sections:

Section	Contents
NAME	This section is required and has a standardized format consisting of a comma-separated list of program or function names, followed by a dash, followed by a short description of the functionality the program (or function, or file) is supposed to provide. By using the makewhatis command, the name sections are inputted into the whatis database files.
SYNOPSIS	This section gives a short overview on available program options. For functions this section lists corresponding include files and the prototype so the programmer knows the type and number of arguments as well as the return type.
DESCRIPTION	This section describes how to use the command or function and what each argument does.
OPTIONS	This section gives a description of how each option affects program behaviour.
FILES	This section lists files the program or function uses.
EXAMPLE	This section gives examples of how to use the command or function.
ENVIRONMENT	This section lists all environment variables that affect the program or function and explains how they do so.
CONFORMING TO	This section lists the eventual standards that the command or function conforms to.
BUGS / TO DO	This section describes the limitations and known inconveniences of the command or function.
EXIT STATUS / RETURN VALUE	This section lists the exit status codes and their meaning.
SEE ALSO	This section provides a list of related man pages in alphabetical order.

Navigation within a manual is accomplished using the following keys:

Key	Function
Space Bar	Move forward one screen
↵ Enter	Move down one line
&uarr;	Move up one line
&darr;	Move down one line
Page&uarr;	Move up (backwards) one half screen
Page&darr;	Move down (forward) one half screen
Home	Move to the beginning of the manual
End	Move to the end of the manual
/	Search for the string that follows the / key. The n key then searches for the next occurrence whilst the N searches for the previous occurrence
Q	Quit the manual

A complete Linux manual is comprised of up to 9 sections:

Section	Contents
1	Executable programs or shell commands
2	System calls (functions provided by the kernel)
3	Library calls (functions within program libraries)
4	Special files (usually found in /dev)
5	File formats and conventions eg /etc/passwd
6	Games
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]

The available section for a specific command are given by the output of the whereis command:

root@debian8:~# whereis passwd
passwd: /usr/bin/passwd /etc/passwd /usr/share/man/man1/passwd.1ssl.gz /usr/share/man/man1/passwd.1.gz /usr/share/man/man5/passwd.5.gz

To consult a specific section of a manual, the section number must be supplied as an argument to the man command:

$ man 5 passwd [Enter]

The -k option of the man command searches for the string supplied as an argument within the list of available manuals:

root@debian8:~# man -k passwd
chgpasswd (8) - update group passwords in batch mode
chpasswd (8) - update passwords in batch mode
exim4_passwd (5) - Files in use by the Debian exim4 packages
exim4_passwd_client (5) - Files in use by the Debian exim4 packages
gpasswd (1) - administer /etc/group and /etc/gshadow
grub-mkpasswd-pbkdf2 (1) - generate hashed password for GRUB
lppasswd (1) - add, change, or delete digest passwords.
mkpasswd (1) - Overfeatured front end to crypt(3)
pam_localuser (8) - require users to be listed in /etc/passwd
passwd (1) - change user password
passwd (1ssl) - compute password hashes
passwd (5) - the password file
update-passwd (8) - safely update /etc/passwd, /etc/shadow and /etc/groupword

The output is a list of the available manuals complete with their description. The number between parentheses indicates the manual section number.

Command Line Switches

The switches associated with the man command are:

root@debian8:~# man --help
Usage: man [OPTION...] [SECTION] PAGE...
-C, --config-file=FILE use this user configuration file
-d, --debug emit debugging messages
-D, --default reset all options to their default values
--warnings[=WARNINGS] enable warnings from groff
Main modes of operation:
-f, --whatis equivalent to whatis
-k, --apropos equivalent to apropos
-K, --global-apropos search for text in all pages
-l, --local-file interpret PAGE argument(s) as local filename(s)
-w, --where, --path, --location
print physical location of man page(s)
-W, --where-cat, --location-cat
print physical location of cat file(s)
-c, --catman used by catman to reformat out of date cat pages
-R, --recode=ENCODING output source page encoded in ENCODING
Finding manual pages:
-L, --locale=LOCALE define the locale for this particular man search
-m, --systems=SYSTEM use manual pages from other systems
-M, --manpath=PATH set search path for manual pages to PATH
-S, -s, --sections=LIST use colon separated section list
-e, --extension=EXTENSION limit search to extension type EXTENSION
-i, --ignore-case look for pages case-insensitively (default)
-I, --match-case look for pages case-sensitively
--regex show all pages matching regex
--wildcard show all pages matching wildcard
--names-only make --regex and --wildcard match page names only,
not descriptions
-a, --all find all matching manual pages
-u, --update force a cache consistency check
--no-subpages don't try subpages, e.g. 'man foo bar' => 'man
foo-bar'
Controlling formatted output:
-P, --pager=PAGER use program PAGER to display output
-r, --prompt=STRING provide the `less' pager with a prompt
-7, --ascii display ASCII translation of certain latin1 chars
-E, --encoding=ENCODING use selected output encoding
--no-hyphenation, --nh turn off hyphenation
--no-justification, --nj turn off justification
-p, --preprocessor=STRING STRING indicates which preprocessors to run:
e - [n]eqn, p - pic, t - tbl,
g - grap, r - refer, v - vgrind
-t, --troff use groff to format pages
-T, --troff-device[=DEVICE] use groff with selected device
-H, --html[=BROWSER] use www-browser or BROWSER to display HTML output
-X, --gxditview[=RESOLUTION] use groff and display through gxditview
(X11):
-X = -TX75, -X100 = -TX100, -X100-12 = -TX100-12
-Z, --ditroff use groff and force it to produce ditroff
-?, --help give this help list
--usage give a short usage message
-V, --version print program version
Mandatory or optional arguments to long options are also mandatory or optional
for any corresponding short options.
Report bugs to cjwatson@debian.org.

The apropos command

apropos searches the whatis database for instances of a keyword passed to the command as an argument. With no further options, the output is identical to man -k:

root@debian8:~# apropos passwd
chgpasswd (8) - update group passwords in batch mode
chpasswd (8) - update passwords in batch mode
exim4_passwd (5) - Files in use by the Debian exim4 packages
exim4_passwd_client (5) - Files in use by the Debian exim4 packages
gpasswd (1) - administer /etc/group and /etc/gshadow
grub-mkpasswd-pbkdf2 (1) - generate hashed password for GRUB
lppasswd (1) - add, change, or delete digest passwords.
mkpasswd (1) - Overfeatured front end to crypt(3)
pam_localuser (8) - require users to be listed in /etc/passwd
passwd (1) - change user password
passwd (1ssl) - compute password hashes
passwd (5) - the password file
update-passwd (8) - safely update /etc/passwd, /etc/shadow and /etc/group

Command Line Switches

The switches associated with the apropos command are:

root@debian8:~# apropos --help
Usage: apropos [OPTION...] KEYWORD...
-d, --debug emit debugging messages
-v, --verbose print verbose warning messages
-e, --exact search each keyword for exact match
-r, --regex interpret each keyword as a regex
-w, --wildcard the keyword(s) contain wildcards
-a, --and require all keywords to match
-l, --long do not trim output to terminal width
-C, --config-file=FILE use this user configuration file
-L, --locale=LOCALE define the locale for this search
-m, --systems=SYSTEM use manual pages from other systems
-M, --manpath=PATH set search path for manual pages to PATH
-s, --sections=LIST, --section=LIST
search only these sections (colon-separated)
-?, --help give this help list
--usage give a short usage message
-V, --version print program version
Mandatory or optional arguments to long options are also mandatory or optional
for any corresponding short options.
The --regex option is enabled by default.
Report bugs to cjwatson@debian.org.

The mandb and whatis commands

The whatis database is maintained by root by using the /bin/mandb or /usr/bin/mandb command.

Once again, the use of mandb is very simple:

root@debian8:~# mandb
Purging old database entries in /usr/share/man...
Processing manual pages under /usr/share/man...
Purging old database entries in /usr/share/man/ug...
Processing manual pages under /usr/share/man/ug...
Purging old database entries in /usr/share/man/tr...
Processing manual pages under /usr/share/man/tr...
Purging old database entries in /usr/share/man/el...
Processing manual pages under /usr/share/man/el...
Purging old database entries in /usr/share/man/ja...
Processing manual pages under /usr/share/man/ja...
...
0 man subdirectories contained newer manual pages.
0 manual pages were added.
0 stray cats were added.
0 old database entries were purged.

Now the whatis command can be used to check which manual sections are available for a specific command:

root@debian8:~# whatis passwd
passwd (1) - change user password
passwd (5) - the password file
passwd (1ssl) - compute password hashes

Command Line Switches

The switches associated with the mandb command are:

root@debian8:~# mandb --help
Usage: mandb [OPTION...] [MANPATH]
-c, --create create dbs from scratch, rather than updating
-C, --config-file=FILE use this user configuration file
-d, --debug emit debugging messages
-f, --filename=FILENAME update just the entry for this filename
-p, --no-purge don't purge obsolete entries from the dbs
-q, --quiet work quietly, except for 'bogus' warning
-s, --no-straycats don't look for or add stray cats to the dbs
-t, --test check manual pages for correctness
-u, --user-db produce user databases only
-?, --help give this help list
--usage give a short usage message
-V, --version print program version
Mandatory or optional arguments to long options are also mandatory or optional
for any corresponding short options.
Report bugs to cjwatson@debian.org.

The switches associated with the whatis command are:

root@debian8:~# whatis --help
Usage: whatis [OPTION...] KEYWORD...
-d, --debug emit debugging messages
-v, --verbose print verbose warning messages
-r, --regex interpret each keyword as a regex
-w, --wildcard the keyword(s) contain wildcards
-l, --long do not trim output to terminal width
-C, --config-file=FILE use this user configuration file
-L, --locale=LOCALE define the locale for this search
-m, --systems=SYSTEM use manual pages from other systems
-M, --manpath=PATH set search path for manual pages to PATH
-s, --sections=LIST, --section=LIST
search only these sections (colon-separated)
-?, --help give this help list
--usage give a short usage message
-V, --version print program version
Mandatory or optional arguments to long options are also mandatory or optional
for any corresponding short options.
Report bugs to cjwatson@debian.org.

The info command

In addition to the man system, information regarding programs and commands can also be found using the info system. Background information, tutorials, and detailed information on using the program in question are sometimes lacking in man pages and as a result the info system was created to resolve that issue.

Within the info system, multiple individual pages of information make up the entire set of info pages for each specific application. These pages of information are referred to as nodes. You can page through nodes one at a time, or you can jump to specific pages through the use of hypertext links.

To help node navigation, info pages all display a header across the top of the page. This header includes information such as the current node, next and previous nodes, and information regarding the parent node. Navigation is accomplished using the following keys :

Key	Function
n	Next node.
p	Previous node.
u	Parent node.
Space	Scroll down one page at a time.
Del	Scroll up one page at a time.
b	Return to the beginning of the current node.
Tab ⇆	Select next hypertext link.
m <link text>	Performs a direct jump to the specified subnode. Pressing Tab displays all available subnodes.
↵ Enter	Follow current hypertext link. Hypertext links are designated by an asterisk * at the beginning of the link, and a colon : at the end of the link.
q	Quit the info system.

To access the top node of the info system, use the following command :

root@debian8:~# info
...
File: dir, Node: Top This is the top of the INFO tree
This (the Directory node) gives a menu of major topics.
Typing "q" exits, "?" lists all Info commands, "d" returns here,
"h" gives a primer for first-timers,
"mEmacs<Return>" visits the Emacs manual, etc.
In Emacs, you can click mouse button 2 on a menu item or cross reference
to select it.
* Menu:
Basics
* Common options: (coreutils)Common options.
* Coreutils: (coreutils). Core GNU (file, text, shell) utilities.
* Date input formats: (coreutils)Date input formats.
* File permissions: (coreutils)File permissions.
Access modes.
* Finding files: (find). Operating on files matching certain criteria.
Compression
* Gzip: (gzip). General (de)compression of files (lzw).
-----Info: (dir)Top, 197 lines --Top--------------------------------------------------------
Welcome to Info version 5.2. Type h for help, m for menu item.

Command Line Switches

The switches associated with the info command are:

root@debian8:~# info --help
Usage: info [OPTION]... [MENU-ITEM...]
Read documentation in Info format.
Options:
-a, --all use all matching manuals.
-k, --apropos=STRING look up STRING in all indices of all manuals.
-d, --directory=DIR add DIR to INFOPATH.
--dribble=FILE remember user keystrokes in FILENAME.
-f, --file=MANUAL specify Info manual to visit.
-h, --help display this help and exit.
--index-search=STRING go to node pointed by index entry STRING.
-n, --node=NODENAME specify nodes in first visited Info file.
-o, --output=FILE output selected nodes to FILE.
-R, --raw-escapes output "raw" ANSI escapes (default).
--no-raw-escapes output escapes as literal text.
--restore=FILE read initial keystrokes from FILE.
-O, --show-options, --usage go to command-line options node.
--strict-node-location (for debugging) use Info file pointers as-is.
--subnodes recursively output menu items.
-v, --variable VAR=VALUE assign VALUE to Info variable VAR.
--vi-keys use vi-like and less-like key bindings.
--version display version information and exit.
-w, --where, --location print physical location of Info file.
-x, --debug=NUMBER set debugging level (-1 for all).
The first non-option argument, if present, is the menu entry to start from;
it is searched for in all `dir' files along INFOPATH.
If it is not present, info merges all `dir' files and shows the result.
Any remaining arguments are treated as the names of menu
items relative to the initial node visited.
For a summary of key bindings, type h within Info.
Examples:
info show top-level dir menu
info info show the general manual for Info readers
info info-stnd show the manual specific to this Info program
info emacs start at emacs node from top-level dir
info emacs buffers start at buffers node within emacs manual
info --show-options emacs start at node with emacs' command line options
info --subnodes -o out.txt emacs dump entire manual to out.txt
info -f ./foo.info show file ./foo.info, not searching dir
Email bug reports to bug-texinfo@gnu.org,
general questions and discussion to help-texinfo@gnu.org.
Texinfo home page: http://www.gnu.org/software/texinfo/

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Copyright © 2004-2018 Hugh Norris.<br><br>

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