linux:shell_commands:var_and_arr [Eureka Moment]

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====== Examples of Variables and Arrays in Bash ======
<code bash Create a variable, ${var} is the same as $var, but not ambiguous.>var="http://wiki.plecko.hr/"
echo $var
http://wiki.plecko.hr/
</code>

<code bash Return string length>
echo ${#var}
31
</code>

<code bash Execute the contents of $var (same as 'eval $$var')>
echo ${!var}
</code>

<code bash Returns variable names beginning with 'U'>
echo ${!U*}
UID USER USERNAME
</code>

<code bash Returns the text from position 7>
echo ${var:7}
wiki.plecko.hr/
</code>

</code bash Returns 6 characters from position 12>
echo ${var:11:6}
plecko
</code>

<code bash Cut 'http://' from beginning of string>
echo ${var#http://}
wiki.plecko.hr//
</code>

<code bash Cut '.hr/' from end of string>
echo ${var%.hr/}
http://wiki.plecko
</code>

<code bash Replaces 'plecko' with 'eurekamoment' once>
echo ${var/plecko/eurekamoment} 
http://wiki.eurekamoment.hr/
</code>

<code bash Replaces 'o' with 'O', always">
echo ${var//o/O}
http://wiki.pleckO.hr/
</code>

<code bash If string starts with 'http', replace 'http' with 'Site'>
echo ${var/#http/Site}
Site://wiki.plecko.hr/
</code>

<code bash If string ends with 'st', replace 'st' with 'STING'>
var2="test"
echo ${var2/%st/STING}
teSTING
</code>

<code bash "" (double quotation marks) protect a string, but recognize $, \ and ` as specials>
echo "$var"
http://wiki.plecko.hr/
</code>

<code bash '' (single quotes) protects a string, but recognizes $, \ and ` as specials>
echo '$var'
$var
</code>

<code bash $'…' (dollar sign before single quotes) protects a string completely, but interprets \n, \t, \a, etc.>
echo $'$var\n'
$var
</code>

<code bash  ... (between crases) Execute commands in a subshell, returning the result>
echo `ls`
Desktop Documents Downloads Images Templates Music Public Videos
</code>

<code bash Execute commands on a subshell>
(ls)
Desktop Documents Downloads Images Templates Music Public Videos
</code>

<code bash Execute commands on a subshell, returning the result>
echo $(ls)
Desktop Documents Downloads Images Templates Music Public Videos
</code>

<code bash Tests an arithmetic operation, returning 0 or 1>
((11>9))
</code>

<code bash Returns the result of an arithmetic operation.>
echo $((11-9))
2
</code>

<code bash Tests an expression, returning 0 or 1 (alias of command ‘test’)>
[[ $var ]] && echo 'Its bigger'
Its bigger
</code>

<code bash Tests an expression, returning 0 or 1 (can use && and **	 	** )>
[[ $var ]] && echo 'Is there this variable'
Is there this variable
</code>

<code bash Special Variables>
Variable	Positional Parameters
$0	Parameter Number 0 (Name of Command or Function)
$1	Parameter Number 1 (from command line or function)
...	Parameter number N …
$9	Parameter Number 9 (from command line or function)
${10}	Parameter Number 10 (from command line or function)
...	Parameter number NN …
$#	Total number of command line or function parameters
$*	All parameters as a single string
$@	All parameters, such as multiple protected strings
Variable	Miscellania
$$	PID number of current process (from script itself)
$!	PID number of last background job
$_	Last argument of last command executed
$?	Return code from last command executed
</code>

<code bash Special escapes to use at prompt (PS1)>
Escape	Reminder	Expands to …
\a	Alert	Alert (beep)
\d	Date	Date in "Weekday Month Day" format (Sat Jan 15)
\e	Escape	Esc Character
\h	Hostname	Machine Name Without Domain (dhcp11)
\H	Hostname	Full Machine Name (dhcp11.company)
\j	Jobs	Number of Active Jobs
\l	Tty	Current Terminal Name (ttyp1)
\n	Newline	Newline
\r	Return	Return by car
\s	Shell	Name of the shell (basename $ 0)
\t	Time	Time in 24-hour format HH: MM: SS
\T	Time	12-hour format HH: MM: SS
\@	At	Time in 12-hour format HH: MM am/pm
\A	At	Time in 24-hour format HH:MM
\u	User	Current user login
\v	Version	Bash Version (2.00)
\V	Version	Bash Version Subversion (2.00.0)
\w	Working Dir	Current directory, full path ($PWD)
\W	Working Dir	Current directory, only the last one (basename $PWD)
\!	History ico	Current command number in history
\#	Number	Current command number
\$	ID >	Show "#" if root, "$" if normal user
\nnn	Octal	Character whose octal is nnn
\\	Backslash	Backslash \ literal
\[	Escapes	Starts a sequence of escapes (color coded type)
\]	Escapes	Ends an escape sequence
</code>

<code bash Formatters of the date Command>
Format	Description
%a	Abbreviated Weekday Name (Sun..Sab)
%A	Name of the day of the week (Sunday..Saturday)
%b	Abbreviated Month Name (Jan. Dec)
%B	Name of the month (January.December)
%c	Complete date (Sat Nov 04 12:02:33 EST 1989)
%y	Year (two digits)
%Y	Year (four digits)
%m	Month (01..12)
%d	Day (01..31)
%j	Day of the year (001..366)
%H	Hours (00..23)
%M	Minutes (00..59)
%S	Seconds (00..60)
%s	Seconds since January 1, 1970
%%	A % literal
%t	One TAB
%n	A line break
</code>

<code bash Printf Command Formatters>
Format	Description
%d	Decimal number
%o	Octal Number
%x	Hexadecimal Number (a-f)
%X	Hexadecimal Number (A-F)
%f	Floating-point number
%e	Number in scientific notation (e + 1)
%E	Number in scientific notation (E + 1)
%s	String
</code>

<code bash To know all local variables, execute>
set
#or, and open txt to see later
set > VariablesLocations.txt
</code>

<code bash Global Variables, to know all global variables, execute>
env
#or
printenv
</code>

<code bash To assign a value to a local variable>
LINUX=free
echo $LINUX
free
</code>

<code bash check if the variable created in the previous item appears in the list of local variables>
set | grep LINUX
LINUX=free
</code>

<code bash Now let’s make this local variable a global variable.>
export LINUX
env | grep LINUX
LINUX=free
</code>

<code bash Delete a ‘local environment variable’ from memory using unset command>
unset LINUX
echo $LINUX
</code>

<code bash Create an alias (nickname for a command or program), you can still include it in your ~/.bashrc>
alias list='ls -la color=auto'
</code>

<code bash Destroying an aliase>
unalias list
</code>

<code bash Verify all commands entered>
history
#run command by his number in history
!468
#execute last command typed
!!
#they stay in bash_history
cat ~/.bash_history
#clear history
history -c
</code>

<code bash Command Line Interpreters>
    $ - Shell of an normal user;
    # - Superuser shell root (administrator)
#Check of Available Shells
cat /etc/shells
#Variable that shows the SHELL you use
echo $SHELL
</code>

<code bash Creating an Array (Array) "Variable Set">
DISTROS=("Debian" "Trisquel" "Ubuntu" "RedHat")
#If you print the DISTROS Array as a variable, it prints variable 0, the array displays the variables contained in it starting at 0 (zero), so it would be the same as printing at ${DISTROS[0]}
echo $DISTROS
</code>

<code bash Print element 1 of array DISTROSS>
echo ${DISTROS[1]}
Trisquel
</code>

<code bash You can also create an array by inserting element one at a time.>
DISTROS[0]="Debian"
DISTROS[1]="Trisquel"
DISTROS[2]="Ubuntu"
DISTROS[3]="RedHat"
</code>

<code bash Change element 2 of array DISTROSS>
DISTROS[2]="Linux Mint"
echo ${DISTROS[2]}
Linux Mint
</code>

<code bash Display Element 2 Distro to End>
echo ${DISTROS[@]:2} 
Ubuntu RedHat
</code>

<code bash This displays the elements starting at position 1 followed by two more elements consecutive to position 1>
echo ${DISTROS[@]:1:2} 
Trisquel Ubuntu
</code>

<code bash Know how many elements the array has DISTROSS>
echo ${#DISTROS[@]} 
4
</code>