Examples of Variables and Arrays in Bash

Create a variable, ${var} is the same as $var, but not ambiguous.
echo $var
Return string length
echo ${#var}
Execute the contents of $var (same as 'eval $$var')
echo ${!var}
Returns variable names beginning with 'U'
echo ${!U*}
Returns the text from position 7
echo ${var:7}

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

Cut 'http://' from beginning of string
echo ${var#http://}
Cut '.hr/' from end of string
echo ${}
Replaces 'plecko' with 'eurekamoment' once
echo ${var/plecko/eurekamoment}
Replaces 'o' with 'O', always"
echo ${var//o/O}
If string starts with 'http', replace 'http' with 'Site'
echo ${var/#http/Site}
If string ends with 'st', replace 'st' with 'STING'
echo ${var2/%st/STING}
"" (double quotation marks) protect a string, but recognize $, \ and ` as specials
echo "$var"
'' (single quotes) protects a string, but recognizes $, \ and ` as specials
echo '$var'
$'…' (dollar sign before single quotes) protects a string completely, but interprets \n, \t, \a, etc.
echo $'$var\n'
... (between crases) Execute commands in a subshell, returning the result
echo `ls`
Desktop Documents Downloads Images Templates Music Public Videos
Execute commands on a subshell
Desktop Documents Downloads Images Templates Music Public Videos
Execute commands on a subshell, returning the result
echo $(ls)
Desktop Documents Downloads Images Templates Music Public Videos
Tests an arithmetic operation, returning 0 or 1
Returns the result of an arithmetic operation.
echo $((11-9))
Tests an expression, returning 0 or 1 (alias of command ‘test’)
[[ $var ]] && echo 'Its bigger'
Its bigger
Tests an expression, returning 0 or 1 (can use && and ** ** )
[[ $var ]] && echo 'Is there this variable'
Is there this variable
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
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 (
\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
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
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
To know all local variables, execute
#or, and open txt to see later
set > VariablesLocations.txt
Global Variables, to know all global variables, execute
To assign a value to a local variable
echo $LINUX
check if the variable created in the previous item appears in the list of local variables
set | grep LINUX
Now let’s make this local variable a global variable.
export LINUX
env | grep LINUX
Delete a ‘local environment variable’ from memory using unset command
unset LINUX
echo $LINUX
Create an alias (nickname for a command or program), you can still include it in your ~/.bashrc
alias list='ls -la color=auto'
Destroying an aliase
unalias list
Verify all commands entered
#run command by his number in history
#execute last command typed
#they stay in bash_history
cat ~/.bash_history
#clear history
history -c
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
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]}
Print element 1 of array DISTROSS
echo ${DISTROS[1]}
You can also create an array by inserting element one at a time.
Change element 2 of array DISTROSS
DISTROS[2]="Linux Mint"
echo ${DISTROS[2]}
Linux Mint
Display Element 2 Distro to End
echo ${DISTROS[@]:2} 
Ubuntu RedHat
This displays the elements starting at position 1 followed by two more elements consecutive to position 1
echo ${DISTROS[@]:1:2} 
Trisquel Ubuntu
Know how many elements the array has DISTROSS
echo ${#DISTROS[@]} 
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  • linux/shell_commands/var_and_arr.txt
  • Last modified: 2021-04-19 13:56
  • by tplecko