linux:shell_commands:var_and_arr

Examples of Variables and Arrays in Bash

Create a variable, ${var} is the same as $var, but not ambiguous.
var="http://wiki.plecko.hr/"
echo $var
http://wiki.plecko.hr/
Return string length
echo ${#var}
31
Execute the contents of $var (same as 'eval $$var')
echo ${!var}
Returns variable names beginning with 'U'
echo ${!U*}
UID USER USERNAME
Returns the text from position 7
echo ${var:7}
wiki.plecko.hr/

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

Cut 'http://' from beginning of string
echo ${var#http://}
wiki.plecko.hr//
Cut '.hr/' from end of string
echo ${var%.hr/}
http://wiki.plecko
Replaces 'plecko' with 'eurekamoment' once
echo ${var/plecko/eurekamoment} 
http://wiki.eurekamoment.hr/
Replaces 'o' with 'O', always"
echo ${var//o/O}
http://wiki.pleckO.hr/
If string starts with 'http', replace 'http' with 'Site'
echo ${var/#http/Site}
Site://wiki.plecko.hr/
If string ends with 'st', replace 'st' with 'STING'
var2="test"
echo ${var2/%st/STING}
teSTING
"" (double quotation marks) protect a string, but recognize $, \ and ` as specials
echo "$var"
http://wiki.plecko.hr/
'' (single quotes) protects a string, but recognizes $, \ and ` as specials
echo '$var'
$var
$'…' (dollar sign before single quotes) protects a string completely, but interprets \n, \t, \a, etc.
echo $'$var\n'
$var
... (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
(ls)
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
((11>9))
Returns the result of an arithmetic operation.
echo $((11-9))
2
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 (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
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
set
#or, and open txt to see later
set > VariablesLocations.txt
Global Variables, to know all global variables, execute
env
#or
printenv
To assign a value to a local variable
LINUX=free
echo $LINUX
free
check if the variable created in the previous item appears in the list of local variables
set | grep LINUX
LINUX=free
Now let’s make this local variable a global variable.
export LINUX
env | grep LINUX
LINUX=free
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
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
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]}
echo $DISTROS
Print element 1 of array DISTROSS
echo ${DISTROS[1]}
Trisquel
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"
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