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45 Examples of Variables and Arrays in Shell Script


45 Examples of Variables and Arrays in Shell Script

1 - Create a variable, ${var} is the same as $var, but not ambiguous.

var="http://www.terminalroot.com.br/"
echo $var
http://www.terminalroot.com.br/

2 - Return string length

echo ${#var}
31

3 - Execute the contents of $var (same as ‘eval $$var’)

echo ${!var}

4 - Returns variable names beginning with ‘U’

echo ${!U*}
UID USER USERNAME

5 - Returns the text from position 7

echo ${var:7}
www.terminalroot.com.br/

6 - Returns 8 characters from position 11

echo ${var:11:8}
terminal

7 - Cut ‘http://’ from beginning of string

echo ${var#http://}
www.terminalroot.com.br/

8 - Cut ‘.com.br/’ from end of string

echo ${var%.com.br/}
http://www.terminalroot

9 - Replaces ‘terminalroot’ with ‘aurelio’ once

echo ${var/terminalroot/aurelio} 
http://www.aurelio.com.br/

10 - Replaces ‘o’ with ‘O’, always

echo ${var//o/O}
http://www.terminalrOOt.cOm.br/

11 - If string starts with ‘http’, replace ‘http’ with ‘Site’

echo ${var/#http/Site}
Site://www.terminalroot.com.br/

12 - If string ends with ‘st’, replace ‘st’ with ‘STING’

var2="test"
echo ${var2/%st/STING}
teSTING

13 - ”“ (double quotation marks) protect a string, but recognize $, \ and ` as specials

echo "$var"
http://www.terminalroot.com.br/

14 - ’‘ (single quotes) protects a string, but recognizes $, \ and ` as specials

echo '$var'
$var

15 - $’…‘ (dollar sign before single quotes) protects a string completely, but interprets \n, \t, \a, etc.

echo $'$var\n'
$var

16 - ... (between crases) Execute commands in a subshell, returning the result

echo `ls`
Desktop Documents Downloads Images Templates Music Public Videos

17 - Execute commands on a subshell

(ls)
Desktop Documents Downloads Images Templates Music Public Videos

18 - Execute commands on a subshell, returning the result

echo $(ls)
Desktop Documents Downloads Images Templates Music Public Videos

19 - Tests an arithmetic operation, returning 0 or 1

((11>9))

20 - Returns the result of an arithmetic operation.

echo $((11-9))
2

21 - Tests an expression, returning 0 or 1 (alias of command ‘test’)

[[ $var ]] && echo 'Its bigger'
Its bigger
22 - Tests an expression, returning 0 or 1 (can use && and **   ** )
[[ $var ]] && echo 'Is there this variable'
Is there this variable

23 - 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

25 - 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

27 - 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

28 - 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

29 - To know all local variables, execute

set

or, and open txt to see later

set > VariablesLocations.txt

30 - Global Variables, to know all global variables, execute

env

or

printenv

31 - To assign a value to a local variable

LINUX=free
echo $LINUX
free

32 - check if the variable created in the previous item appears in the list of local variables

set | grep LINUX
LINUX=free

33 - Now let’s make this local variable a global variable.

export LINUX
env | grep LINUX
LINUX=free

34 - Delete a ‘local environment variable’ from memory using unset command

unset LINUX
echo $LINUX

35 - Create an alias (nickname for a command or program), you can still include it in your ~/.bashrc

alias list='ls -la color=auto'

36 - Destroying an aliase

unalias list

37 - 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

38 - 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

39 - 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

40 - Print element 1 of array DISTROSS

echo ${DISTROS[1]}
Trisquel

41 - 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"

42 - Change element 2 of array DISTROSS

DISTROS[2]="Linux Mint"
echo ${DISTROS[2]}
Linux Mint

43 - Display Element 2 Distro to End

echo ${DISTROS[@]:2} 
Ubuntu RedHat

44 - This displays the elements starting at position 1 followed by two more elements consecutive to position 1

echo ${DISTROS[@]:1:2} 
Trisquel Ubuntu

45 - Know how many elements the array has DISTROSS

echo ${#DISTROS[@]} 
4

Thanks!



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