variable related thing added

Author: ArnobKumarSaha

basics/01-variable1.bash

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+#!/bin/bash
+
+# AUTHOR : Arnob kumar saha
+# Date created : 06-11-2021
+# Lase modified : 06-11-2021
+
+# DESCRIPTION : This script will show some basic stuffs we can do with variables.
+# USAGE : ./01-variable.bash
+
+
+# no need do declare like, x="hello" if it doesn't contain any special character
+x=hello
+y=world
+
+# $ is used to extract the value of a variable
+# Note : $var & ${var}  are almost same. second style used to remove ambiguity.
+# The difference between these two is, ${var} is more flexible & we can do some stuffs (has been shown in 03-variable2.bash) ,
+# which we could not do uisng $var style.
+: '
+    for example , 
+    v=foo
+    echo $vbar
+    # Prints nothing because there is no variable 'vbar'
+    echo ${v}bar
+    # prints 'foobar'
+'
+echo "$x $y"
+
+
+

basics/02-arithmatic.bash

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+#!/bin/bash
+
+# AUTHOR : Arnob kumar saha
+# Date created : 06-11-2021
+# Lase modified : 06-11-2021
+
+# DESCRIPTION : This script will show some way to evaluate arithmetic operations:  $(( )), let, expr
+# USAGE : 02-./arithmatic.bash
+
+
+# first way
+n1=5
+n2=8
+
+: '
+    $(( EXPRESSION )) style is used for evaluating some arithmatic expression
+    inside $(( )), using $ before the varibleName & not using $, is exactly same.
+'
+echo $(($n1+$n2))
+
+echo $((n1+n2))
+
+
+
+# second way
+: '
+    We can use "let <arithmetic expression>" to evalutae expression also
+'
+
+let "a = 5 + 4"
+let a++
+echo "a = $a"
+
+let "4 + 5" # does nothing
+
+
+
+# third way
+: '
+expr is similar to let except instead of saving the result to a variable it instead prints the answer. 
+Unlike let you dont need to enclose the expression in quotes. You also must have spaces between 
+the items of the expression. It is also common to use expr within command substitution to save the output to a variable.
+'
+expr 5 + 4
+expr "5 + 4"
+expr 5+4
+
+expr 5 \* 3  # * is special character, so We need to escape its special meaning.
+a=$( expr 10 - 3 )
+echo $a 

basics/03-variable2.bash

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+#!/bin/bash
+
+# AUTHOR : Arnob kumar saha
+# Date created : 06-11-2021
+# Lase modified : 06-11-2021
+
+# DESCRIPTION : This script will show some interesting stuffs we can do with variables.
+# USAGE : ./03-variable.bash
+
+
+
+# there is a special character space, so using double quote is needed.
+var="heLlo WoRld"
+
+echo ${var,}  # 1st letter to lowercase
+echo ${var,,} # all letters to lowercase
+echo ${var^}  # 1st letter to uppercase
+echo ${var^^} # all letters to uppercase
+echo ${#var} # length
+
+echo ${var::5} # first 5 characters
+echo ${var:4} # all excluding first 4 characters
+echo ${var: -3} # last 3 characters,  space before minus is required.
+echo ${var:2:5} # staring from 2nd letter, cut a srring of length 5.
+
+
+echo "-------------------------"
+
+: '
+    We have seen when to use ${var} style over $var.
+    Now we will see the difference between $var, "$var" & "${var}"
+    When using quoting "" , we tell the bash to remove the special meaning of all the special characters except $ and  `(backtick)
+
+'
+
+var="foo bar"
+touch $var # expands to touch foo bar. will create two files named foo & bar.
+touch "$var" # expands to touch "foo bar". will create a single file name "foo bar".
+
+: '
+    "${var}" is used for removing ambiguity of "$var" , just like ${var} does so for $var.
+'
+
+echo "$varxyz" # prints nothing
+echo "${var}xyz" # prints..  foo barxyz