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bash scripting

#! /bin/bash
#this is a comment
echo "hello world"
print to screen : echo
echo 'hey'
echo hello
echo "hey "

variables are not parsed in single quoted echo

system variables  : env, printenv , set command
echo $HOME
echo $BASH
echo $PWD
echo $PATH
user defined variable : varname=valueofvar
echo my name is $name

variable naming:not starting with number and no special characters except _

reading user input: read
echo enter name
read name
echo hello $name
echo enter age and sex
read name1 name2 name3
echo  $name1 $name2 $name3

read -p 'what is your user name: ' u_name #input with prompt
read -sp 'password: ' pass #silent/hidden input

reading array
read -a names
echo ${names[0]} ${names[1] }

default variable name : REPLY
echo $REPLY
passing arguments 
 # arguments are stored in $1 $2 ............
echo $1 $2 $3
$0  stores script name arg1 arg2 agr3 ... argn
[email protected] stores passed arguments as array
echo [email protected]

echo #
#prints number of arguments passed

 IF- Else- ELseif

if [ condition ]

if [$count -lt 3 ]
echo its less than 3

integer comparision

-eq =equal to
-ne =not equal to
-gt =greater than
-ge = greater tahn equal to
-lt = less than
-le = less than equal to

to use ==,!=,<,<=,>,>= we need double (()) parenthesis

example: if (($var<10 p="">
string comparision
= :equal to [ "$a" = "$b" ]
== :equal to [ "$a" == "$b" ]
!= :not equal [ "$a " !=  "$b" ]
<:less a="" alphabetical="" ascii="" b="" br="" in="" nbsp="" order="" than="">
>:greater than in ascii order  [["$a" > "$b"]]
-z :is null string

# using angle brackets in string needs double [[]] parenthesis

else and elseif condition 

if [ conditiion ]
elif [ condition ]
<10 p="">
<10 p="">
<10 p="">
<10 p="">
<10 p="">
<10 p="">
<10 p="">
<10 p="">
file test operator

<10 p="">
<10 p="">
<10 p="">
<10 p="">
echo -e "enter the name of file: \c"
<10 p="">read file_name
<10 p="">if [ -e $file_name ] 
<10 p="">then 
<10 p="">echo "file found"
<10 p="">else 
<10 p="">echo "file not found"
<10 p="">fi

<10 p="">-e enables escape sequence interpretation   
<10 p="">\c keep cursor in same line


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