bash scripting

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

variables are not parsed in single quoted echo

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

variable naming:not starting with number and no special characters except _
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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
read
echo $REPLY
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passing arguments 
 # arguments are stored in $1 $2 ............
echo $1 $2 $3
$0  stores script name

hello.sh arg1 arg2 agr3 ... argn
$@ stores passed arguments as array
echo $@

echo #
#prints number of arguments passed

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 IF- Else- ELseif

syntax
if [ condition ]
then
statement
fi

count=2
if [$count -lt 3 ]
then
echo its less than 3
fi

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 ]
then
statement
elif [ condition ]
then
statement
else
statement
fi
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file test operator

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<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