shell-script.txt

		==>.tn.txt
#!/bin/bash

#Simple Script
#Author:

echo $1 >> ~/notes.txt

echo $* >> ~/notes.txt

echo $(date): $* >> ~/notes.txt

echo $(date): $note >> ~/notes.txt 
echo Note saved : $note
					cmd line execute= './tn added date to note'
........................................

Shebang
- First line of file.
- Starts with #!.
- specify which interpreter should run the code.
- specify options for interpreter.

Bash Scripts
- #!/bin/bash

- don't name your script "test","if" or "ls".

type command:
- type test

.............................................

:~Variables~:

- a="Ram"
- echo $a
- files="file1 file2"
- touch $files
- ls -l $files
	
-Variable name letter,number and underscore.
-First character should be _ or letter.
-Case sensitive

*man



# Ask for input
	read -p "your note: " note

-echo $date : $note >> ~/${topic}notes.txt
-echo Note saved: $note
////////////////////////////////////////////////////
#get the date
date = $(date)

#get the topic
topic=$1

#filename to write to
filename="${HOME}/${topic}notes.txt"   #~/${topic}notes.txt

#ask user to read
read -p "your note: " note

echo "$date: $note" >> "$filename"
echo "Note \'$note\' saved to $filename"

		

/////////////////////////////////////////////////////


Good Habits:
- use "$x" instead of $x.
- prevent surprises when it contains spaces
- use double quotes : keeping meaning intact.

Debugging: set -x(enable) .... set +x(disable)


................................................................

::~If,Then,Else~::

if mkdir a;then echo "ok";else echo "error"; fi

if testcode;then
	#code here gets executed
fi

if testcode;then
	#code here gets executed
else
	#some code
fi
////////////////////////////////////////////////////
#get the date
date = $(date)

#get the topic
topic=$1

#filename to write to
filename="${HOME}/${topic}notes.txt"   #~/${topic}notes.txt

#ask user to read
read -p "your note: " note


if [[ $note ]];then
	echo "$date: $note" >> "$filename"
	echo "Note \'$note\' saved to $filename"
else
	echo "Note wasn't saved"

fi
/////////////////////////////////////////////////////

return code or exit status
 - value returned by program upon exit
 - 0..255
 - 0 means success other are error codes

conditional expression:
 -tests on files and directories.
 -tests on strings.
 -arithmetic tests.

* [[ $str ]]  str is not empty
  [[ $str="something" ]] str equals to something
  [[ -e $filename ]]  file $filename exists
  [[ -d $dirname ]]  $dirname is a directory                   
 
Arithematic tests

[[arg1 op arg2]]

op:
 -> -eq,-ne,-lt,-gt
 -> $#,$?


if [[ $1 = "cat" ]];then
	echo "meow"
elif [[ $1 = "dog" ]];then
	echo "woof"
elif [[ $1 = "cow" ]];then
	echo "mooo"
else 
	echo "unknown animal"
fi

 ->  && for and || for or ! for not


..................................................................
Inputs & Output

- Printf:
  - more sophisticated output than echo.
  - printf "hello\n".
  - printf "hello %s, how are you\n"$USER
  - printf "p%st\n" a e i o u
  - printf "|%20s |%20s |%20s |\n" $(ls)
  - For help: help printf, man printf

- read:
   - reads input into a variable
   - "read x"
   - no variable specified? will use REPLY
   - -n or -N number of chararcters to read
   - -s will suppress output
   - -r disallows escape seq, line continuation
   - can splits into multiple varible
	-read x y
        -input "1 2 3"

   read;echo $REPLY
//////////////////////////
	#!/bin/bash
	echo -n "Are you sure (Y/N)? "
	answered=
	while [[ ! $answered ]]; do
		read -r -n 1 -s answer
		if [[ $answer =[Yy] ]];then
		answered="yes"
		elif [[ $answer =[Nn] ]];then
		answered="No"
		fi
	done
	printf "\n%s\n" $answered

Standard Stream
 - input,output,error
 - Represented by number(file descriptor) or special file.
 - 0: standard Input(stdin)
 - 1: standard Output(stdout)
 - 2: standard Error(stderr)

	Redirection
	- Input direction: <	
	- Output direction: >
	- pipes ls | grep x
	Redirection 2
	- specific stream with N>
		"cmd 2>dev/null" discards all errors
	- specific stream with >&N
	- sending both error and output to single file
		cmd > logfile 2>&1


::~Control Flow~::

-Loops
-Break and continue
-case
-compound stmts

	->while:
		while test;do                      
			code to repeat
		done
	->untill:
		untill test;do
			code to repeat
		done


target=$(($Random % 100))
guess=
untill [[$guess -eq $target ]];do
    read -p "Take a guess" guess
    if [[$guess -lt $target]];then
	echo "Higher!"
    elif [[$guess -gt $target]];then
	echo "Lower!"
    else
	echo "You found it!!"
    fi
done

exit 0

	->for
	 for var in words;do
		//code to repeat
	 done
	
		mvext::
			if [[ $# -ne 2]];then
				echo "you need exactly tow arguments"
			fi
			for i in *$1;do
				base=$(basename "$f" "$1")
				echo mv "$f" "#{base}$2"
			done

	touch {a..h}.txt
	

	->for ((INIT;TEST;UPDATE));do
		//loop code
	  done
	

	->Break
	->Continue

	->case WORD in
		PATTERN1)
		code for pat1;;
		PATTERN2)
		code for pat1;;
		.....
		PATTERNN)
		code for patn;;
	 esac
		example::
			case $1 in 
			 cat)
				echo "meow"
			   *)
				echo "unknown animal"
			esac

	Command Groups
	-> will group them in single stmt.
	-> can use I/O redirection for whole group.
	-> return status last command in group.

	|| and &&
	 
	[[ $# -ne 2 ]] && { echo "need exactly two arguments" >&2;}// replacement for IF

	

::~Advance Variables~::
    - integer variables	
	- declare -i num
		num = "5+4"
		echo $num #value 9
	- let n=100/2
	- ((p=x/100))
	- ((p=$(ls|wc -l)*10))

	declare -ir target = $(( ($RANDOM %100) + 1 ))
	declare -i guess=0
	
	untill ((guess == target));do
		read -p "Take a guess: " guess
		((guess)) || continue
		if((guess < target ));then
			echo "Higher!"
		elif((guess > target ));then
			echo "Lower!"
		else
			echo "You Found It!"
		fi
	done

	declare -r var//constant variable

	#exporting variables
	 - declare -x var="outer"
	 - export var

:_Arrays_:
	storing  value
	 -> x[0]="Ram"
	retrive value
	 -> ${x[0]}
	 -> ${x[@]} or ${x[*]}
	 count no. of element in array=${#array[@]}
	
	declare -A array
	arr=(this is array)
	declare -p arr
	declare -a arr='([0]="this" [1]="is" [2]="an" [3]="array")'


:~Special Variables~:
 - postional parameters
   - holds the nth command line argument $1,$2...
   - above 9 use braces : ${10}
 - $0 holds script name

 - $@ equivalent to $1,$2...,$N
 - $* equivalent to $1,$2...,$N
 - $# holds no. of arguments passed

Shift
 - Remove the first argument
 - All postional parameters shift
	- $2 -> $1
 
 - $# lowered by 1.

Getopts
 -parse argument lists
	-OPTARG,-OPTIND

	::~Shell Functions~::

  -Functions
   -declare
   -use
   -return 
   -export

   function name(){...}

   sum(){
	return $(( $1 + $2 ))
   }
   sum 4 5
   echo $?

  export function:: export -f fun   

  fun(){...}>&2 //function redirection

  	::-Fun With Strings-::
 
  ${var#pattern} #remove shortest possible match from begining.	
  ${var##pattern} #remove longest possible match from begining.
	
  ${var%pattern} #remove shortest possible match from ending.	
  ${var%%pattern} #remove longest possible match from ending.

  ${var/pattern/string} #search and replace

  Default value
  -> ${var:-value}
  -> ${var-value}
  
  -> ${var:=value}

  -> =~ does RE matching
  -> ? matches token 0 or 1 time.
  -> * matches token any no. of times.   
  -> ^ matches token start of string.
  -> $ matches token end of string.

   ::ENd of Options::	
	--

:--Many Ways to Run Your Script--:
 - bash myscript
 - bash -x myscript
 - background running with &:: myscript &
 - long-running script: nohup myscript &(keeping script running when you exit terminal)
 - nice myscript(run in low pro=iority)
 - nice nohup myscript &


=> tail -f logfile
 
 - At:exec script at specific time
 - cron: scheduling

 :~Customize Bash Behaviour~:

 -set: set -x print
	   -u
           -n
           -v
           -e .... and more

 -shopt: -s set,-u unset
	shopt -s nocaseglob
		 extglob
		 dotglob 
                 noclobber  
  
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