Raspberry pi Projects

• Bash script loops

  Every programing language has loops and functions to make the program easy to make, today I will try to cover all the bash script loops so let get started.
  

  while loop

  the first loop it's the conditional while loop, "let" this command line to name a variable and it can be string or float or anything else, the "$x" it's present the variable value and the "-lt" means bigger than in bash script.
  the "do" and "done" function to keep the loop continues until complete the condition.
  

  #!/bin/bash
  let x=0 
  while [ $x -lt 10 ] 
  do  
  	sleep 1 
  	let x=$x+1 
  	echo $x 
  done
  

  
  The same loop but in a decreasing way, "-gt" means less than the x variable

  #!/bin/bash
  let x=10
  while [ $x -gt 0 ]
  do
  	sleep 1
  	let x-=1
  	echo $x
  done								 
  

  
  

  case loop

  the case loop it's for make options in the program, "read" it's command for input and put it in the variable I call it "ans" which is a sort to answer
  "case $ans in" its means compare the "ans" if the input equal "y" then print the message, "you say yes" and "n" the same, and if you enter the letter "q"
  then it will be out from the loop by increasing the variable "x" by breaking the while condition
  the "*)" its means anything else, "esac" to close the case loop.

  #!/bin/bash
  x=0
  while [ $x = 0 ]
  do
  	echo 'do like candies? (y/n)- Press "q" to quit'
  	read ans
  	case $ans in
  	y)
  	echo 'you said yes'
  	;;
  	n)
  	echo 'you said no'
  	;;
  	q)
  	echo 'Exit...'
  	x=1
  	sleep 1
  	;;
  	*)
  	clear
  	echo 'that not option'
  	;;
  	esac
  done						 
  

  
  

  select loop

  the select loop I use it to for limiting the options and it's work like the while but with out condition so I make the "quit" option to break the loop
  note: do not make spaces between the letters inside the options the program will not working example "hello world".
  

  #!/bin/bash
  echo 'what do you like?'
  select ans in 'Gold' 'Silver' 'Tin' 'quit'
  do
  	case $ans in
  	quit)
  	echo 'exiting...'
  	sleep 1
  	break
  	;;
  	esac
  	echo "you press:- $ans"
  done						 
  

  
  

  if loop

  the most usable loop in the bash script, as you see in the example below, you can name the variable without "let" command and the bash script will recognize it automatically if it was a string or float or integer.
  so this loop starts with "if" and between the arrows, we put the conditions, the "fi" it's command for ending of the if loop.
  

  #!/bin/bash
  a=0
  b=10
  c=10
  while [ $c -gt 0 ]
  do
  	let a=$a+1
          let b-=1
  	let c-=1
  	if [ $a == $b ];
  	then
  		echo " $a it's = $b"
  	fi
  
  	if [ $a != $b ];
  	then
  		echo " $a don't equal $b"
  	fi
  	sleep 1
  done			 
  

  
  

  if and else if

  this loop 'is exactly the same as if loop but with little modification, I add great than and less than and also else if.

  #!/bin/bash
  a=10;b=0;c=0
  while [ $c -lt 10 ]
  do
  	let a-=1;let b+=1;let c+=1
  	if [ $a -lt $b ];
  	then
  		echo "$a less than $b"
  	elif [ $a -gt $b ];
  	then
  		echo "$a greater then $b"
  	else
  		echo "$a it's equal $b"
  	fi
  	sleep 1
  done						 
  

  
  

  Equal-OR in the if loop

  the equal condition in the if loop writes like this "-eq" and the or write "-a", so as you see in the example below I make two conditions must be happening to break the loop.
  

  #!/bin/bash
  a=0;b=10
  while [ 1 ]
  do
  	if [ $a -eq 5 -a $b -eq 5 ];
  	then
  		echo "the end"
  		break
  	fi
  	let a+=1
  	let b-=1
  	echo "$a : $b"
  	sleep 0.5
  done 
  

  
  

  for loop

  the for loop it's mean the program jam in this loop until it finishes, at the beginning we name variable and input the values, the for loop use "do" "done" to do it.
  

  #!/bin/bash
  for var in 0 1 2 3 4 5 6 7 8 9 10
  do
  	echo $var
  	sleep 1
  done			 
  

  
  

  until loop

  the until loop it is exactly the same as while loop.
  

  #!/bin/bash
  a=0
  until [ ! $a -lt 10 ]
  do
  	echo $a
  	let a+=1
  	sleep 1
  done