for 

Indexed for loop
"Borrowed" from the c-shell.


for (( initial condition[s]; test condition[s]; modifier ))
do
  action list
done


For indexed for loop, bash allows the use of braces {}
  to encapsulate the action body instead of the do and done
  keywords.


for (( initial condition[s]; limiting condition[s]; modifiers ))
{
  action list
}



for - keyword.

((  )) - open and closed double parentheses encapsulate the
parameters for initialization, loop continuation condition, and adjustments.

initial condition[s] - list statement[s] that usually initialize variables.

Run once at start of loop.

assign initial values to variables. 
  i=45

Use comma between assignments to do multiple assignments.
 i=0, j=10 

Use command substitution to generate initial value on fly.
   i=$( ls | wc -l )
# gets the count of files listed in current directory

Assigned values and/or substituted commands do not have to be used in the 
for-loop control section.

; - semi-colon terminates initial action list.

A initial value list with no values, just the semi-colon is valid.

Variable can be initialed before the loop.

Variables changed by initialization, the adjustment statement, or actions
in the body of the for will be kept when loop completes.


limiting condition[s] - conditional test statement evaluated to continue.

for tests the return code of the expression.

test condition[s] evaluated as soon as the initialization statement completes.
  So it is possible that a loop doesn't run at all.

test condition doesn't have to work with initialized variables.

Useful test expressions.

   == !=  equality and inequality
   <= >= < >   comparison
   &&     logical AND
   ||     logical OR

test conditions can be combined using && and || for compound test condition.

   $num1 < 7 && $num2 > 44

test condition can be blank as long as the trailing semi-colon exists.

It is possible to perform command substitution in the test condition.
But, it is the output of the command that is used as the test condtion.

If file testcond contains the following line :

  i < 7

for (( i=1; $( cat testcond ); i++ ))
do
  echo $i
done


will print the values 1 through 6

# This is tricky, so use with caution.



modifier - arithmetic expression performed after each pass through 
action block.

  Run after test condition and for-loop action block executed.

   #The final echo will print out 2
   for (( num=1; num<2; num++ )); {   echo $num; }  
   echo $num


  Usually some adjustment applied to modify value of variable being tested.

  Available mathematical operators
  

  = : set value
  *= : multiply by specified value    i*=2 
  /= : divide by specified value     j/=3
  %= : mod with specified value, place remainder in variable  i%=2 
  += : increment by specified value   i+=2
  ++ : increment by 1
  -= : decrement by specified value
  -- : decrement by 1
  <<= : bit shift left by value   
  >>= : bit shift right by 
  &=  : bit and 
  ^=  : bit xor
  |=  : bit or 
  
 
  
Action[s] consist of mathematical statements.

Does not run until after 1st pass through loop body.

Multiple comma delimited adjustments can be listed.

Modifier section can be left blank.

for (( sum=0; sum < 1000; ))
do
   read -p "Number : " val
   sum=$( expr $sum + $val )
done

Arithmetic adjustments don't have to be applied to variable being tested.

j=20

for (( i=1; i <= 10; i++,j-- ))
do 
  echo $i $j
done



#!/bin/bash

# Get a random number from 0-99
target=$( expr $RANDOM % 100 )
# get first guess
read -p "Guess : " guess

# set guess count to 1
# test guess against target
#  if mis-guess then test for too high or low
# increment guess count if missed.
for (( i = 1; "$guess" != "$target"; i++ ))
do
  if [ "$guess" -gt "$target" ]
  then
     echo "too high"
  else
     echo "too low"
  fi

  read -p "Guess : " guess
done

# show target and number of guesses.
echo "$target in $i guesses"