FOR AND WHILE LOOP IN BASH WITH EXAMPLE

OPIC: For and while LOOP in BASH
FOR LOOP: In computer science, a for-loop or for loop is a control flow statement for specifying iteration. Specifically, a for loop functions by running a section of code repeatedly until a certain condition has been satisfied. For-loops have two parts: a header and a body.

WHILE LOOP: In most computer programming languages, a while loop is a control flow statement that allows code to be executed repeatedly based on a given Boolean condition. The while loop can be thought of as a repeating if statement.

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# for loop syntax

for VARIABLE in 1 2 3 4 5 .. N
do
    command1
    command2
    commandN
done

or

for VARIABLE in file1 file2 file3
do
    command1 on $VARIABLE
    command2
    commandN
done

or

for OUTPUT in $(Linux-Or-Unix-Command-Here)
do
    command1 on $OUTPUT
    command2 on $OUTPUT
    commandN
done

# Examples


#!/bin/bash
#This type of for loop is characterized by counting
for i in 1 2 3 4 5
do
   echo "Welcome $i times"


done


#!/bin/bash
#Sometimes you may need to set a step value (allowing one to count by two’s or to count backwards for instance).
for i in {1..5}
do
   echo "Welcome $i times"
done


#!/bin/bash
#has inbuilt support for setting up a step value using {START..END..INCREMENT} syntax
echo "Bash version ${BASH_VERSION}..."
for i in {0..10..2}
do
  echo "Welcome $i times"
done


#!/bin/bash
#The seq command can be used as follows. A representative example in seq is as follows
for i in $(seq 1 2 20)
do
   echo "Welcome $i times"
done


#!/bin/bash
#A representative three-expression example in bash as follows
# set counter 'c' to 1 and condition
# c is less than or equal to 5
for (( c=1; c<=5; c++ ))
do
   echo "Welcome $c times"
done


#!/bin/bash
## $@ expands to the positional parameters, starting from one.  ##
#./script one foo bar two three
for i in $@
do
    echo "Script arg is $i"
done


#!/bin/bash
#find Png time
# Purpose: Ping all of the IP addresses associated with my domain name www.cyberciti.biz
ips="$(host -t a www.cyberciti.biz | awk '{ print $4}')"
for i in $ips; do ping -q -c 4 "$i"; done


# WHILE LOOP


The syntax is as follows:

while [ condition ]
do
   command1
   command2
   command3
done


OR

while [ condition ]; do commands; done
while control-command; do COMMANDS; done


# EXAMPLE


#!/bin/bash
#loop will print welcome 5 times on screen
x=1
while [ $x -le 5 ]
do
  echo "Welcome $x times"
  x=$(( $x + 1 ))
done


#!/bin/bash
# one liner for the above script
x=1; while  [ $x -le 5 ]; do echo "Welcome $x times" $(( x++ )); done



#!/bin/bash
# Here is a sample shell code to calculate factorial using while loop
counter=$1
factorial=1
# check if bash variable $counter set or not
# see https://www.cyberciti.biz/faq/see-check-if-bash-variable-defined-in-script-on-linux-unix-macos/
if [  -v $counter ]
then
    echo "Syntax: $0 <number>"
else
    while [ $counter -gt 0 ]
    do
        factorial=$(( factorial * counter ))
        counter=$(( counter - 1 ))
    done
    echo "$factorial"
fi



#!/bin/bash
# While loops are frequently used for reading data line by line from file
FILE="$1"
#
# Check for input file else die with an error/syntax
# message
if [ -v $FILE ]
then
    echo "Syntax: $0 <filename>"
    exit 1
else
    # read $FILE using the file descriptors (FDs)
    exec 3<&0
    exec 0<"$FILE"
    while read -r line
    do
	    # use $line variable to process line
    	echo "$line"
    done
    exec 0<&3
fi



#!/bin/bash
# Infinite for while can be created with empty expressions
while :
do
	echo "infinite loops [ hit CTRL+C to stop]"
done



#!/bin/bash
# In this example, the break statement will skip the while loop when user enters -1, otherwise it will keep adding two numbers:
while :
do
	read -p "Enter two numnbers ( - 1 to quit ) : " a b
	if [ $a -eq -1 ]
	then
		break
	fi
	ans=$(( a + b ))
	echo "$ans"
done



# UNTIL loop syntax

until [CONDITION]
do
  [COMMANDS]
done


#!/bin/bash
# In the example below, on each iteration the loop prints the current value of the variable counter and increments the variable by one.
counter=0
until [ $counter -gt 5 ]
do
  echo Counter: $counter
  ((counter++))
done


#!/bin/bash
# The following script may be useful when your git host has downtime
until git pull &> /dev/null
do
    echo "Waiting for the git host ..."
    sleep 1
done

echo -e "\nThe git repository is pulled."



#!/bin/bash
# Bash Until Loop example with a single condition

i=1
until [ $i -gt 10 ]
do
echo $i
((i++))
done



#!/bin/bash
# Bash Until Loop example with multiple conditions

max=5
a=1
b=0

until [[ $a -gt $max || $b -gt $max ]];
do
echo "a = $a & b = $b."
((a++))
((b++))
done


#!/bin/bash
# You can create an infinite loop using a false statement as an expression.
count=0
until false
do
	echo "Counter = $count"
	((count++))
	sleep 2
done



#!/bin/bash
# one liner for above
until false; do echo "Counter = $count"; ((count++)); sleep 2; done


#!/bin/bash
# Using the same infinite loop example. Here when the count is equal to five continue
count=0
until false
do
  ((count++))
  if [[ $count -eq 5 ]]
  then
    continue
  elif [[ $count -ge 10 ]]
  then
    break
  fi
  echo "Counter = $count"
done



#!/bin/bash
# Create a Bash script and paste the following lines
#!/bin/bash
until (( num > 8 ))
do
    # Generate a random number between 1 and 10
    num=$(( (RANDOM % 10) + 1 ))
    echo "Generated number: $num"
done
echo "Loop finished!"



#!/bin/bash
# until Loop with Multiple Conditions
#!/bin/bash
count=1
until (( count > 10 ))
do
    if (( count % 2 == 0 ))
    then
        echo "Even number: $count"
    else
        echo "Odd number: $count"
    fi
    if (( count == 5 ))
    then
        echo "Reached 5! Continuing to next iteration..."
        (( count++ ))
        continue
    fi
    if (( count == 8 ))
    then
        echo "Reached 8! Exiting the loop..."
        break
    fi
    (( count++ ))
done
echo "Loop finished!"