Let's take Input from user as 9. So the Fibonacci series upto 9 is as follow's 0,1,1,2,3,5,8,13,21

Expected Output : 0,*,1,*,3,*,8,*,21

Below is the following code which i work around to implement logic


echo "The Fibonacci series is  : "

for (( i=0; i<UserInput; i++ )) 
    if [ $i -eq 2 ]
        echo -n "$a "
        sn=$((a + b)) 
  • Pleased share the shell script you have developed so far and indicate the point you are stuck.
    – FedKad
    Nov 14, 2020 at 7:44
  • @FedonKadifeli code is shared have a look at it Nov 14, 2020 at 8:11
  • Where you say every consecutive number. Did you mean every other field? You example does not show the *s on the consecutive numbers. Nov 18, 2020 at 11:12
  • @ctrl-alt-delor every consecutive means every 2nd number of series should be mask with special character '*' , please see the Expected output : 0,*,1,*,3,*,8,*,21 Nov 18, 2020 at 13:37
  • I did see the example, that is why I asked. Nov 18, 2020 at 14:48

2 Answers 2


Mandatory awk-based solution (where for simplicity I will assume n>2):

awk -v n=9 'BEGIN{q=1; printf "0,*,"; for (i=2;i<n;i++) {s=q+r;r=q;q=s; printf "%s%s",i%2?"*":s,i==n-1?"\n":","}}'

The user input is stored in the variable n and passed to awk via the command-line argument -v n=number.

In order to exit immediately if n<3:

awk -v n=9 'BEGIN{if (n<3) exit; q=1; printf "0,*,"; for (i=2;i<n;i++) {s=q+r;r=q;q=s; printf "%s%s",i%2?"*":s,i==n-1?"\n":","}}'


awk is a text-processing tool, we are "creatively misusing" it. Therefore, everything happens inside the BEGIN block which usually contains code executed before the first input file is processed.

The syntax itself is very C-like, so we

  • print the first two terms of the series (which are fixed since we assume n>2): printf "0,*,"
  • loop from 2 to n-1 and calculate the Fibonaccy number s as sum of the two previous ones q and r, and update q and r
  • print either * if i is even, or the current Fibonacci number s if it is not (i%2?"*":s)
  • and print either a , or a newline after that, depending on whether we have reached the end of the loop (i==n-1) or not.
  • i seen most of the answers from you is in awk is their any special about awk. May be i required to brush up with awk basic and internally working .. i need to always ask you for detail explanation Nov 17, 2020 at 17:30

The closest to your code, using Bash and arithmetic operators is this:

let UserInput=9 
let a=0 
let b=1
echo -n "The Fibonacci series is: 0"
for (( i=1; i<UserInput; i++ )) ; do
  let sn=a+b
  let a=b
  let b=sn
  if (( i % 2 == 0 )) ; then 
    echo -n ",$a"
    echo -n ",*"

In the if statement you have to use the % modulo (remainder) operator. Using it you can test whether i is divisible to two (i.e. an even number) and print the value of $a or print a * accordingly.

The output will be:

The Fibonacci series is: 0,*,1,*,3,*,8,*,21

To change the script, so that it gets the number of steps from its argument, please modify the first line of the script like this:

let UserInput="$1"
  • Thank you for the solution , I was trying to store it into array and replace with sed command Nov 14, 2020 at 11:02

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