Generating the Fibonacci Sequence in One Line of Bash

John M. HigginsFeb. 16, 2024

During the later half of my undergrad I spent a lot of time doodling with bash scripts when I got bored in class. One day I challenged myself to write an algorithm that generates the fibonacci sequence using only a single line of bash.

Here's the whole thing without any formatting.

printf "1\n1\n" | tee -a fib_out && seq $(expr $1 - 2) | xargs -n 1 sh -c 'echo $(expr $(tail -n 1 fib_out) + $(tail -n 2 fib_out | head -n 1)) | tee -a fib_out' && rm fib_out

This is a lot to take in all in one line, so let's break it down piece by piece. Here's what it looks like when you display every command between the `&&` operators on their own separate lines.

printf "1\n1\n" | tee -a fib_out && seq $(expr $1 - 2) | xargs -n 1 \
    sh -c 'echo $(expr $(tail -n 1 fib_out) + $(tail -n 2 fib_out | head -n 1)) \
    | tee -a fib_out' && rm fib_out

Here's the first segment of the command before the first && operator.

printf "1\n1\n" | tee -a fib_out

This writes two ones separated by a newline to the beginning of the fib_out file. After this we are setup to calculate the next Fibonacci numbers because we have the first two defined.

seq $(expr $1 - 2)

The expr command allows us to perform mathematical operations on the command line. This allows us to invoke the seq command (which generates numbers from 1 to N) with a calculated N value. We subtract 2 from N because we have already calculated the first two Fibonacci numbers with our printf invocation.

After we have setup the fib_out file with the first two numbers we xargs command to calculate all further numbers.

xargs -n 1 sh -c '...'

The xargs command will run the specified command on every line read from stdin until an EOT or EOF is encountered. We use the -n flag here to specify that only one line will be read by each process. So what's in the specified command after the -n flag? Here's the command we use to parse two numbers from the file and calculate the next Fibonacci number:

echo $(expr $(tail -n 1 fib_out) + $(tail -n 2 fib_out | head -n 1)) | tee -a fib_out

We use the expr command again to add the last two numbers in the file together. The last number is retrieved by using tail to print the last n lines of the file. When we invoke tail -n 1 we are specifying that we only need to see the very last line of the file. To get the second to last line of the file, we can combine head and tail. The head command serves as the inverse of the tail command. Instead of printing the last n lines, it prints the first n lines of the specified file. The expression on the right hand side of the operator is created by grabbing the last two lines of the fib_out file and then using the head command to read the first line from STDIN. Once we have calculated the sum of the last two Fibonacci numbers, we can append the result to the fib_out file using the tee operator.

Once we have finished writing all the numbers to the console, we remove the fib_out script with the rm command and exit the program.