A recursive function is a function which calls itself more times. It can be useful for example when it's necessary to repeat many times the same operation on data. Here there's an example, based on the Euclid's algorithm for computing the greatest common divisor between two numbers M and N. The algorithm has this structure:

`if [ N = 0 ] `

M is the GCD

else

compute the remainder R of M/N

if [ R = 0 ]

GCD = R

else

exchange M and N

repeat from beginning

fi

fi

and it can be implemented with this script:

`#!/bin/bash`

# Exit variables

EXIT_SUCCESS=0

EXIT_FAILURE=1

usage()

{

echo

echo "Function usage:"

echo "$(basename $0) <first number> <second number>"

echo

exit $EXIT_FAILURE

}

mcd ()

{

# It sets as new m and n the values passed by the last call

m=$1

n=$2

# It verifies that m and n are valid, or that

# the end of the algorithm was not reached

if [ $n -eq 0 -a $m -eq 0 ]; then

echo "gcd(0,0) is not defined!"

exit $EXIT_FAILURE

elif [ $m -eq 0 ]; then

return $n

elif [ $n -eq 0 ]; then

return $m

fi

# Recursive call of the function

# The first parameter is the residue of $n / $m,

# The second parameter is $m

# the third parameter is $n

# Note how $m and $n are continuously inverted

mcd $(( $n % $m )) $m

}

################

# Main Program #

################

# Security control: if the number of passed values

# is not 2, it shows the usage of this script

if [ $# -ne 2 ]; then

usage

fi

# It calls the function mcs, and it passes it as parameters

# the values written at the command line when the script

# was launched

mcd $1 $2

# It prints the results

MCD=$?

echo "The gcd of \"$1\" and \"$2\" is: $MCD"

exit $EXIT_SUCCESS

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