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Recursive bash function

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While teaching a class on the Linux Command-Line (CLI), the book gave an example of generating a list of random US telephone numbers into a file. The book uses the RANDOM function to generate segments of the telephone number, and then the grep command to identify malformed telephone numbers.

My students wanted me to explain why the numbers were malformed. I had to explain that the RANDOM function returns a random number between 1 and 99,999. The RANDOM function may return a 1 to 5 digit random number, which means you may get a 1-digit or 2-digit number when you request a 3-digit random number or a 1- to 3-digit number when you request a 4-digit random number.

The author’s example is:

for i in {1..10}; do
  echo "(${RANDOM:0:3}) ${RANDOM:0:3}-${RANDOM:0:4}" >> list.txt
done

They asked if there was a way to write a shell script that guaranteed random but well-formed US telephone numbers. I said yes, however, you need to write a recursive bash shell function and assign the result to a global variable set in the shell script.

They seemed doubtful, so I wrote it for them. Here’s the script if you’re interested in learning more about bash shell scripting. While I implemented it with an bash array, that’s optional.

#!/usr/bin/bash
 
# ============================================================
#  Name:   telephone.sh
#  Author: Michael McLaughlin
#  Date:   05-May-2020
# ------------------------------------------------------------
#  Purpose: Demonstrate how to generate random telehpone
#           numbers. The RANDOM function returns a random
#           number between 1 and 99999; and while you can
#           easily shave off a extra digit guarnteeing a
#           value above 100 is impossible without logic.
# ============================================================
 
targetLength()
{
  # Declare variable in function-level scope.
  randomString=''
 
  # Check the number of parameters to process.
  if [[ ${#} = 2 ]]; then
    # Assign value to function-level and local variables.
    randomString=${1}
    formatLength=${2}
 
    # Get the length of the telephone number as integer.
    length=`echo -n ${randomString} | wc -c`
 
    # Calculate any shortfall.
    short=$((${formatLength}-${length}))
 
    # Check if the telephone number is too short.
    if [[ ${short} > 0 ]]; then
      randomString=`echo "${randomString}${RANDOM:0:${short}}"`
    fi
  fi
 
  # Check if the combination of random numbers equals the target length
  # and assign the value to the global variable, or repeat processing
  # by making a recursive function call.
  if [[ `echo -n ${randomString} | wc -c` = ${formatLength} ]]; then
    result=${randomString}
  else
    targetLength ${randomString} ${formatLength}
  fi
}
 
# Declare global variable to support targetLength().
result=''
 
# Declare an array of strings.
declare -A telephone_parts
 
# Generate one hundred random telephone numbers.
for i in {1..100}; do
  # Create random three digit area code.
  targetLength ${RANDOM:0:3} 3
  telephone_parts[1]=${result}
 
  # Create random three digit prefix code.
  targetLength ${RANDOM:0:3} 3
  telephone_parts[2]=${result}
 
  # Create random four digit number code.
  targetLength ${RANDOM:0:4} 4
  telephone_parts[3]=${result}
 
  # Print the telephone numbers.
  echo "[${i}] (${telephone_parts[1]}) ${telephone_parts[2]}-${telephone_parts[3]}"
done

For reference, a recursive function call isn’t required here. It could be done more effectively with the following while loop:

targetLength()
{
  # Declare variable in function-level scope.
  randomString=''
  short=1
 
  # Check the number of parameters to process.
  if [[ ${#} = 2 ]]; then
    # Assign value to function-level and local variables.
    randomString=${1}
    formatLength=${2}
 
    # Check if the telephone number is too short.
    while [[ ${short} > 0 ]]; do
      # Get the length of the telephone number as integer.
      length=`echo -n ${randomString} | wc -c`
 
      # Calculate any shortfall.
      short=$((${formatLength}-${length}))
 
      # Assign new value to randomString.
      randomString=`echo "${randomString}${RANDOM:0:${short}}"`
    done
 
    # Assign randomString to global result variable.
    result=${randomString}
  fi
}

As always, I hope this helps those you want to learn or solve a problem.

Written by maclochlainn

July 1st, 2020 at 11:55 pm

Posted in bash,basic programming,Fedora,Linux,Unix

Tagged with , ,

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