@cassidoo's interview question from September 25, 2022

ordinalize.sh

#!/usr/bin/env bash
set -eu -o pipefail

# initialize vars
ordinal=''

# check input
ARGC="$#" 
if [[ "$ARGC" -ne 1 ]]; then
	echo "Please enter an integer number, e.g. 123"
	exit 1
fi
NUM="$1"
re='^[0-9]+$'
if ! [[ $NUM =~ $re ]] ; then
   echo "error: Not a number" >&2; exit 1
fi

# get ABS(NUM)
NUM=${NUM#-}

# get NUM mod 100
## expr returns 1 if NUM ends in zero, but mod_num is correct, so we trap the error
mod_num=$( expr $NUM % 100 ) || :

# is NUM in the tweens set?
case $mod_num in
	( '11' | '12'| '13' ) ordinal='th';;
	('*') : ;;
esac

# get NUM mod 10
## expr returns 1 if NUM ends in zero, but mod_num is correct, so we trap the error
mod_num=$( expr $NUM % 10 ) || :

# select appropriate suffix
if [ ! "$ordinal" ]; then
	case $mod_num in
		( '0' | '4' | '5' | '6' | '7' | '8' |'9' ) ordinal='th';;
		('1') ordinal='st';;
		('2') ordinal='nd';;
		('3') ordinal='rd';;
		('*') ordinal='error $NUM could not be ordinalized';;
	esac
fi
	
if [[ $ordinal == *"error"* ]]; then
	echo $ordinal
	exit 1
fi

echo "$NUM's ordinal is $NUM$ordinal"

MONTASIO:scripts andrewrich$ for N in {1..30}; do ./ordinalize.sh $N; done
1's ordinal is 1st
2's ordinal is 2nd
3's ordinal is 3rd
4's ordinal is 4th
5's ordinal is 5th
6's ordinal is 6th
7's ordinal is 7th
8's ordinal is 8th
9's ordinal is 9th
10's ordinal is 10th
11's ordinal is 11th
12's ordinal is 12th
13's ordinal is 13th
14's ordinal is 14th
15's ordinal is 15th
16's ordinal is 16th
17's ordinal is 17th
18's ordinal is 18th
19's ordinal is 19th
20's ordinal is 20th
21's ordinal is 21st
22's ordinal is 22nd
23's ordinal is 23rd
24's ordinal is 24th
25's ordinal is 25th
26's ordinal is 26th
27's ordinal is 27th
28's ordinal is 28th
29's ordinal is 29th
30's ordinal is 30th

MONTASIO:scripts andrewrich$ ./ordinalize.sh abc
error: Not a number