#!/bin/bash
# primes2.sh

#  Generating prime numbers the quick-and-easy way,
#+ without resorting to fancy algorithms.

CEILING=10000   # 1 to 10000
PRIME=0
E_NOTPRIME=

is_prime ()
{
  local factors
  factors=( $(factor $1) )  # Load output of `factor` into array.

if [ -z "${factors[2]}" ]
#  Third element of "factors" array:
#+ ${factors[2]} is 2nd factor of argument.
#  If it is blank, then there is no 2nd factor,
#+ and the argument is therefore prime.
then
  return $PRIME             # 0
else
  return $E_NOTPRIME        # null
fi
}

echo
for n in $(seq $CEILING)
do
  if is_prime $n
  then
    printf %5d $n
  fi   #    ^  Five positions per number suffices.
done   #       For a higher $CEILING, adjust upward, as necessary.

echo

exit