loop.sh

#!/usr/bin/env bash

array=(9 8 7)
size=${#array[@]}

# While loop.
j=0
while [ $j -lt "${size}" ]; do
  printf "%s " "${array[j]}" # Prints 9 8 7
  ((j++))
done
echo

# Until loop.
j=0
until [ $j -eq "${size}" ]; do
  printf "%s " "${array[j]}" # Prints 9 8 7
  ((j++))
done
echo

# For loop.
for ((i = 0; i < size; i++)); do
  printf "%s " "${array[i]}" # Prints 9 8 7
done
echo

# For loop with range.
# The values in the range must be literal. {0..$size-1} won't work.
for i in {0..2}; do
  printf "%s " "${array[i]}" # Prints 9 8 7
done
echo

# For-in loop.
for n in "${array[@]}"; do
  printf "%s " "${n}" # Prints 9 8 7
done
echo

# TIP: Assign an array to for-in.
# Don't use comma, just separate the values with a space.
for char in h e l l o; do
  printf "%s " "${char}" # Prints h e l l o
done
echo

# TIP: Iterate over files in a directory.
j=1
for file in /var/log/*.log; do
  echo "$((j++)): ${file}"
done