Loops: for, while, until

for over a list

for f in *.txt; do
  echo "processing $f"
done

The list is anything: glob results, an explicit list, command substitution, an array.

for over a range

# C-style (bash/zsh)
for ((i=0; i<10; i++)); do echo $i; done
# brace expansion
for i in {1..10}; do echo $i; done
for i in {1..10..2}; do echo $i; done   # step

for over array

files=("a.txt" "b.txt" "c.txt")
for f in "${files[@]}"; do echo "$f"; done

"${arr[@]}" expands to one quoted token per element. "${arr[*]}" joins them into one string with IFS — almost always wrong.

while

i=0
while [[ $i -lt 5 ]]; do
  echo $i
  ((i++))
done

# Read a file line by line
while IFS= read -r line; do
  echo "> $line"
done < input.txt

IFS= read -r is the safe form — preserves leading/trailing whitespace and backslashes. Use it for any line-by-line file read.

until — the inverse of while

until ping -c1 server.local; do
  echo 'waiting...'
  sleep 5
done

break and continue

break exits the current loop. break 2 exits two nested levels. continue skips to the next iteration.