Loops

for runs a block once for each word in a list. The list can be written out, come from a wildcard or come from a command.

for duck in mallard teal eider; do
  echo "Hello, $duck"
done

Looping over files with a wildcard is safe even when names have spaces, as long as you quote the variable:

for f in *.jpg; do
  cp "$f" "backup/$f"
done

For numbers, {1..5} gives 1 2 3 4 5, so for i in {1..5} counts. Bash also has the C-style form: for ((i = 0; i < 5; i++)).

while repeats as long as a command succeeds. The usual way to handle a file one line at a time is while read:

while read -r line; do
  echo "Got: $line"
done < names.txt

break leaves a loop early and continue skips to the next round. Don't loop over $(ls) or $(cat file): they split on spaces, so a file called my notes.txt becomes two words. Use a wildcard for files and while read for lines.