for Loops

A for loop runs commands once for each item in a list.

Syntax

for variable in item1 item2 item3; do
  commands
done

Looping over words, ranges, and files

for fruit in apple banana cherry; do
  echo "I like $fruit"
done

for i in {1..3}; do
  echo "Pass $i"
done

touch jan.csv feb.csv mar.csv
for file in *.csv; do
  echo "Found $file"
done

I like apple
I like banana
I like cherry
Pass 1
Pass 2
Pass 3
Found feb.csv
Found jan.csv
Found mar.csv

Looping over a wildcard is the safe way to process files — each file name arrives intact, even with spaces, as long as you quote "$file" when you use it.

A practical example: batch rename

touch "IMG 001.jpg" "IMG 002.jpg" "IMG 003.jpg"
for file in IMG*.jpg; do
  new="holiday-${file#IMG }"
  mv "$file" "$new"
done
ls

holiday-001.jpg
holiday-002.jpg
holiday-003.jpg

${file#IMG } removes IMG from the start of the name — one of Bash's string operations, covered in Level 2.

C-style loop

for (( i = 10; i > 0; i -= 3 )); do
  echo "$i"
done

10
7
4
1

Common mistakes

• Looping over $(ls) or $(find ...) — names with spaces get split into pieces. Loop over a wildcard, or use the find/while read patterns in Level 2.
• Forgetting to quote "$file" inside the loop.

Practice

Print the 5 times table from 5 x 1 = 5 to 5 x 10 = 50.

for n in {1..10}; do
  echo "5 x $n = $(( 5 * n ))"
done

5 x 1 = 5
5 x 2 = 10
5 x 3 = 15
5 x 4 = 20
5 x 5 = 25
5 x 6 = 30
5 x 7 = 35
5 x 8 = 40
5 x 9 = 45
5 x 10 = 50