Loops

Bash has three loop forms — for, while, and until — and which one reads most naturally usually depends on whether you're working through a known list or watching a condition change.

for: looping over a list

bash fruits.sh

#!/bin/bash
for fruit in apple banana cherry; do
    echo "I like $fruit"
done

Terminal output

I like apple
I like banana
I like cherry

for: looping over files

A bare glob like *.txt in a for loop expands to every matching filename — the right way to process a set of files:

bash list-txt.sh

#!/bin/bash
for file in *.txt; do
    echo "Found: $file"
done

Terminal output

Found: notes.txt
Found: report.txt

Note: loop over *.txt directly, never over $(ls *.txt). Parsing ls's output breaks on filenames with spaces and adds nothing a glob doesn't already do — it's a well-known anti-pattern in shell scripting.

while and until

while keeps looping as long as its condition is true; until is the mirror image — it loops as long as its condition is false:

bash countdown.sh

#!/bin/bash
count=5
while [[ $count -gt 0 ]]; do
    echo "$count..."
    count=$((count - 1))
done
echo "Liftoff!"

Terminal output

5...
4...
3...
2...
1...
Liftoff!

break and continue

bash skip-and-stop.sh

#!/bin/bash
for n in 1 2 3 4 5 6; do
    if [[ $n -eq 3 ]]; then
        continue
    fi
    if [[ $n -eq 5 ]]; then
        break
    fi
    echo "n = $n"
done

Terminal output

n = 1
n = 2
n = 4

Caution: while true; do ... done loops forever unless something inside explicitly breaks out or the script is killed. Double-check every open-ended loop has a real exit path before you run it.

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