Bash Loops: for, while and until

Loops repeat commands. They are the main reason scripts save so much time: one loop can process a thousand files.

for Loop over a List

Example

bash

for city in Karachi Lahore Quetta Peshawar; do
    echo "Branch in $city"
done

Output

Branch in Karachi
Branch in Lahore
Branch in Quetta
Branch in Peshawar

The code between do and done runs once for each item.

for Loop over a Range

{1..5} makes the numbers 1 to 5. A third number sets the step.

Example

bash

for i in {1..5}; do
    echo -n "$i "
done
echo

for i in {0..20..5}; do
    echo -n "$i "
done
echo

for ((i = 3; i > 0; i--)); do
    echo "$i..."
done
echo "Go!"

Output

1 2 3 4 5 
0 5 10 15 20 
3...
2...
1...
Go!

echo -n prints without a new line. The (( )) form works like the for loop in C or Java.

Looping over Files

Combine for with a wildcard to work on many files at once:

Example

bash

mkdir -p photos && cd photos
touch img1.jpg img2.jpg notes.txt

for file in *.jpg; do
    mv "$file" "karachi-$file"
done
ls

Output

karachi-img1.jpg  karachi-img2.jpg  notes.txt

This renames every .jpg file in one go. Doing that by hand for 500 photos would take ages.

Always write "$file" with quotes. Without them, a file called my photo.jpg would be split into two words and the command would fail.

while Loop

while repeats as long as the condition is true:

Example

bash

balance=1000
months=0

while [ "$balance" -lt 2000 ]; do
    balance=$((balance + 300))
    months=$((months + 1))
done

echo "Balance $balance after $months months"

Output

Balance 2200 after 4 months

until Loop

until is the opposite of while: it repeats until the condition becomes true.

Example

bash

count=1
until [ "$count" -gt 3 ]; do
    echo "Attempt $count"
    ((count++))
done

Output

Attempt 1
Attempt 2
Attempt 3

Reading a File Line by Line

while read is the standard way to process a file one line at a time:

Example

bash

printf "Ali 85\nSara 92\nHamza 78\n" > marks.txt

while read -r name score; do
    echo "$name scored $score"
done < marks.txt

Output

Ali scored 85
Sara scored 92
Hamza scored 78

read splits each line at spaces, so the first word goes into name and the second into score. -r keeps backslashes as they are.

break and continue

Keyword	What it does
`break`	Stops the loop completely
`continue`	Skips to the next round

Example

bash

for n in {1..10}; do
    if [ $((n % 2)) -eq 0 ]; then
        continue
    fi
    if [ "$n" -gt 7 ]; then
        break
    fi
    echo -n "$n "
done
echo

Output

1 3 5 7

Exercise

Which word ends the body of a for loop in Bash?

endfidoneesac