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Bash Loops: for, while and until with Examples

Automate repetition in Bash with for and while loops, loop over files and command output, read a file line by line, and use break and continue.

Intermediate8 min readUpdated
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for#

A for loop runs once for each item in a list:

Terminal
for city in Hyderabad Pune Chennai; do
  echo "Office: $city"
done
Output
Office: Hyderabad
Office: Pune
Office: Chennai

Number ranges use braces, or the C-style form:

Terminal
for i in {1..3}; do
  echo "Try $i"
done
for ((i = 0; i < 6; i += 2)); do
  echo "$i"
done
Output
Try 1
Try 2
Try 3
0
2
4

Looping over files#

Use a wildcard directly, and quote the variable, so that names with spaces work:

Terminal
mkdir photos
touch "photos/beach day.jpg" photos/party.jpg
for file in photos/*.jpg; do
  echo "Found: $file"
done
Output
Found: photos/beach day.jpg
Found: photos/party.jpg

A common job is renaming files:

Terminal
touch a.txt b.txt
for f in *.txt; do
  mv "$f" "${f%.txt}.md"
done
ls *.md
Output
a.md
b.md

${f%.txt} removes .txt from the end. See string handling.

while#

while repeats as long as its condition succeeds:

Terminal
count=3
while (( count > 0 )); do
  echo "$count..."
  ((count--))
done
echo "Go"
Output
3...
2...
1...
Go

Read a file line by line#

This is the correct way to process a file:

Terminal
printf 'asha,admin\nravi,user\n' > users.csv
while IFS=, read -r name role; do
  echo "$name has role $role"
done < users.csv
Output
asha has role admin
ravi has role user

IFS=, splits each line on commas, and -r keeps backslashes as they are. The file is fed in with < users.csv at the end of the loop.

To process a command's output, use a pipe, or < <(command):

Terminal
total=0
while read -r size; do
  ((total += size))
done < <(printf '10\n20\n30\n')
echo "Total: $total"
Output
Total: 60

With a plain pipe, the loop runs in a subshell, and total would be lost when it ends.

until#

until is the opposite of while. It runs until the condition succeeds, which suits waiting for something:

Terminal
tries=0
until [[ -f ready.txt ]]; do
  ((tries++))
  (( tries == 2 )) && touch ready.txt
done
echo "Ready after $tries tries"
Output
Ready after 2 tries

break and continue#

break leaves the loop, and continue moves to the next item:

Terminal
for n in 1 2 3 4 5 6; do
  (( n == 5 )) && break
  (( n % 2 == 0 )) && continue
  echo "$n"
done
Output
1
3

Retry a command#

This pattern is useful in deployment scripts:

Terminal
attempt=1
until false || (( attempt == 3 )); do
  echo "Attempt $attempt failed, retrying"
  ((attempt++))
  sleep 0.1
done
echo "Giving up after $attempt"
Output
Attempt 1 failed, retrying
Attempt 2 failed, retrying
Giving up after 3

Replace false with a real command, such as curl -sf http://localhost:8000/health.

Common mistakes#

Practice#

Practice exercise

Use a loop to create log1.txt, log2.txt and log3.txt, each containing its own number, then print the total of all the numbers by reading the files in a second loop.

Show solution
Terminal
for i in 1 2 3; do
  echo "$i" > "log$i.txt"
done
sum=0
for f in log*.txt; do
  ((sum += $(cat "$f")))
done
echo "Sum: $sum"
Output
Sum: 6

Key takeaway#

Next: Functions.

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