while Loops and Reading Files

A while loop repeats as long as its condition succeeds.

count=1
while (( count <= 3 )); do
  echo "Count: $count"
  (( count++ ))
done

Count: 1
Count: 2
Count: 3

until is the opposite: it loops until the condition succeeds.

Reading a file line by line

The most important while pattern in Bash:

printf '%s\n' "Ada,London" "Linus,Helsinki" "Grace,New York" > people.csv

while IFS=, read -r name city; do
  echo "$name lives in $city"
done < people.csv

Ada lives in London
Linus lives in Helsinki
Grace lives in New York

• < people.csv at the end feeds the file into the loop.
• read -r reads one line per pass; the loop ends at the end of the file.
• IFS=, splits each line on commas into name and city. (Use IFS= alone to keep each whole line exactly as it is.)

Processing command output

Pipe a command's output into while read:

printf '%s\n' 3 9 4 > numbers.txt
total=0
while read -r n; do
  (( total += n ))
done < numbers.txt
echo "Total: $total"

Total: 16

Retrying something

# illustrative
attempt=1
until curl -fs https://example.com > /dev/null; do
  if (( attempt >= 5 )); then
    echo "Giving up after $attempt attempts" >&2
    exit 1
  fi
  echo "Attempt $attempt failed, retrying in 2s..."
  (( attempt++ ))
  sleep 2
done
echo "Site is up"

Common mistakes

• cat file | while read ... runs the loop in a subshell, so variables changed inside (like total) are lost afterwards. Redirect with done < file instead.
• Forgetting -r, so backslashes in the data disappear.
• A missing last newline in the file makes read skip the final line; while IFS= read -r line || [[ -n $line ]] handles it.