while / until

Repeat with a while loop.

while condition; do
    commands
done

Repeat with an until loop (repeats while the condition is false).

until condition; do
    commands
done

Read a file line by line.

while IFS= read -r line; do
    commands
done < filename

Read from a pipe (runs in a subshell — be aware of variable scope).

command | while IFS= read -r line; do
    commands
done

Infinite loop.

while true; do
    commands
done

Pattern	Description
while [ condition ]	Repeats the loop while the condition is true.
while (( condition ))	Repeats while the arithmetic condition is non-zero.
until [ condition ]	Repeats the loop while the condition is false.
while true	Loops indefinitely until an explicit break exits the loop.
while IFS= read -r line	Safely reads a file one line at a time.
break	Exits the loop immediately.
continue	Skips the rest of the current iteration.

A basic while loop that counts from 1 to 5.

while_count.sh

#!/bin/bash
count=1
while [ $count -le 5 ]; do
    echo "Count: $count"
    (( count++ ))
done

bash while_count.sh
Count: 1
Count: 2
Count: 3
Count: 4
Count: 5

You can also type a while loop directly in the terminal. After pressing Enter following do, a > prompt appears indicating more input is expected. Type done to execute.

count=1; while [ $count -le 5 ]; do
    echo "Count: $count"
    (( count++ ))
done
Count: 1
Count: 2
Count: 3
Count: 4
Count: 5

until loops while the condition is false — the opposite behavior of while.

until_count.sh

n=0
until [ $n -ge 5 ]; do
    echo "n = $n"
    (( n++ ))
done

bash until_count.sh
n = 0
n = 1
n = 2
n = 3
n = 4

The standard pattern for reading a file line by line. IFS= preserves leading and trailing whitespace, and -r prevents backslash interpretation.

read_lines.sh

while IFS= read -r line; do
    echo "Line: $line"
done < /etc/hosts

bash read_lines.sh
Line: 127.0.0.1	localhost
Line: 255.255.255.255	broadcasthost
Line: ::1             localhost

An infinite loop is useful for process monitoring. Use break to exit explicitly.

monitor.sh

while true; do
    if ! pgrep -x "myapp" > /dev/null; then
        echo "myapp is not running. Restarting..."
        ./myapp &
    fi
    sleep 60
done

Split CSV fields by changing IFS to a comma.

csv_parse.sh

while IFS=',' read -r name score grade; do
    echo "Name: $name, Score: $score, Grade: $grade"
done <<'EOF'
Alice,95,A
Bob,78,C
Charlie,88,B
EOF

bash csv_parse.sh
Name: Alice, Score: 95, Grade: A
Name: Bob, Score: 78, Grade: C
Name: Charlie, Score: 88, Grade: B

Process the output of a command one line at a time using a pipe.

find_lines.sh

find . -name "*.php" | while IFS= read -r file; do
    lines=$(wc -l < "$file")
    echo "$file: $lines lines"
done

bash find_lines.sh
./index.php: 120 lines
./ajax.php: 85 lines

When processing a file line by line, while IFS= read -r line; do ... done < file is the standard pattern. Without IFS=, leading and trailing whitespace is stripped from each line. Without -r, backslashes are interpreted and expanded.

A while loop fed by a pipe runs in a subshell. This means any variables updated inside the loop are not accessible in the outer shell after the loop ends. To share variables, use process substitution (< <(command)) or a temporary file instead.

For iterating over a list, for is usually a better fit.