Loops

Iteration patterns in bash: for, while, until, and loop control.

For Loops

Basic Syntax

for variable in list; do
    commands
done

Iterating Over Words

for name in Alice Bob Charlie; do
    echo "Hello, $name"
done

Hello, Alice
Hello, Bob
Hello, Charlie

Iterating Over Files

for file in *.txt; do
    echo "Processing: $file"
done

Empty Glob

If no files match, the glob itself becomes the value:

for file in *.xyz; do
    echo "$file"    # Prints "*.xyz" if no match
done

Fix with nullglob:

shopt -s nullglob
for file in *.xyz; do
    echo "$file"    # Loop doesn't run if no match
done

Iterating Over Array

fruits=("apple" "banana" "cherry")
for fruit in "${fruits[@]}"; do
    echo "$fruit"
done

Iterating Over Command Output

for user in $(cat /etc/passwd | cut -d: -f1); do
    echo "User: $user"
done

Better - avoid word splitting issues:

while IFS=: read -r user _; do
    echo "User: $user"
done < /etc/passwd

C-Style For Loop

for ((i=0; i<5; i++)); do
    echo "Count: $i"
done

Count: 0
Count: 1
Count: 2
Count: 3
Count: 4

Range with Brace Expansion

for i in {1..5}; do
    echo "Number: $i"
done

With step:

for i in {0..10..2}; do
    echo "Even: $i"
done

Even: 0
Even: 2
Even: 4
Even: 6
Even: 8
Even: 10

Range with seq

for i in $(seq 1 5); do
    echo "Number: $i"
done

# With step
for i in $(seq 0 2 10); do
    echo "Even: $i"
done

While Loops

Basic Syntax

while condition; do
    commands
done

Counter Loop

count=0
while [[ $count -lt 5 ]]; do
    echo "Count: $count"
    ((count++))
done

Infinite Loop

while true; do
    echo "Press Ctrl+C to stop"
    sleep 1
done

Or:

while :; do
    # commands
done

Reading Lines from File

while IFS= read -r line; do
    echo "Line: $line"
done < file.txt

Components explained:

• IFS= - Preserve leading/trailing whitespace
• -r - Don't interpret backslashes
• line - Variable to store each line

Reading with Process Substitution

while IFS= read -r line; do
    echo "$line"
done < <(command)

Reading Multiple Fields

# /etc/passwd format: user:x:uid:gid:desc:home:shell
while IFS=: read -r user _ uid gid desc home shell; do
    echo "$user has UID $uid"
done < /etc/passwd

Reading from Pipe

# Warning: runs in subshell, variables don't persist
echo -e "a\nb\nc" | while read -r line; do
    echo "Got: $line"
done

To preserve variables, use process substitution or here string.

Until Loops

Run until condition is true (opposite of while):

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

Waiting for Condition

# Wait for file to appear
until [[ -f /tmp/ready ]]; do
    echo "Waiting..."
    sleep 1
done
echo "File found!"

Loop Control

break

Exit the loop immediately:

for i in {1..10}; do
    if [[ $i -eq 5 ]]; then
        break
    fi
    echo "$i"
done

1
2
3
4

continue

Skip to next iteration:

for i in {1..5}; do
    if [[ $i -eq 3 ]]; then
        continue
    fi
    echo "$i"
done

1
2
4
5

Nested Loop Control

Break/continue outer loops with a number:

for i in {1..3}; do
    for j in {1..3}; do
        if [[ $j -eq 2 ]]; then
            break 2    # Break both loops
        fi
        echo "$i,$j"
    done
done

1,1

Common Patterns

Process All Arguments

for arg in "$@"; do
    echo "Argument: $arg"
done

Find and Process Files

# Using find with -exec
find . -name "*.txt" -exec echo "Found: {}" \;

# Using while read (safer for filenames with spaces)
find . -name "*.txt" -print0 | while IFS= read -r -d '' file; do
    echo "Found: $file"
done

Retry Until Success

max_attempts=5
attempt=1

while [[ $attempt -le $max_attempts ]]; do
    if some_command; then
        echo "Success on attempt $attempt"
        break
    fi
    echo "Attempt $attempt failed, retrying..."
    ((attempt++))
    sleep 2
done

if [[ $attempt -gt $max_attempts ]]; then
    echo "Failed after $max_attempts attempts"
fi

Progress Counter

total=100
for ((i=1; i<=total; i++)); do
    printf "\rProgress: %d%%" "$((i * 100 / total))"
    sleep 0.1
done
echo

Parallel Processing

Simple parallel with &:

for file in *.txt; do
    process_file "$file" &
done
wait    # Wait for all background jobs

With controlled parallelism:

max_jobs=4
for file in *.txt; do
    while [[ $(jobs -r -p | wc -l) -ge $max_jobs ]]; do
        sleep 0.1
    done
    process_file "$file" &
done
wait

Menu Loop

while true; do
    echo "1) Option A"
    echo "2) Option B"
    echo "3) Quit"
    read -rp "Choice: " choice

    case "$choice" in
        1) echo "You chose A" ;;
        2) echo "You chose B" ;;
        3) break ;;
        *) echo "Invalid choice" ;;
    esac
done

Accumulator Pattern

total=0
for num in 1 2 3 4 5; do
    ((total += num))
done
echo "Total: $total"    # 15

Loop Performance

Avoid External Commands in Loops

# Slow - calls external command each iteration
for i in {1..1000}; do
    result=$(echo "$i * 2" | bc)
done

# Fast - use bash arithmetic
for i in {1..1000}; do
    ((result = i * 2))
done

Use While Read for Large Files

# Bad - loads entire file into memory
for line in $(cat huge.txt); do
    echo "$line"
done

# Good - reads line by line
while IFS= read -r line; do
    echo "$line"
done < huge.txt

Select Loop

Create simple menus:

PS3="Choose a color: "
select color in "Red" "Green" "Blue" "Quit"; do
    case "$color" in
        Red|Green|Blue)
            echo "You chose: $color"
            ;;
        Quit)
            break
            ;;
        *)
            echo "Invalid option"
            ;;
    esac
done

1) Red
2) Green
3) Blue
4) Quit
Choose a color: 2
You chose: Green
Choose a color: 4

Try It

1. Basic for loop:

   for name in Alice Bob Charlie; do
       echo "Hello, $name!"
   done
   

2. File loop:

   for file in /etc/*.conf; do
       echo "Config: $file"
   done | head -5
   

3. C-style loop:

   for ((i=1; i<=5; i++)); do
       echo "Square of $i is $((i * i))"
   done
   

4. While with counter:

   n=1
   while [[ $n -le 3 ]]; do
       echo "Iteration $n"
       ((n++))
   done
   

5. Read file:

   echo -e "line1\nline2\nline3" | while read -r line; do
       echo "Got: $line"
   done
   

Summary

Loop Type	Use Case
for x in list	Known list of items
for ((i=0;...))	Counter-based iteration
while condition	Until condition is false
until condition	Until condition is true
select x in list	User menus

Control	Effect
break	Exit loop
break N	Exit N nested loops
continue	Skip to next iteration
continue N	Continue outer loop

Best practices:

• Quote array expansion: "${arr[@]}"
• Use while IFS= read -r for files
• Use -print0 / -d '' for filenames with spaces
• Avoid external commands inside loops
• Use wait after background jobs