for / while

# ===== for (list) =====
for f in *.txt; do
  echo "=== $f ==="
  head -3 "$f"
done

# Range:
for i in {1..10}; do echo $i; done
for i in {0..100..5}; do echo $i; done   # step of 5

# Brace expansion + variable:
n=10
for i in $(seq 1 $n); do echo $i; done

# ===== for (C-style) =====
for ((i=0; i<10; i++)); do
  echo $i
done

# ===== for ... in command output =====
for user in $(cut -d: -f1 /etc/passwd); do
  echo "user: $user"
done

# Better: read lines (handles spaces):
while IFS= read -r line; do
  echo "got: $line"
done < file.txt

# ===== while =====
i=0
while [ $i -lt 5 ]; do
  echo $i
  i=$((i + 1))
done

# Read stdin line by line:
while IFS= read -r line; do
  echo "got: $line"
done

# ===== until =====
n=5
until [ $n -eq 0 ]; do
  echo $n
  n=$((n - 1))
done

# ===== select (interactive menu) =====
PS3='Pick an option: '
select opt in start stop quit; do
  case "$opt" in
    start) echo 'starting'; ;;
    stop) echo 'stopping'; ;;
    quit) break ;;
  esac
done

# ===== break + continue =====
for i in 1 2 3 4 5; do
  [ $i -eq 3 ] && continue
  [ $i -eq 5 ] && break
  echo $i
done

# ===== Infinite loop =====
while true; do
  do_thing
  sleep 1
done

# ===== Loop over arguments =====
for arg in "$@"; do
  echo "$arg"
done

# ===== Common one-liners =====
# Resize many images:
for img in *.png; do convert "$img" -resize 50% "resized-$img"; done

# Run a command N times:
for i in {1..10}; do curl -sf https://example.com > /dev/null; done

# Process files with spaces in names (use find + -print0 + xargs -0):
find . -name '*.log' -print0 | while IFS= read -r -d '' f; do
  echo "$f"
done

# ===== Patterns =====
# - Always quote "$var" in loops; never bare
# - while IFS= read -r line for line-by-line reading
# - find -print0 + read -d '' for filenames with spaces
# - Use {n..m} range for fixed sequences

# ===== Pitfalls =====
# - for f in $(ls *.txt) -> breaks on filenames with spaces
# - Unquoted variables -> word splitting
# - Forgetting IFS= -> leading/trailing whitespace stripped
# - 'break N' to exit nested loops; default is 1

for f in *.log; do
    gzip "$f"
done

while read line; do
    echo "$line"
done < file.txt