Loops

Bash · Reference cheat sheet

📋 Overview

Bash offers for, while, until, and C-style for (( )). Prefer iterating arrays or null-delimited streams over unquoted globs. Use break / continue to control flow; avoid parsing ls.

🔧 Core concepts

Read lines safely with while IFS= read -r line (and -d '' for NUL). Process substitution or pipes run the loop in a subshell unless you use lastpipe / redirect.

💡 Examples

Array and C-style:

files=(*.txt)
for f in "${files[@]}"; do
  echo "file: $f"
done

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

Read file line by line:

while IFS= read -r line || [[ -n "$line" ]]; do
  printf '%s\n' "$line"
done < "$input"

Find with NUL (safe for spaces):

while IFS= read -r -d '' path; do
  echo "$path"
done < <(find . -type f -print0)

While with counter:

n=0
while (( n < 3 )); do
  ((n++)) || true
  echo "$n"
done

⚠️ Pitfalls

• for f in $(ls) breaks on spaces/newlines—use globs or find -print0.
• Pipeline cmd | while read runs while in a subshell; variables won’t persist.
• Unquoted for f in $list word-splits; use arrays: "$\{arr[@]\}".
• for f in *.txt leaves literal *.txt if no match unless nullglob is set.
• ((n++)) returns status 1 when expression is 0—watch with set -e.
• Infinite loops: ensure the condition or break can fire.

🔗 Related

• arrays.md
• globbing.md
• conditionals.md
• pipes_redirection.md
• subshells.md