Quoting and word splitting in bash: why “$var” and arrays save you

Most mysterious bash bugs come from one mechanism: after the shell expands $variables and $(commands), it splits the result into words on spaces, tabs and newlines, and then expands any * or ? against the file system. Quotes control which of those steps happen.

The three kinds of quoting

name="my report.txt"

echo '$name'      # single quotes: literally $name, nothing expands
echo "$name"     # double quotes: expands, result stays one word
echo $name        # no quotes: expands, then splits into: my  report.txt

touch "$name"    # creates one file
touch $name       # creates two files: my  and  report.txt

• Single quotes protect everything. Use them for fixed strings, regular expressions and awk or sed programs.
• Double quotes expand $var, $(cmd) and \ escapes but keep the result as one word. This is the default you want around every variable.
• No quotes means word splitting and glob expansion happen. It is only right when you want a list, for example for f in *.log.

The special case of "$@"

Inside double quotes, "$@" expands to each argument as its own word, preserving spaces inside them. "$*" joins all arguments into one word. Unquoted $@ re-splits everything. Pass arguments on with "$@" almost always.

show() { printf '<%s>\n' "$@"; }

show "a b" c      # <a b>  <c>
show $(echo "a b" c)   # <a> <b> <c>: command output was split

Arrays for lists

When you need a list of things, use a bash array instead of a space-separated string. Arrays keep elements with spaces intact and expand cleanly with "${arr[@]}".

files=("/tmp/a b.txt" "/tmp/c.txt")
files+=("/tmp/d.txt")

for f in "${files[@]}"; do
  echo "processing: $f"
done

echo "${#files[@]} files"      # count
echo "${files[0]}"            # first element

# Build a command line safely
opts=(-v --color=auto)
[[ -n "${PATTERN:-}" ]] && opts+=(-e "$PATTERN")
grep "${opts[@]}" -- *.log

Collect command output into an array with mapfile (also called readarray), which splits on newlines only, or with a null-separated find for file names that may contain anything.

mapfile -t hosts < hosts.txt

mapfile -d '' logs < <(find /var/log -name '*.log' -print0)
echo "${#logs[@]} log files"

Globs and when to disable them

An unquoted * matches files in the current directory. That is useful in for f in *.jpg and dangerous in echo $pattern when pattern contains a star. If a glob matches nothing it stays as the literal text; set shopt -s nullglob in scripts so that an empty match becomes an empty list instead of a literal *.jpg.

shopt -s nullglob
for f in *.jpg; do
  echo "$f"
done

# Stop option-looking file names from being read as options
rm -- "$f"
ls -l -- *

Checklist

• Quote every expansion: "$var", "$(cmd)", "${arr[@]}", "$@".
• Use arrays for lists of files or options; never a string you later split.
• Use -- before file arguments that come from variables.
• Use [[ ]] rather than [ ] in bash: it does not word-split its operands.
• Run shellcheck script.sh; warnings SC2086 and SC2046 point exactly at missing quotes.

Official documentation

• Bash manual: Quoting
• Bash manual: Word Splitting
• Bash manual: Arrays