Globbing, Brace Expansion, and Safe Bulk Operations

1. The shell transforms a command before the program sees it

When a command line contains braces, variables, command substitutions, spaces, or wildcard characters, Bash performs a sequence of expansions. The external program receives the resulting argument vector. It usually never sees the original quotes or wildcard expression.

flowchart TD
  C["Typed command line"] --> B["Brace expansion"]
  B --> V["Parameter and command expansion"]
  V --> W["Word splitting when applicable"]
  W --> G["Pathname expansion"]
  G --> Q["Quote removal"]
  Q --> A["Final argv passed to command"]

2. Pathname expansion uses glob patterns, not regular expressions

lab="$HOME/devops-academy/linux/chapter04/lesson04"
rm -rf -- "$lab"
mkdir -p -- "$lab"
printf '%s\n' alpha > "$lab/app-1.log"
printf '%s\n' beta  > "$lab/app-2.log"
printf '%s\n' gamma > "$lab/app-a.log"
printf '%s\n' delta > "$lab/app-10.log"
printf '%s\n' hidden > "$lab/.app-hidden.log"

cd -- "$lab"
printf 'single-character suffix matches:\n'
printf '  %s\n' app-?.log

printf '\nnumeric single-character matches:\n'
printf '  %s\n' app-[0-9].log

A normal * does not match a leading dot at the start of a path component. This protects . and .. from many patterns but also means a command such as cp -a source/* destination/ omits hidden entries. Copying source/. is often the clearer complete-tree form.

3. Quoting decides whether metacharacters remain active

pattern='*.log'

printf 'Unquoted variable may split and glob:\n'
printf '  %s\n' $pattern

printf '\nQuoted variable is one literal argument:\n'
printf '  %s\n' "$pattern"

printf '\nQuoted wildcard is literal:\n'
printf '  %s\n' '*.log'

printf '\nEscaped wildcard is literal:\n'
printf '  %s\n' \*.log

Double quotes permit parameter and command substitution but suppress word splitting and pathname expansion of the resulting value. Single quotes preserve nearly every character literally. A wildcard typed outside quotes remains eligible for pathname expansion.

4. Choose what an unmatched pattern means

By default, Bash leaves an unmatched pathname pattern unchanged. A loop over *.log can therefore process the literal text *.log when no files match. Shell options let scripts choose safer behavior.

empty="$lab/empty"
mkdir -p -- "$empty"
cd -- "$empty"

shopt -s nullglob
logs=( *.log )
printf 'nullglob match count=%d\n' "${#logs[@]}"
shopt -u nullglob

# Demonstrate failglob in a subshell so the parent shell continues.
if ( shopt -s failglob; printf '%s\n' *.log ) 2>"$lab/failglob.err"; then
  printf 'unexpected match\n'
else
  printf 'failglob rejected unmatched pattern\n'
fi
cat -- "$lab/failglob.err"

5. Brace expansion generates text before pathname matching

Brace expansion is textual. It does not inspect the filesystem. Bash expands alternatives such as {dev,test,prod} and sequences such as {01..05} before parameter expansion and globbing.

root="$lab/environments"
mkdir -p -- "$root"/{dev,test,prod}/{config,logs}

printf 'Generated environment directories:\n'
printf '  %s\n' "$root"/{dev,test,prod}/{config,logs}

printf '\nSequence-generated candidate names:\n'
printf '  node-%02d.conf\n' {1..5}

# Variables are not expanded early enough to define brace syntax.
start=1
end=3
printf '\nThis remains literal brace text:\n'
printf '  %s\n' {$start..$end}

Brace expansion can generate nonexistent names, which is valuable for creation commands but risky if mistaken for a match query. Use globbing to select existing paths and brace expansion to construct predictable text combinations.

6. Use arrays for a reviewable bulk target set

cd -- "$lab"
shopt -s nullglob
candidates=( app-[0-9].log app-10.log )
shopt -u nullglob

printf 'Candidate count: %d\n' "${#candidates[@]}"
printf 'Quoted candidate argv:\n'
printf '  %q\n' "${candidates[@]}"

if ((${#candidates[@]} == 0)); then
  printf 'No matching logs; nothing to do.\n'
else
  mkdir -p -- archive
  cp -vn -- "${candidates[@]}" archive/
fi

printf '\nArchive verification:\n'
find archive -maxdepth 1 -type f -printf '%f\n' | sort

"${array[@]}" expands to one quoted argument per element. By contrast, ${array[*]} and unquoted expansions can collapse or re-split boundaries. Always use the [@] form for command arguments.

7. For recursive selection, prefer find with null-safe execution

Pathnames may contain spaces, tabs, newlines, wildcard characters, and leading dashes. Parsing find output line by line is therefore fragile unless a null delimiter is used. For direct execution, -exec ... {} + passes names as arguments without text parsing.

tree="$lab/tree"
mkdir -p -- "$tree/a" "$tree/b"
printf x > "$tree/a/old build.log"
printf y > "$tree/b/-leading-dash.log"
printf z > "$tree/b/keep.txt"

printf 'Preview selected logs:\n'
find "$tree" -type f -name '*.log' -printf '  %p\n'

mkdir -p -- "$lab/recursive-archive"
find "$tree" -type f -name '*.log' \
  -exec cp -vn -- {} "$lab/recursive-archive/" +

printf '\nCopied basenames:\n'
find "$lab/recursive-archive" -maxdepth 1 -type f -printf '%f\n' | sort

8. Hands-on lab: archive logs with preview and exact argv preservation

set -u
root="$HOME/devops-academy/linux/chapter04/lesson04-bulk"
source_dir="$root/incoming"
archive_dir="$root/archive"
rm -rf -- "$root"
mkdir -p -- "$source_dir" "$archive_dir"

printf 'ok\n' > "$source_dir/api 01.log"
printf 'warn\n' > "$source_dir/api 02.log"
printf 'ignore\n' > "$source_dir/api.tmp"
printf 'hidden\n' > "$source_dir/.internal.log"
printf 'dash\n' > "$source_dir/-special.log"

cd -- "$source_dir"
shopt -s nullglob
logs=( *.log )
shopt -u nullglob

printf 'Dry-run target list (%d files):\n' "${#logs[@]}"
printf '  %q\n' "${logs[@]}"

if ((${#logs[@]})); then
  cp -vn -- "${logs[@]}" "$archive_dir/"
fi

printf '\nArchived files:\n'
find "$archive_dir" -maxdepth 1 -type f -printf '%f\n' | sort

printf '\nHidden source file remains excluded by normal glob:\n'
test -f "$source_dir/.internal.log" && printf 'yes\n'

9. Common expansion mistakes

10. Knowledge check

11. Summary

Bash constructs command arguments before external programs run. Globs select existing pathnames, brace expansion generates text combinations, and quoting controls whether metacharacters remain active. Safe bulk operations materialize targets in arrays, preview them with quoted output, handle zero matches explicitly, terminate option parsing with --, and use null-safe recursive tools.

12. Further reading

• GNU Bash Reference Manual — shell expansions, pattern matching, arrays, glob options, and quoting.
• GNU Coreutils manuals — argument handling and -- conventions.
• GNU findutils manual — find expressions, -exec, null delimiters, and filename safety.
• POSIX Shell Command Language — pathname expansion, field splitting, and quoting.
• ShellCheck documentation — diagnostics for unsafe expansions and loops over command output.