Linux Basic — Glob Patterns

Glob Patterns

“Globs” are Bash’s pattern-matching language for filenames. They’re not regex, they’re not brace expansion, and they have a few hidden powers (extended globs, **) that most scripters never use.

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The Basic Operators

Pattern	Matches
*	any string (zero or more chars), no /, no leading .
?	exactly one char
[abc]	one char from the set
[!abc] or [^abc]	one char not in the set
[a-z]	one char in the range
[[:alpha:]]	POSIX class (alpha/digit/space/alnum/lower/upper/punct/xdigit)

ls *.txt              # all .txt files
ls report?.log        # report1.log, reportA.log (exactly one char)
ls [abc]*             # files starting with a, b, or c
ls [!._]*             # files NOT starting with . or _
ls [[:digit:]]*       # files starting with a digit

Notes:

• * does not cross / — * in /tmp/*.log matches names in /tmp only, not subdirectories
• * does not match leading . unless shopt -s dotglob
• ? is exactly one char (not zero-or-one as in regex)

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Brace Expansion ≠ Globs

# Brace expansion: text generation (file may not exist)
echo file{1,2,3}.txt
# file1.txt file2.txt file3.txt
echo {a..c}{1..3}
# a1 a2 a3 b1 b2 b3 c1 c2 c3

# Globs: file matching (must exist on disk)
ls file*.txt

Brace expansion runs before glob expansion. It generates words from a literal pattern, not from filesystem contents. Mix them:

ls *.{txt,md,log}     # expand braces first → *.txt *.md *.log → then glob

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** — globstar

shopt -s globstar

# All .py under cwd, recursively
ls **/*.py

# Same with hidden dirs included
shopt -s dotglob
ls **/*.py

** matches across directory levels. Without globstar enabled, ** is just *. Default off → no surprise breakage for old scripts.

Performance: globstar is fine for hundreds-of-thousands of files. For million-scale trees, prefer find.

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Extended Globs — extglob

Turn them on with shopt -s extglob. Then you get five PCRE-like operators:

Pattern	Means
?(p)	zero or one of p
*(p)	zero or more of p
+(p)	one or more of p
@(a\|b)	exactly one of a or b
!(p)	anything except p

shopt -s extglob

# Files NOT ending in .bak
ls !(*.bak)

# Files ending in .jpg or .png
ls @(*.jpg|*.png)

# A name that's all digits
ls +([0-9])

!(p) is especially useful as “all except”:

# Delete every backup except 'good.bak'
rm !(good).bak

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Glob Anchoring — There Is None

Unlike regex, globs are fully-anchored to the whole filename:

ls *.log              # filename must END with .log
ls log*               # filename must START with log
ls *log*              # filename must CONTAIN log

You don’t need ^ and $.

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Bracket Expressions

POSIX character classes inside […]:

[[:alpha:]]    [[:digit:]]    [[:alnum:]]
[[:space:]]    [[:upper:]]    [[:lower:]]
[[:punct:]]    [[:xdigit:]]   [[:cntrl:]]

Useful examples:

ls [[:upper:]]*       # start with uppercase
ls *[[:digit:]]       # end with a digit
ls [[:alnum:]]*       # start with letter or digit

Inside […], a literal - must be first or last; ] must be first; !/^ mean negation only if first.

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Globs Inside [[ … == … ]]

Bash’s == operator inside [[ ]] does glob matching (not regex):

file="report.txt"

[[ "$file" == *.txt    ]] && echo "is text"
[[ "$file" == report.* ]] && echo "is a report"
[[ "$file" != *.bak    ]] && echo "not a backup"

This is the cleanest way to test “does this string match this pattern?” — no fork to grep. Use it freely.

Extended globs also work here when shopt -s extglob is on:

shopt -s extglob
file="image.png"
[[ "$file" == @(*.png|*.jpg) ]] && echo "is image"

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case Uses Globs Too

case "$file" in
    *.gz)        gunzip "$file"  ;;
    *.tar)       tar xf  "$file" ;;
    *.tar.gz)    tar xzf "$file" ;;
    *.@(jpg|png|gif))  echo "image — skipping" ;;
    *)           echo "unknown: $file" ;;
esac

case patterns are also globs. They’re more readable than chained if [[ … == … ]] ladders.

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Globs vs Regex — A Cheat Sheet

Concept	Glob	Regex
Any sequence	*	.*
Single char	?	.
Char class	[abc]	[abc]
Negated class	[!abc]	[^abc]
Zero or more of x	*(x) (extglob)	x*
One or more of x	+(x) (extglob)	x+
Optional x	?(x) (extglob)	x?
Alternation	@(a\|b) (extglob)	(a\|b)
Anchors	implicit (whole-string match)	^…$

Globs are for filenames and string-matches-pattern tests. Regex (=~ from Chapter 10) is for richer matching needs.

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A Real Example — Categorise Files

shopt -s nullglob extglob

images=( *.@(jpg|jpeg|png|gif|webp) )
videos=( *.@(mp4|mov|avi|mkv) )
docs=(   *.@(md|txt|pdf|doc|docx) )

echo "Images: ${#images[@]}"
echo "Videos: ${#videos[@]}"
echo "Docs:   ${#docs[@]}"

A single glob does what would take a long find -name chain.

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Common Mistakes

❌ Wrong	✅ Right	Issue
[[ "$f" =~ *.log ]]	[[ "$f" == *.log ]]	=~ is regex, == is glob
**/*.log without globstar	shopt -s globstar first	** is just * otherwise
Quoting the glob ("*.log")	leave unquoted	quoting disables expansion
[a-Z] (mixed case range)	[a-zA-Z] or [[:alpha:]]	locale-dependent surprises
Counting on * to match dotfiles	shopt -s dotglob or explicit .* *	dotfiles excluded by default

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Next → find & xargs