BASH - Globs

Glob or Globstar is the common name for a set of Bash features that match or expand specific types of patterns.

Some synonyms for globbing (depending on the context in which it appears) are pattern matching, pattern expansion, filename expansion, and so on.

A glob may look like *.txt and, when used to match filenames, is sometimes called a “wildcard”.

Traditional shell globs use a very simple syntax, which is less expressive than a Regular Expression.

Most characters in a glob are treated literally, but a * matches 0 or more characters, a ? matches precisely one character, and […] matches any single character in a specified set (see Ranges below).

All globs are implicitly anchored at both start and end.

Bash expands globs which appear unquoted in commands, by matching filenames relative to the current directory.

The expansion of the glob results in 1 or more words (0 or more, if certain options are set), and those words (filenames) are used in the command.

tar xvf *.tar
# Expands to: tar xvf file1.tar file2.tar file42.tar ...

Even if a file contains internal whitespace, the expansion of a glob that matches that file will still preserve each filename as a single word.

For example,

# This is safe even if a filename contains whitespace:
for f in *.tar; do
  tar tvf "$f"
done
 
# But this one is not:
for f in $(ls | grep '\.tar$'); do
  tar tvf "$f"
done

Globs are also used to match patterns in a few places in Bash.

The most traditional is in the case command:

case "$input" in
  [Yy]|'') confirm=1;;
  [Nn]*) confirm=0;;
  *) echo "I don't understand.  Please try again.";;
esac

Bash also allows globs to appear on the right-hand side of a comparison inside a [[ command:

if [[ $output = *[Ee]rror* ]]; then ...

Globs are used during parameter expansion to indicate patterns which may be stripped out, or replaced, during a substitution.

filename=${path##*/}    # strip leading pattern that matches */ (be greedy)
dirname=${path%/*}      # strip trailing pattern matching /* (non-greedy)
 
printf '%s\n' "${arr[@]}"          # dump an array, one element per line
printf '%s\n' "${arr[@]/error*/}"  # dump array, removing error* if matched

Globs can specify a range or class of characters, using square brackets.

This gives you the ability to match against a set of characters.

For example:

In addition to the traditional globs (supported by all Bourne-family shells) that we've seen so far, Bash (and Korn Shell) offers extended globs, which have the expressive power of regular expressions.

Korn shell enables these by default; in Bash, you must run the command in your shell (or at the start of your script – see note on parsing below) to use them.

shopt -s extglob

The pattern matching reference describes the syntax, which is reproduced here:

?(pattern-list)
    Matches zero or one occurrence of the given patterns. 
*(pattern-list)
    Matches zero or more occurrences of the given patterns. 
+(pattern-list)
    Matches one or more occurrences of the given patterns. 
@(pattern-list)
    Matches one of the given patterns. 
!(pattern-list)
    Matches anything except one of the given patterns. 

Extended globs allow you to solve a number of problems which otherwise require a rather surprising amount of ugly hacking; for example,

# To remove all the files except ones matching *.jpg:
rm !(*.jpg)
 
# All except *.jpg and *.gif and *.png:
rm !(*.jpg|*.gif|*.png)

or

# To copy all the MP3 songs except one to your device.
cp !(04*).mp3 /mnt

To use an extglob in a parameter expansion (this can also be done in one BASH statement with read):

# To trim leading and trailing whitespace from a variable.
x=${x##+([[:space:]])}; x=${x%%+([[:space:]])}

Extended glob patterns can be nested, too.

[[ $fruit = @(ba*(na)|a+(p)le) ]] && echo "Nice fruit"

Therefore, if you use this option in a script, it is best put right under the shebang line.

#!/usr/bin/env bash
shopt -s extglob   # and others, such as nullglob dotglob.

If your code must be sourced and needs extglob, ensure it preserves the original setting from your shell:

# Remember whether extglob was originally set, so we know whether to unset it.
shopt -q extglob; extglob_set=$?
# Set extglob if it wasn't originally set.
((extglob_set)) && shopt -s extglob
# Note, 0 (true) from shopt -q is "false" in a math context.
 
# The basic concept behind the following is to delay parsing of the globs until evaluation.
# This matters at group commands, such as functions in { } blocks.
 
declare -a s='( !(x) )'
echo "${s[@]}"
 
echo "${InvalidVar:-!(x)}"
 
eval 'echo !(x)'  # using eval if no other option.
 
# Unset extglob if it wasn't originally set.
((extglob_set)) && shopt -u extglob
 
This should also apply for other shell options.

nullglob expands non-matching globs to zero arguments, rather than to themselves.

$ ls *.c
ls: cannot access *.c: No such file or directory
 
# With nullglob set.
shopt -s nullglob
ls *.c
# Runs "ls" with no arguments, and lists EVERYTHING

Typically, nullglob is used to count the number of files matching a pattern:

shopt -s nullglob
files=(*)
echo "There are ${#files[@]} files in this directory."

Without nullglob, the glob would expand to a literal * in an empty directory, resulting in an erroneous count of 1.

shopt -s nullglob
unset array[1]
#unsets nothing
 
unset -v "array[1]"
#correct

shopt -s nullglob
array=([1]=*)
# Results in an empty array.

Apart from few builtins that use modified parsing under special conditions (e.g. declare) always use Quotes when arguments to simple commands could be interpreted as globs.

Enabling failglob, nullglob, or both during development and testing can help catch mistakes early.

To prevent pathname expansion occurring in unintended places, you can set failglob. However, you must then guarantee all intended globs match at least one file. Also note that the result of a glob expansion does not always differ from the glob itself. failglob won't distinguish echo ? from echo '?' in a directory containing only a file named ?. nullglob will.

“null globbing” is not specified by POSIX.

In portable scripts, you must explicitly check that a glob match was successful by checking that the files actually exist.

# POSIX
 
for x in *; do
  [ -e "$x" ] || break
  ...
done
 
f() {
  [ -e "$1" ] || return 1
 
  for x do
    ...
  done
}
 
f * || echo "No files found"

Some modern POSIX-compatible shells allow null globbing as an extension.

# Bash
shopt -s nullglob

In ksh93, there is no toggle-able option. Rather, that the “nullglob” behavior is to be enabled is specified inline using the “N” option to the ∼() sub-pattern syntax.

# ksh93
 
for x in ~(N)*; do
  ...
done

In zsh, an toggle-able option(NULL_GLOB) or a glob qualifier(N) can be used.

# zsh
for x in *(N); do ...; done # or setopt NULL_GLOB

mksh doesn't yet support nullglob (maintainer says he'll think about it).

By convention, a filename beginning with a dot is “hidden”, and not shown by ls.

Globbing uses the same convention – filenames beginning with a dot are not matched by a glob, unless the glob also begins with a dot.

Bash has a dotglob option that lets globs match “dot files”:

shopt -s dotglob nullglob
files=(*)
echo "There are ${#files[@]} files here, including dot files and subdirs"

It should be noted that when dotglob is enabled, * will match files like .bashrc but not the . or .. directories.

This is orthogonal to the problem of matching “just the dot files” – a glob of .* will match . and .., typically causing problems.

(since bash 4.0-alpha)

globstar recursively repeats a pattern containing **.

shopt -s globstar; tree
.
├── directory2
│   ├── directory3
│   ├── file1.c
│   └── file2
├── file1
└── file2.c

Suppose that for the following examples.

Matching files:

$ files=(**)
# equivalent to: files=(* */* */*/*)
# finds all files recursively
 
$ files=(**/*.c)
# equivalent to: files=(*.c */*.c */*/*.c)
# finds all *.c files recursively
# corresponds to: find -name "*.c"
# Caveat: **.c will not work, as it expands to *.c/*.c/…

shopt -u globstar

Assume you have a folder structure:

.
├── bar
│   ├── foo
│   │   └── baz
│   │       └── hurz
│   │           └── lolz
│   │               └── hello.txt
│   └── poit.txt
└── fnord.txt

Then ls with single star * would list:

ls *.txt
fnord.txt

The double star operator ** will work on the subfolders. The output will look like:

ls **/*.txt
bar/foo/baz/hurz/lolz/hello.txt  bar/poit.txt  fnord.txt

files=(**/)
# Finds all subdirectories.
 
files=(. **/)
# Finds all subdirectories, including the current directory.
# Corresponds to: find -type d.

If a pattern fails to match, bash reports an expansion error.

This can be useful at the commandline:

# Good at the command line!
$ > *.foo # creates file '*.foo' if glob fails to match
$ shopt -s failglob
$ > *.foo # doesn't get executed
-bash: no match: *.foo

The Bash variable (not shopt) GLOBIGNORE allows you to specify patterns a glob should not match.

This lets you work around the infamous “I want to match all of my dot files, but not . or ..” problem:

$ echo .*
. .. .bash_history .bash_logout .bashrc .inputrc .vimrc
$ GLOBIGNORE=.:..
$ echo .*
.bash_history .bash_logout .bashrc .inputrc .vimrc

$ GLOBIGNORE=
$ echo .*
. .. .bash_history .bash_logout .bashrc .inputrc .vimrc

Globs inside [[ and case commands are matched case-insensitive:

foo() {
  local f r=0 nc=0
  shopt -q nocasematch && nc=1 || shopt -s nocasematch
  for f; do
    [[ $f = *.@(txt|jpg) ]] || continue
    cmd -on "$f" || r=1
  done
  ((nc)) || shopt -u nocasematch
  return $r
}

This is conventionally done this way:

case $f in
  *.[Tt][Xx][Tt]|*.[Jj][Pp][Gg]) : ;;
  *) continue
esac

and in earlier versions of bash we'd use a similar glob:

[[ $f = *.@([Tt][Xx][Tt]|[Jj][Pp][Gg]) ]] || continue

or with no extglob:

[[ $f = *.[Tt][Xx][Tt] ]] || [[ $f = *.[Jj][Pp][Gg] ]] || continue

Here, one might keep the tests separate for maintenance; they can be easily reused and dropped, without having to concern oneself with where they fit in relation to an internal ||.

Note also:

[[ $f = *.@([Tt][Xx][Tt]|[Jj][Pp]?([Ee])[Gg]) ]]

Variants left as an exercise.

(since bash 2.02-alpha1)

This option makes pathname expansion case-insensitive.

In contrast, nocasematch operates on matches in [[ and case commands.