"Glob" 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 RegularExpression. 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. For example:

*

Matches any string, of any length

foo*

Matches any string beginning with foo

*x*

Matches any string containing an x (beginning, middle or end)

*.tar.gz

Matches any string ending with .tar.gz

*.[ch]

Matches any string ending with .c or .h

foo?

Matches foot or foo$ but not fools

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. For example:

tar xvf *.tar
# Expands to: tar xvf file1.tar file2.tar file42.tar ...
# (which is generally not what one wants)

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

In the second example above, the output of ls is filtered, and then the result of the whole pipeline is divided into words, to serve as iterative values for the loop. This word-splitting will occur at internal whitespace within each filename, which makes it useless in the general case. The first example has no such problem, because the filenames produced by the glob do not undergo any further word-splitting. For more such examples, see BashPitfalls.

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

Patterns (which are separated by | characters) are matched against the first word after the case itself. The first pattern which matches, "wins", causing the corresponding commands to be executed.

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

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

Finally, globs are used during parameter expansion to indicate patterns which may be stripped out, or replaced, during a substitution. Simple examples (there are many more on the previously referenced page):

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

(Reference: Arrays Quotes printf.)

Ranges

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:

[abcd]

Matches a or b or c or d

[a-d]

The same as above, if your locale is C or POSIX. Otherwise, implementation-defined.

[!aeiouAEIOU]

Matches any character except a, e, i, o, u and their uppercase counterparts

[[:alnum:]]

Matches any alphanumeric character in the current locale (letter or number)

[[:space:]]

Matches any whitespace character

[![:space:]]

Matches any character that is not whitespace

[[:digit:]_.]

Matches any digit, or _ or .

Implementation-defined means it may work as you expect on one machine, but give completely different results on another machine. Do not use the m-n syntax unless you have explicitly set your locale to C first, or you may get unexpected results. The POSIX character class expressions should be preferred whenever possible.

Options which change globbing behavior

extglob

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

shopt -s extglob

in your shell (or at the start of your script -- see note on parsing below) to use them. 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.

Patterns in a list are separated by | characters.

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)

# 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'

Because the extglob option changes the way certain characters are parsed, it is necessary to have a newline (not just a semicolon) between the shopt command and any subsequent commands that use extended globs. Likewise, you cannot put shopt -s extglob inside a statement block that uses extended globs, because the block as a whole must be parsed when it's defined; the shopt command won't take effect until the block is evaluated, at which point it's too late. In fact as bash parses the entire statement block before evaluating any of it, you need to set extglob outside of the outermost block.

Therefore, if you use this option in a script, it's best to put it right under the shebang line, or as close as you can get it while still making your boss happy.

#!/usr/bin/env bash
# Copyright (c) 2012 Foo Corporation
shopt -s extglob   # and others, such as nullglob dotglob

If your code isn't a script, but is instead being sourced, and must set extglob itself:

SourcedFile.sh
    if ! shopt extglob; then
      ClearExtGlob_SourcedFile_sh=1
      shopt -s extglob
    fi
   # The basic Concept behind the following options is to delay parsing of the 
   # extglob until evaluation.

   declare -a s='( !(x) )'
   echo "${s[@]}"

   echo "${InvalidVar:-!(x)}"

   eval 'echo !(x)'  # using eval if no other option.

   if [ "${ClearExtGlob_SourcedFile_sh}" == "1" ]; then
     unset ClearExtGlob_SourcedFile_sh
     shopt -u extglob
   fi

test.sh
   shopt -u extglob
   if true; then
     source ./SourcedFile.sh
   fi

nullglob

If a glob fails to match any filenames, the shell normally leaves it alone. This means the raw glob will be passed on to the command, as in:

$ ls *.ttx
ls: cannot access *.ttx: No such file or directory

This allows the command to see the glob you used, and to use it in an error message. If the Bash option nullglob is set, however, a glob which matches no files will be removed entirely. This is useful in scripts, but somewhat confusing at the command line, since it "breaks" the expectations of many of the standard tools (see failglob below for a better alternative):

# Good in scripts!
shopt -s nullglob
oggs=(*.ogg)
for ogg in "${oggs[@]}"; do ...

# Bad at the command line!
shopt -s nullglob
ls *.ttx
# Runs "ls" with no arguments, and lists EVERYTHING

failglob

If a pattern fails to match, bash reports an expansion error. This can be useful at the commandline:

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

dotglob

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. See next section.

GLOBIGNORE

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

Unset GLOBIGNORE

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


CategoryShell

glob (last edited 2013-08-16 12:41:45 by mailbizz02)