glob (programming)

In computer programming, the verb glob or globbing is used to refer to an instance of pattern matching behavior. The noun "glob" is sometimes used to refer to a particular pattern, e.g. "use the glob *.log to match all those log files".

Many command line interpreters (shells) such as Unix shells, cmd.exe and Windows PowerShell, provide globbing on filenames at the command line and in shell scripts.

Technicals: Expansion

Technically, Unix globbing operates by parameter expansion – the glob pattern (say, *.log) is expanded and replaced by the list of all matches. For example, if a directory contains two files, a.log and b.log then the command cat *.log will be expanded by the shell to cat a.log b.log which is then evaluated (in this case, displaying the files). The order of arguments to a command often matters – for example, cat a.log b.log prints first a.log and then b.log, while cat b.log a.log prints first b.log and then a.log. Thus, while "filenames that match the pattern" is an (unordered) set, the actual expanded list of matching files is an ordered list, a sequence, and thus an order must be chosen, conventionally alphabetical order, however defined by the shell.

Other languages

The term "glob" is also used to refer more generally to limited pattern-matching facilities of this kind, in other contexts:

• Larry Wall's book Programming Perl discusses glob in the context of the Perl language.
• Tcl contains both true regular expression matching facilities and a more limited kind of pattern matching often described as globbing.
• Python has a glob module in the standard library which performs wildcard pattern matching on filenames. Guido van Rossum, author of the Python programming language wrote and contributed a glob() routine to BSD Unix in 1986. There were previous implementations of glob, e.g., in the ex and ftp programs in previous releases of BSD.
• Perl has a Glob extension which mimics the BSD glob routine.
• Ruby has a glob method for the Dir class which performs wildcard pattern matching on filenames. Several libraries such as Rant and Rake provide a FileList class which has a glob method or use the method FileList.[] identically.
• PHP has a glob function.

Syntax

Although there is no definite syntax for globs, common features include:

Task	Example	Unix shells	COMMAND.COM	cmd.exe	Windows Powershell	SQL
Match one unknown character	?at matches Cat, cat or bat	?	?	?	?	_
Match any number of unknown characters	Law* matches Lawyer or Lawmaker	*	*	*	*	%
Match a character as part of a group of characters	[CB]at matches Cat or Bat but not cat or mat	[characters]	N/A	N/A	[characters]	N/A
Escape character	Law\* will only match Law*	\	N/A	^	`	N/A

Globs do not include syntax for the Kleene star which allows multiple repetitions of the preceding part of the expression; thus they are not considered regular expressions, which can describe a larger set of regular languages over any given finite alphabet.

Etymology

The command interpreters of the early versions of Unix (1st through 6th Editions, 1969-75) did not expand wildcard characters in file path arguments to a command; a separate program, /etc/glob, performed the expansion and supplied the expanded list of file paths to the command for execution.

glob() is a Unix library function that expands file names using a pattern-matching notation reminiscent of regular expression syntax but without the expressive power of true regular expressions.

See also

• regular expression
• wildcard character
• wildmat