glob (programming)

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

Origin

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.

Technical

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.

Implementations

Unix shells such as Bash, tcsh, and zsh provide globbing on filenames at the command line and in shell scripts.

Windows shells such as cmd.exe and Windows PowerShell do not glob, but instead rely on the called program or Cmdlet to perform globbing.

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

• Go has a Glob function in the filepath module
• Perl has both a glob function (as discussed in Larry Wall's book Programming Perl) and a Glob extension which mimics the BSD glob routine. Perl's angle brackets can be used to glob as well: <*.log>.
• PHP has a glob function.
• 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.
• 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.
• Tcl contains both true regular expression matching facilities and a more limited kind of pattern matching often described as globbing.

Syntax

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

Task	Example	Unix shells	COMMAND.COM	cmd.exe	Windows PowerShell	SQL (within string literals)	SAP
Match one unknown character	?at matches Cat, cat, Bat or bat	?	?	?	?	_	+
Match any number of unknown characters	Law* matches Law, Laws, or Lawyer	*	*	*	*	%	*
Match a character as part of a group of characters	[CB]at matches Cat or Bat but not cat or bat	[characters]	N/A	N/A	[characters]	N/A	N/A
Escape character	Law\* will only match Law*	\	N/A	^	`	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.^{[citation needed]}

See also

• Regular expression
• Wildcard character
• Wildmat