glob (programming)

In computer programming, in particular in a Unix-like environment, glob patterns specify sets of filenames with wildcard characters. For example, the Unix command mv *.txt textfiles/ moves (mv) all files with names ending in .txt from the current directory to the directory textfiles. Here, * is a wildcard standing for "any string of characters" and *.txt is a glob pattern. The other common wildcard is the question mark (?), which stands for one character.

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,^[1] performed the expansion and supplied the expanded list of file paths to the command for execution. Its name is an abbreviation for "global command".^[2] Later, this functionality was provided as a library function, glob(), used by programs such as the shell.

Technical

Unix shell globbing operates by parameter expansion – the glob pattern (e.g., *.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.^[3]

Implementations

Unix shells such as Bash, tcsh, and zsh provide globbing on filenames at the command line and in shell scripts.^[4]

The Windows command interpreter cmd.exe relies on a runtime function in applications to perform globbing.^[5][6] Windows PowerShell Cmdlets support globbing.^[7]

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

• D has a globMatch function in the std.path module.^[8]
• Go has a Glob function in the filepath package.^[9]
• Java has a Files class containing methods that operate on glob patterns.^[10]
• Haskell has a Glob package with the main module System.FilePath.Glob. The pattern syntax is based on a subset of Zsh’s. It tries to optimize the given pattern and should be noticeably faster than a naïve character-by-character matcher.^[11]
• 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.^[12] Perl's angle brackets can be used to glob as well: <*.log>.
• PHP has a glob function.^[13]
• Python has a glob module in the standard library which performs wildcard pattern matching on filenames.^[14] and an fnmatch module with functions or matching strings or filtering lists based on these same wildcard patterns ^[15] Guido van Rossum, author of the Python programming language, wrote and contributed a glob() routine to BSD Unix in 1986.^[16] 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.^[17] 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.^[18]

Syntax

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

Some shells (like the C shell) support additional syntax including alternation or brace expansion, also known as extended globbing.

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 the full set of regular languages over any given finite alphabet.

Standard SQL uses a glob-like syntax for simple string matching in its LIKE operator. The percent sign (%) matches zero or more characters, and the underscore matches exactly one character. The term "glob" is not generally used in the SQL community, however. Many implementations of SQL have extended the LIKE operator to allow a richer pattern-matching language incorporating elements of regular expressions.

See also

• Regular expression
• Wildcard character
• Wildmat

References