glob (programming)

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In computer programming, in particular in a Unix-like environment, the term globbing is sometimes used to refer to pattern matching based on wildcard characters.[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] Its notation is simpler than regular expressions, and without their expressive power.

Origin[edit]

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[edit]

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[edit]

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:

  • Go has a Glob function in the filepath package.[8]
  • Java has a Files class containing methods that operate on glob patterns. [9]
  • 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.[10] Perl's angle brackets can be used to glob as well: <*.log>.
  • PHP has a glob function.[11]
  • Python has a glob module in the standard library which performs wildcard pattern matching on filenames.[12] Guido van Rossum, author of the Python programming language wrote and contributed a glob() routine to BSD Unix in 1986.[13] 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.[14] 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.[15]

Syntax[edit]

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

Task Example Unix shells COMMAND.COM cmd.exe Windows PowerShell SQL (LIKE operator) SAP
Match one or zero unknown characters ?at matches at, Cat, cat, Bat or bat N/A ? ? N/A N/A N/A
Match exactly one unknown character ?at matches Cat, cat, Bat or bat, but not at ? N/A N/A ? _ +
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[16]
Match any character but the one specified [!C]at matches Bat, bat, or cat but not Cat [!characters] N/A N/A 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 the full set of regular languages over any given finite alphabet.[citation needed]

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[edit]

References[edit]

  1. ^ "First Edition Unix manual 'Miscellaneous' section (PDF)" (PDF). Retrieved 2011-05-11. 
  2. ^ 1st Edition UNIX, code.google.com, src/cmd/glob.c 
  3. ^ "The Open Group Base Specifications Issue 6, IEEE Std 1003.1, 2004 Edition, 2.13.3 'Patterns Used for Filename Expansion'". 
  4. ^ The "Advanced Bash-Scripting Guide, Chapter 19.2: Globbing" (Mendel Cooper, 2003) has a concise set of examples of filename globbing patterns.
  5. ^ "Wildcard Expansion". Microsoft Developer Network. 2013. 
  6. ^ "Expanding Wildcard Arguments". Microsoft Developer Network. 2013. 
  7. ^ "Supporting Wildcard Characters in Cmdlet Parameters". Microsoft Developer Network. 2013. 
  8. ^ "Package filepath - The Go Programming Language". Golang.org. Retrieved 2011-05-11. 
  9. ^ "File Operations". Oracle. Retrieved 2013-12-16. 
  10. ^ Contact details. "File::Glob - Perl extension for BSD glob routine". perldoc.perl.org. Retrieved 2011-05-11. 
  11. ^ "glob - Manual". PHP. 2011-05-06. Retrieved 2011-05-11. 
  12. ^ "10.7. glob — Unix style pathname pattern expansion — Python v2.7.1 documentation". Docs.python.org. Retrieved 2011-05-11. 
  13. ^ "'Globbing' library routine". Archived from the original on 2007-12-19. Retrieved 2011-05-11. 
  14. ^ "Class: Dir". Ruby-doc.org. Retrieved 2011-05-11. 
  15. ^ "TCL glob manual page". Retrieved 16 November 2011. 
  16. ^ Some proprietary extensions such as Transact-SQL provide this functionality, e.g., [characters] in as discussed in LIKE (Transact-SQL)