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NAME
glob - globbing pathnames
DESCRIPTION
Long ago, in UNIX V6, there was a program /etc/glob that would expand wildcard patterns.
Soon afterward this became a shell built-in.
These days there is also a library routine glob(3) that will perform this function for a
user program.
The rules are as follows (POSIX.2, 3.13).
Wildcard matching
A string is a wildcard pattern if it contains one of the characters '?', '*' or '['.
Globbing is the operation that expands a wildcard pattern into the list of pathnames
matching the pattern. Matching is defined by:
A '?' (not between brackets) matches any single character.
A '*' (not between brackets) matches any string, including the empty string.
Character classes
An expression "[...]" where the first character after the leading '[' is not an '!'
matches a single character, namely any of the characters enclosed by the brackets. The
string enclosed by the brackets cannot be empty; therefore ']' can be allowed between the
brackets, provided that it is the first character. (Thus, "[][!]" matches the three
characters '[', ']' and '!'.)
Ranges
There is one special convention: two characters separated by '-' denote a range. (Thus,
"[A-Fa-f0-9]" is equivalent to "[ABCDEFabcdef0123456789]".) One may include '-' in its
literal meaning by making it the first or last character between the brackets. (Thus,
"[]-]" matches just the two characters ']' and '-', and "[--0]" matches the three
characters '-', '.', '0', since '/' cannot be matched.)
Complementation
An expression "[!...]" matches a single character, namely any character that is not
matched by the expression obtained by removing the first '!' from it. (Thus, "[!]a-]"
matches any single character except ']', 'a' and '-'.)
One can remove the special meaning of '?', '*' and '[' by preceding them by a backslash,
or, in case this is part of a shell command line, enclosing them in quotes. Between
brackets these characters stand for themselves. Thus, "[[?*\]" matches the four
characters '[', '?', '*' and '\'.
Pathnames
Globbing is applied on each of the components of a pathname separately. A '/' in a
pathname cannot be matched by a '?' or '*' wildcard, or by a range like "[.-0]". A range
cannot contain an explicit '/' character; this would lead to a syntax error.
If a filename starts with a '.', this character must be matched explicitly. (Thus, rm *
will not remove .profile, and tar c * will not archive all your files; tar c . is better.)
Empty lists
The nice and simple rule given above: "expand a wildcard pattern into the list of matching
pathnames" was the original UNIX definition. It allowed one to have patterns that expand
into an empty list, as in
xv -wait 0 *.gif *.jpg
where perhaps no *.gif files are present (and this is not an error). However, POSIX
requires that a wildcard pattern is left unchanged when it is syntactically incorrect, or
the list of matching pathnames is empty. With bash one can force the classical behavior
using this command:
shopt -s nullglob
(Similar problems occur elsewhere. E.g., where old scripts have
rm `find . -name "*~"`
new scripts require
rm -f nosuchfile `find . -name "*~"`
to avoid error messages from rm called with an empty argument list.)
NOTES
Regular expressions
Note that wildcard patterns are not regular expressions, although they are a bit similar.
First of all, they match filenames, rather than text, and secondly, the conventions are
not the same: for example, in a regular expression '*' means zero or more copies of the
preceding thing.
Now that regular expressions have bracket expressions where the negation is indicated by a
'^', POSIX has declared the effect of a wildcard pattern "[^...]" to be undefined.
Character classes and internationalization
Of course ranges were originally meant to be ASCII ranges, so that "[ -%]" stands for
"[ !"#$%]" and "[a-z]" stands for "any lowercase letter". Some UNIX implementations
generalized this so that a range X-Y stands for the set of characters with code between
the codes for X and for Y. However, this requires the user to know the character coding
in use on the local system, and moreover, is not convenient if the collating sequence for
the local alphabet differs from the ordering of the character codes. Therefore, POSIX
extended the bracket notation greatly, both for wildcard patterns and for regular
expressions. In the above we saw three types of items that can occur in a bracket
expression: namely (i) the negation, (ii) explicit single characters, and (iii) ranges.
POSIX specifies ranges in an internationally more useful way and adds three more types:
(iii) Ranges X-Y comprise all characters that fall between X and Y (inclusive) in the
current collating sequence as defined by the LC_COLLATE category in the current locale.
(iv) Named character classes, like
[:alnum:] [:alpha:] [:blank:] [:cntrl:]
[:digit:] [:graph:] [:lower:] [:print:]
[:punct:] [:space:] [:upper:] [:xdigit:]
so that one can say "[[:lower:]]" instead of "[a-z]", and have things work in Denmark,
too, where there are three letters past 'z' in the alphabet. These character classes are
defined by the LC_CTYPE category in the current locale.
(v) Collating symbols, like "[.ch.]" or "[.a-acute.]", where the string between "[." and
".]" is a collating element defined for the current locale. Note that this may be a
multicharacter element.
(vi) Equivalence class expressions, like "[=a=]", where the string between "[=" and "=]"
is any collating element from its equivalence class, as defined for the current locale.
For example, "[[=a=]]" might be equivalent to "[aáàäâ]", that is, to "[a[.a-acute.][.a-
grave.][.a-umlaut.][.a-circumflex.]]".
SEE ALSO
sh(1), fnmatch(3), glob(3), locale(7), regex(7)
COLOPHON
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