glob - globbing pathnames
Long ago, in UNIX V6, there was a program /etc/glob
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).
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.
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 '!'.)
There is one special convention: two characters separated by '-' denote a range.
(Thus, " [A-Fa-f0-9]
" is equivalent to "
".) 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
" matches the three characters '-', '.', '0', since
'/' cannot be matched.)
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 '\'.
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
". A range containing an explicit '/' character is
syntactically incorrect. (POSIX requires that syntactically incorrect patterns
are left unchanged.)
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 .
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
one can force the classical behavior using this command:
shopt -s nullglob
(Similar problems occur elsewhere. For example, 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.)
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
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
", 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.]
", 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 "
", that is, to "
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