glob - Globbing pathnames
DESCRIPTION
Long ago, in Unix V6, there was a program /etc/glob that
would expand wildcard patterns. Soon afterwards 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 1003.2, 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, pro
vided 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 charac
ter between the brackets. (Thus, `[]-]' matches just the
two characters `]' and `-', and `[--/]' matches the three
characters `-', `.', `/'.)
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
brackets these characters stand for themselves. Thus,
`[[?*\]' matches the four characters `[', `?', `*' and
`\'.
PATHNAMES
Globbing is applied on each of the components of a path
name 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 .pro
file, 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 orig
inal 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 pat
tern is left unchanged when it is syntactically incorrect,
or the list of matching pathnames is empty. With bash one
can force the classical behaviour by setting
allow_null_glob_expansion=true.
(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 argu
ment 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: e.g., 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.
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 cod
ing in use on the local system, and moreover, is not con
venient if the collating sequence for the local alphabet
differs from the ordering of the character codes. There
fore, POSIX extended the bracket notation greatly, both
for wildcard patterns and for regular expressions. In the
above we saw three types of item 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 currect 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 ele
ment defined for the current locale. Note that this may be
a multi-character 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]' (warning: Latin-1 here), that is, to `[a[.a-
acute.][.a-grave.][.a-umlaut.][.a-circumflex.]]'.
SEE ALSO
sh(1), glob(3), fnmatch(3), locale(7), regex(7)
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