The shells provide a number of wildcards that you can use to abbreviate filenames or refer to groups of files. For example, let's say you want to delete all filenames in the current directory (1.21) ending in .txt. You could delete these files one by one, but that would be boring if there were only five and very boring if there were a hundred. Instead, you can use a wildcarded name to say, "I want all files whose names end with .txt, regardless of what the first part is." The wildcard is the "regardless" part. Like a wildcard in a poker game, a wildcard in a filename can have any value.
The wildcard you see most often is *
(asterisk), but we'll start with
something simpler: ?
(question mark).
When it appears in a filename, the
?
matches any single character. For example,
letter?
refers to any filename that begins with
letter and has one character after that. This would
include letterA, letter1, as well as filenames with a
non-printing character as their last letter, like
letter^C.
The *
wildcard matches any character or group of zero or more
characters. For example, *.txt
matches all files whose
names end with .txt, *.c
matches all files
whose names end with .c (by convention, source code for programs
in the C language), and so on.
The *
and ?
wildcards are sufficient for 90 percent of the
situations that you will find. However, there are some situations that
they can't handle. For example, you may want to list files whose
names end with .txt, mail, or let. There's no way
to do this with a single *
; it won't let you exclude the files you don't
want. In this situation, use a separate *
with each filename ending:
*.txt *mail *let
Sometimes you need to match a particular group of characters.
For example,
you may want to list all filenames that begin with digits, or all
filenames that begin with uppercase letters. Let's assume that you want
to work with the files program.
n
, where n
is a
single-digit number. Use the filename:
program.[0123456789]
In other words, the wildcard
[
character-list
]
matches any
single character that appears in the list. The character list can be
any group of ASCII characters; however, if they are consecutive (e.g.,
A-Z, a-z, 0-9, or 3-5, for that matter), you can use a hyphen as
shorthand for the range. For example, [a-zA-Z]
means any alphabetic
character.
There is one exception to these wildcarding rules. Wildcards
never match /
, which is both the name of the
filesystem root (1.19)
and
the character used to separate directory names in a
path (1.21).
If you are new to computers, you probably will catch on to UNIX
wildcarding quickly. If you have used any other computer system, you
have to watch out for one very important detail. Virtually all
computer systems except for UNIX consider a period (.
) a special
character
within a filename. Many operating systems even require a filename to
have a period in it. With these operating systems, a *
does not
match a period; you have to say *.*
. Therefore, the equivalent of
rm *
does virtually nothing on most operating systems. Under
UNIX, it is very dangerous: it means "delete all the files in the
current directory, regardless of their name." You only want to give
this command when you really mean it.
But here's the exception to the exception. The shells and the ls
command consider a
.
special if it is the first character of a filename. This is
often used to hide initialization files and other files that you
aren't normally concerned with; the ls command doesn't show
these files unless you
ask (16.11)
for them.
If a file's name begins with
.
,
you always have to type the .
explicitly. For
example, .*rc
matches all files whose names begin with .
and end with rc. This is a common convention for the names of
UNIX initialization files.
Table 1-1 has a summary of the different sorts of wildcards available.
Wildcard | Matches |
---|---|
? | Any single character |
* | Any group of zero or more characters |
[ab] | Either a or b |
[a-z] | Any character between a and z, inclusive |
Wildcards can be used at any point or points within a path.
Remember, wildcards only match names that already exist.
You can't use them to
create new files (9.4)-
though some shells have
curly braces ({}
) (9.5, 15.3)
for doing that.
Article
1.18
has more about how wildcards are handled.
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