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Filename globbing allows you to provide more than one
filename to a command without having to write all filenames in full. You
use special characters for this, called 'wildcards'.
Say you want to delete all files in a directory that
end with the string '.bak' using the 'rm' command'. Instead of typing each
filename as an argument to 'rm', you use the '*' wildcard:
rm *.bak
'*' matches zero or more characters. In this example,
you tell the shell to expand the argument to the 'rm' command to "all file
names ending on or consisting of the string '.bak'", the shell does so and
passes that expanded argument to the 'rm' command.
As you will see, it is important to note that the shell
reads and interprets the command line before the command does it.
This has the advantage that you can use wildcards with (almost) all shell
commands which take strings (file names, directory names, search strings
etc) for an argument.
Let's play a bit more with the '*' wildcard. You have
a directory which contains the files '124.bak', '346.bak' and '583.bak'.
You want to keep '583.bak'. What you do is this
rm *4*.bak
The shell expands '*4*.bak' to "all file names which
contain the number '4' and the end on the string '.bak'".
Notice that rm 4*.bak would not have worked, since this would
only have encompassed file names beginning with the number '4'.
Since there are no such files in this directory, the shell expands this pattern
to an empty string and 'rm' issues an error message:
rm: cannot remove `4*.bak': No such file or directory
Now you want to keep the file '346.bak' but delete '124.bak'
and '583.bak'. That's trickier since the files which are to be deleted have
nothing in common except the ending. But lucky as you are, you can also define
files by what they not have:
rm *[!6].bak
This reads: delete all files which end on the string
'.bak' exceptfor those which end on the string '6.bak'. You have
to put the negation sign '!' and the character to be negated (here '6') into
brackets, because otherwise the shell interprets the exclamation mark as
the beginning of a history substitution . Negation works with all globbing
patterns introduced in this article.
Notice that it is very easy to shoot yourself
in the foot with the '*' wildcard and negation. Guess what
rm *[!6]*.bak
does? It deletes all the files, even the one
which does contain a '6' in its filename. If you put '*' wildcards before
and after a negation, it renders the negation practically useless, because
the shell expands this to "all file names which do not have that character
at any given position". In our example, the only
file name on which that pattern would not have matched, would have been '666.bak'.
The second wildcard is the question mark, '?'. In a
globbing pattern, a question mark represents exactly one character. To demonstrate
its use, let's add two new files to the three example files, '311.bak~' and
'some.text'. Now list all files, which have exactly four characters after
the dot:
ls *.????
does this. The question mark wildcard is also a useful
means to avoid the 'negation trap' mentioned above:
rm *[!4]?.*
This expands to "all files except for those with a '4'
in the second to last position before the dot" and deletes all files except
for '346.bak'.
Is there more? You bet. So far, you've only seen globbing
patterns which match one character at a certain position. But nothing keeps
you from matching more than one:
ls [13]*
lists all files which either begin with the character
'1' or the character '3'; in our test case the file '124.bak', '311.bak~'
and '346.bak' match. Notice that you have to enclose the pattern in brackets,
otherwise the pattern would match only files which begin with the string
'13'.
Now all that's left for ultimate happiness to ensue
is the possibility to define ranges of matches:
ls *[3-8]?.*
lists all files whose second to last character before
the dot is a number between '3' and '8'. In our example, this matches the
files '346.bak' and '583.bak'.
Quoting Special Shell Characters
These powerful mechanisms have one drawback, though:
the shell will always try to expand them, and it will do so before the command.
There are several cases in which this can get in your way:
-
File names with special characters. Assume you have
another file in that directory with the name '!56.bak'. Try to match it with
a globbing pattern:
rm !*
rm
rm: too few arguments
The shell interprets '!*' as a history substitution
('insert all arguments from previous command'), not as a globbing pattern.
-
Commands which take special characters as arguments
themselves. A row of Linux command line tools like (e)grep, sed, awk, find
and locate for example use their own set of what is then called 'regular
expressions'. These expressions may look strikingly similar to globbing patterns
but are in some cases interpreted differently.
But in order to enable the command to interpret them in the first place,
the shell must be prevented from interpreting them as globbing patterns first:
find . -name [1-9]* -print
find: paths must precede expression
Properly:
find . -name '[1-9]*' -print
./346.bak
./124.bak
./583.bak
./311.bak~
You can quote such special characters like !, $, ? or
the empty space either with a back slash:
ls \!*
!56.bak
or with (single) quotes
ls '!'*
!56.bak
Notice that using quotes may need some deliberation
on where to put them.ls '!*' would look for file called '!*' since
the second wildcard is now quoted, too, and thus interpreted literally.
 section index  top
The Unix philosophy is to have many small programs,
each excelling at a certain task. Complex tasks are not fulfilled by complex
programs but by tying together a bunch of programs with a handful of shell
mechanisms. One of them is redirecting output.
Redirecting between two or more commands
This is done via 'pipes', denoted by the pipe symbol
|. The syntax is
command1 | command2 | command3
etc
You've certainly seen them already. They are often used
to direct the output of a program to a pager like 'more' or 'less'.
ls -l | less
The first command provides the directory listing and
the second displays it in a scrollable manner. A more complex example:
rpm -qa | grep ^x | less
The first command puts together a list of all installed
RPMs, the second filters ('grep') those that start ('^') with an 'x' and
the third displays the results in a paged and scrollable list.
Redirecting from or into files
Sometimes you want to save the output of a command in
a file or feed it from a file. This is done via the operators '>' and
'<'.
command > file
saves the output of command in file
overwriting all previous content of file:
ls > dirlist
saves the listing of the current directory to a file
called 'dirlist'.
command < file
uses file as the input for command:
sort < dirlist > sdirlist
feeds the content of 'dirlist' to the 'sort' command,
which sorts it and puts the sorted output into the 'sdirlist' file. Of course,
if you're clever, you'd do that in one step:
ls | sort > sdirlist
A special case is 'command 2> file'.
This puts just the error messages of command into file.
You may need that from time to time...
Another operator is '>>'. This one appends the output to a
existent file:
echo "string" >> file
This would append string to the content of
the filefile. A quick way to edit a file without opening an editor
first!
There is an important restriction to the '<' and '>' operators, though:
Something like
command < file1 >file1
will erase the content of file1. However
command < file1 >>
file1
will work fine and append the processed content of file1
to the same file.
That's a lot isn't it? ;-) No need to panic, you can
learn everything step by step at your own pace. Practice makes perfect ...
Once you are familiar with the most common shell mechanisms, you might feel
the urge to customize your environment. You will find some ideas on the next
two pages. On the last page, you'll also find a short FAQ dealing with the
most common shell error messages and some minor configuration settings.
section index top
 Customizing:
Shell configuration files, the prompt, $PATH
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