Linux 版 (精华区)
发信人: netiscpu (说不如做), 信区: Linux
标 题: [B] Red Hat Linux Unleashed (8)
发信站: 紫 丁 香 (Sat Jul 25 03:09:28 1998), 转信
The Linux File System
_________________________________________________________________
o Files An Overview
# Common Types of Files
# Filenames
o Directories An Overview
# Parent Directories and Subdirectories
# The Root Directory
# How Directories Are Named
# The Home Directory
o Navigating the Linux File System
# The pwd Command Where Am I?
# Absolute and Relative Filenames
# Going Places The cd Command
# There's No Place Like Home
o Creating and Deleting Files
# cat That Useful Feline
# Creating Directories
# Moving and Copying Files
# Moving and Copying with Wildcards
# Moving Directories
o Removing Files and Directories
# Removing Directories
o File Permissions and Ownership
# File and Directory Ownership
# File Permissions
# Changing File Permissions
# Changing Directory Permissions
o Miscellaneous File Commands
# Fear of Compression The Zipless File
# How to tar Without Feathering
o Important Directories in the Linux File System
# /
# /home
# /bin
# /usr
# /usr/bin
# /var/spool
# /dev
# /usr/sbin
# /sbin
# /etc
o Summary
_________________________________________________________________
8
The Linux File System
In this chapter, you learn about:
* Files: what they are, types of files, filenames
* Directories: what they are, parent directories and subdirectories,
directory names, your home directory
* Absolute and relative file and directory names
* Moving between directories using the cd command
* Using the cat command to create a new file
* Creating directories
* Moving and copying files
* Removing files and directories
* File and directory ownership, using chown and chgrp to change
ownership
* File and directory permissions, using chmod to change permissions
* Using the gunzip command to uncompress .gz files compressed by
gzip
* The tar command
* The standard Linux directories and directory structure
To understand how Linux works, and to use the system beyond a
superficial level, you must be familiar with the Linux notion of files
and the file system into which they are organized.
Files An Overview
The most basic concept of a file, and one you may already be familiar
with from other computer systems, defines a file as a distinct chunk
of information that is found on your hard drive. Distinct means that
there can be many different files, each with its own particular
contents. To keep files from getting confused with each other, every
file must have a unique identity. In Linux, you identify each file by
its name and location. In each location or directory, there can be
only one file by a particular name. So, for instance, if you create a
file called novel, and you get a second great idea, you will either
have to call it something different, such as novel2, or put it in a
different place, to keep from overwriting the contents already in your
original novel.
Common Types of Files
Files can contain various types of information. The following three
types will become the most familiar to you:
* User data: Information that you create and update. The very
simplest user data is plain text or numbers. You learn to create
these simple files later in this chapter. More complicated user
data files might have to be interpreted by another program to make
sense. For instance, a spreadsheet file looks like gibberish if
you look at it directly. To work with a spreadsheet, you have to
start up the spreadsheet program and read in the spreadsheet file.
* System data: Information, often in plain text form, that is read
and used by the Linux system—to keep track of which users are
allowed on the system, for instance. As a system administrator,
you are responsible for changing system data files. For instance,
when you create a new user, you modify the file /etc/passwd, which
contains the user information. Ordinary users of the system are
usually not concerned with system data files, except for their
private startup files.
* Executable files: These files contain instructions that your
computer can perform. This set of instructions is often called a
program. When you tell the computer to perform them, you're
telling it to execute the instructions given to it. To human eyes,
executable files contain meaningless gibberish—obviously your
computer doesn't think the way you do! Creating or modifying
executable files takes special tools. You learn how to use these
programming tools in Part V, "Linux for Programmers."
Filenames
Linux allows filenames to be up to 256 characters long. These
characters can be lower- and uppercase letters, numbers, and other
characters, usually the dash (-), the underscore (_), and the dot (.).
They can't include reserved metacharacters such as the asterisk,
question mark, backslash, and space, because these all have meaning to
the shell. We met some metacharacters when we discussed wildcards in
the previous chapter. Other metacharacters will be introduced in the
Linux shell chapters.
Directories An Overview
Linux, like many other computer systems, organizes files in
directories. You can think of directories as file folders and their
contents as the files. However, there is one absolutely crucial
difference between the Linux file system and an office filing system.
In the office, file folders usually don't contain other file folders.
In Linux, file folders can contain other file folders. In fact, there
is no Linux "filing cabinet"—just a huge file folder that holds
some files and other folders. These folders contain files and possibly
other folders in turn, and so on.
Parent Directories and Subdirectories
Imagine a scenario in which you have a directory, A, that contains
another directory, B. Directory B is then a subdirectory of directory
A, and directory A is the parent directory of directory B. You will
see these terms often, both in this book and elsewhere.
The Root Directory
In Linux, the directory that holds all the other directories is called
the root directory. This is the ultimate parent directory; every other
directory is some level of subdirectory.
From the root directory, the whole structure of directory upon
directory springs and grows like some electronic elm. This is called a
tree structure because, from the single root directory, directories
and subdirectories branch off like tree limbs.
How Directories Are Named
Directories are named just like files, and they can contain upper- and
lowercase letters, numbers, and characters such as -, ., and _.
The slash (/) character is used to show files or directories within
other directories. For instance, usr/bin means that bin is found in
the usr directory. Note that you can't tell, from this example,
whether bin is a file or a directory, although you know that usr must
be a directory because it holds another item—namely, bin. When
you see usr/bin/grep, you know that both usr and bin must be
directories, but again, you can't be sure about grep. The ls program
shows directories with a following /—for example, fido/. This
notation implies that you could have, for instance, fido/file;
therefore, fido must be a directory.
The root directory is shown simply by the symbol / rather than
mentioned by name. It's very easy to tell when / is used to separate
directories and when it's used to signify the root directory. If / has
no name before it, it stands for the root directory. For example, /usr
means that the usr subdirectory is found in the root directory, and
/usr/bin means that bin is found in the usr directory and that usr is
a subdirectory of the root directory. Remember, by definition the root
directory can't be a subdirectory.
The Home Directory
Linux provides each user with his or her own directory, called the
home directory. Within this home directory, users can store their own
files and create subdirectories. Users generally have complete control
over what's found in their home directories. Because there are usually
no Linux system files or files belonging to other users in your home
directory, you can create, name, move, and delete files and
directories as you see fit.
______________________________________________________________
NOTE: Your home directory does not provide privacy! Normally, any
user can go into another's home directory and read (and copy!) the
files stored there (although he can't delete or change the files).
When Linux creates your home directory, it in effect provides you
with an open office cubicle whose desk and filing cabinet drawers
are unlocked.
You must lock up everything you want to keep private. (This topic
is covered in the section "File Permissions and Ownership.") It is
generally considered rude or nosy to poke around in someone else's
home directory, just as it's rude or nosy to poke around in
someone's office while they're away from their desk, but the world
is full of nosy and rude people, so you must take precautions!
Note that anyone logged in as root can read and manipulate all the
files on the system, including files that users have locked up. If
you can't trust the system administrator (who usually has the root
password), don't use the system!
______________________________________________________________
The location of a user's home directory is specified by Linux and
can't be changed by the user. This is both to keep things tidy and to
preserve system security.
Navigating the Linux File System
Fortunately, navigating the Linux file system is simple. There are
only two commands to be learned, and one of them has absolutely no
options or parameters!
The pwd Command Where Am I?
Type pwd at the Linux command prompt. You see
darkstar:~$ pwd
/home/fido
darkstar:~$
This tells you that you're currently in the directory /home/fido. (If
you are logged in under a different user name, you will see that name
in place of fido.) This is your home directory. When you log in, Linux
always places you in your home directory.
The letters "pwd" stand for "print working directory." Again, a
command's name or function has been cut down to a few easy-to-type
characters. (You will often see the term current directory used in
place of working directory.)
You might be wondering what "working directory" or "being in a
directory" really means. It simply means that Linux commands, by
default, perform their actions in your working directory. For
instance, when you run ls, you are shown only the files in your
working directory. If you want to create or remove files, they will be
created or removed in your working directory.
Absolute and Relative Filenames
If you specify only the name of a file, Linux looks for that file in
your working directory. For example, more myfile lets you read the
contents of the file myfile. But myfile must be in your current
working directory, or the more command won't find it.
Sometimes you want to specify a file that isn't in your current
directory. You would then specify the name of the directory the file
is in, as well as the name of the file itself.
If, for instance, your current directory has a subdirectory called
novel, which contains a file called chapter_1, you could type more
novel/chapter_1, which tells more that it should look in the
subdirectory novel for the file chapter_1. This is called a relative
filename. You are specifying the location of chapter_1 relative to
where you are now, in the subdirectory novel, which is found in your
current directory. If you changed your working directory, the relative
filename would no longer work.
Two special directory specifications are "." and "..".The
specification "." always stands for the directory you are currently
in, and ".." stands for the parent directory of your current
directory. (You see how "." and ".." are used later in this chapter.)
Any filename that includes "." or ".." is, by definition, a relative
filename.
A filename that is valid from any location is called an absolute
filename. Absolute filenames always begin with /, signifying root. So
if you specify a filename as /home/fido/novel/chapter_1, there is no
doubt as to where the file is located. Every file on your system has a
unique absolute filename.
Someone else on the system might also have a directory called novel in
his or her home directory. Perhaps it even contains a file called
chapter_1. In this case, you can't distinguish the two files by using
the relative filename novel/chapter_1. However, the absolute filenames
will be different—for instance, /home/fido/novel/chapter_1 as
opposed to /home/mary/novel/chapter_1. The novel subdirectory in
/home/fido is not the same directory as the novel directory in
/home/mary! The two are in quite separate locations, and only
coincidentally do they share the same name.
Going Places The cd Command
The cd (change directory) command lets you change your working
directory. You can think of it as moving to another directory.
The syntax of the cd command is
cd <directory specification>
There must be a space between cd and the directory specification.
The directory specification can be an absolute or relative one. For
instance, type cd .. followed by pwd:
darkstar:~$ cd ..
darkstar:/home$ pwd
/home
darkstar:/home$ cd ..
darkstar:/$ pwd
/
darkstar:/$ cd ..
darkstar:/$ pwd
/
There is no parent directory for the root directory, so typing cd ..
when in the root directory simply leaves you in the root directory.
Note that the Linux command prompt shows you which directory you are
currently in, so you don't have to type pwd all the time. (I will
continue to use pwd for clarity.)
You can also use absolute directory names.
darkstar:/$ cd /usr/bin
darkstar:/usr/bin$ pwd
/usr/bin
When you type an absolute directory name, you go to that directory, no
matter where you started from. When you type cd .., where you end up
depends on where you started.
To see the effect of changing your working directory, type ls. The
list of files is so long that the first part scrolls off your screen.
The ls command shows you the contents of your current directory (as
always), but now your current directory is /usr/bin, which contains
many more files than your home directory.
There's No Place Like Home
Type cd without any directory specification:
darkstar:/usr/bin$ cd
darkstar:~$ pwd
/home/fido
Typing cd by itself always returns you to your home directory. When
exploring the file system, you sometimes wind up deep in a blind alley
of subdirectories. Type cd to quickly return home, or type cd / to
return to the root directory.
The ~ in your prompt is another special character. It stands for your
home directory. There's no reason to type cd ~ when cd works just as
well, and is much easier to type! However, try this:
When you type cd ~<user>, you move to that user's home directory. This
is a very useful trick, especially on large systems with many users
and more complicated directory structures than the simple /home/<user>
on your Linux system.
______________________________________________________________
NOTE: When you're changing to a distant directory, it's often a
good idea to take several steps. If you mistype a very long
directory specification, you will have to retype the entire
specification. Sometimes it might not even be clear why cd gave you
an error! Taking a number of shorter steps means less retyping in
case of an error. Consider this example:
darkstar:~$ cd /usr/docs/faq/unix
bash: /usr/docs/faq/unix: No such file or directory
You're pretty sure that this path is correct. Let's change
directories one step at a time:
darkstar:~$ cd /usr
darkstar:/usr$ cd docs
bash: docs: No such file or directory
Aha! There's a problem with docs. The directory is actually named
doc:
darkstar:/usr$ ls
bin/ doc/ games/ info/ man/ sbin/ spool/
darkstar:/usr$ cd doc
darkstar:/usr/doc$ cd faq/unix
darkstar:/usr/doc/faq/unix$ pwd
/usr/doc/faq/unix
______________________________________________________________
Creating and Deleting Files
Linux has many ways to create and delete files. In fact, some of the
ways are so easy to perform that you have to be careful not to
accidentally overwrite or erase files!
______________________________________________________________
NOTE: Go through the following sections very carefully. You should
be logged in as your "ordinary" username, not as root! Only when
you're sure you understand these sections thoroughly should you use
these commands while logged in as root.
There is no "unerase" command in Linux! Be sure you know what
you're doing!
______________________________________________________________
Return to your home directory by typing cd. Make sure you're in your
/home/<user> directory by running pwd.
In the previous chapter, you created a file by typing ls -l /bin >
test. Remember, the > symbol means "redirect all output to the
following filename." Note that the file test didn't exist before you
typed this command. When you redirect to a file, Linux automatically
creates the file if it doesn't already exist.
What if you want to type text into a file, rather than some command's
output? The quick and dirty way is to use the command cat.
cat That Useful Feline
The cat command is one of the simplest, yet most useful, commands in
Linux. It certainly does more than any living feline!
The cat command basically takes all its input and outputs it. By
default, cat takes its input from the keyboard and outputs it to the
screen. Type cat at the command line:
darkstar:~$ cat
The cursor moves down to the next line, but nothing else seems to
happen. Now cat is waiting for some input:
hello
hello
what
what
asdf
asdf
Everything you type is repeated on-screen as soon as you press Enter!
How do you get out of this? At the start of a line, type ^D (Ctrl-D).
(In other words, hold down the Ctrl key and press D.) If you're not at
the beginning of a line, you have to type ^D twice. ^D is the Linux
"end of file" character. When a program such as cat encounters a ^D,
it assumes that it has finished with the current file, and it goes on
to the next one. In this case, if you type ^D by itself on an empty
line, there is no next file to go on to, and cat exits.
______________________________________________________________
NOTE: When you say that a program exits, you mean that it has
finished running and that you are back at the Linux command prompt.
It might seem odd to talk about the program exiting when, from your
point of view as a user, you have exited the program. This turn of
phrase goes back to the early days of UNIX, when it was coined by
the people who were programming the system. They looked at things
from the program's point of view, not the user's!
______________________________________________________________
So how do you use cat to create a file? Simple! You redirect the
output from cat to the desired filename:
darkstar:~$ cat > newfile
Hello world
Here's some text
You can type as much as you want. When you are finished, press ^D by
itself on a line; you will be back at the Linux prompt.
Now you want to look at the contents of newfile. You could use the
more or less commands, but instead, let's use cat. Yes, you can use
cat to look at files simply by providing it with a filename:
darkstar:~$ cat newfile
Hello world
Here's some text
darkstar:~$
Neat! You can also add to the end of the file by using >>. Whenever
you use >>, whether with cat or any other command, the output is
always appended to the specified file. (Note that the ^D character
does not appear on-screen. I show it in the examples for clarity.)
darkstar:~$ cat >> newfile
Some more lines
^D
darkstar:~$ cat newfile
Hello world
Here's some text
Some more lines
darkstar:~$
To discover what cat actually stands for, let's first create another
file.
darkstar:~$ cat > anotherfile
Different text
^D
darkstar:~$
Now, try this:
darkstar:~$ cat newfile anotherfile> thirdfile
darkstar:~$ cat thirdfile
Hello world
Here's some text
Some more lines
Different text
darkstar:~$
cat stands for concatenate; cat takes all the specified inputs and
regurgitates them in a single lump. This by itself would not be very
interesting, but combine it with the forms of input and output
redirection available in Linux and you have a powerful and useful
tool.
Sometimes you want to change just one line of a file, or perhaps you
are creating a large and complicated file. For this you should use one
of the editing programs available in Linux. They are discussed in
Chapter 16, "Text Editors."
Creating Directories
To create a new directory, use the mkdir command. The syntax is mkdir
<name>, where <name> is replaced by whatever you want the directory to
be called. This creates a subdirectory with the specified name in your
current directory:
darkstar:~$ ls
anotherfile newfile thirdfile
darkstar:~$ mkdir newdir
darkstar:~$ ls
anotherfile newdir/ newfile thirdfile
______________________________________________________________
NOTE: The mkdir command is already familiar to you if you have used
MS-DOS systems. In MS-DOS, you can abbreviate mkdir as md. You
might think that md would work in Linux, because, after all, most
of the commands we've seen have extremely concise names. However,
Linux doesn't recognize md; it insists on the full mkdir.
If you frequently switch between Linux and MS-DOS, you might want
to use mkdir for both systems. However, be warned that you might
start typing other Linux commands in MS-DOS—for example,
typing ls instead of dir!
______________________________________________________________
Moving and Copying Files
You often need to move or copy files. The mv command moves files, and
the cp command copies files. The syntax for the two commands is
similar:
mv <source> <destination>
cp <source> <destination>
As you can see, mv and cp are very simple commands. Here's an example:
darkstar:~$ ls
anotherfile newdir/ newfile thirdfile
darkstar:~$ mv anotherfile movedfile
darkstar:~$ ls
movedfile newdir/ newfile thirdfile
darkstar:~$ cp thirdfile xyz
darkstar:~$ ls
anotherfile newdir/ newfile thirdfile xyz
You can use cat (or more or less) at any time to verify that
anotherfile became movedfile, and that the contents of file xyz are
identical to the contents of thirdfile.
It can get more confusing if you're moving or copying files from one
directory to another. This is because a file's real name includes its
absolute path—for instance, /home/fido/newfile. However, Linux
lets you leave off parts of the file's name, because it's more
convenient to refer to newfile rather than /home/fido/newfile.
For instance, suppose you want to move newfile into the newdir
subdirectory. If you want the file to keep the same name, you type
darkstar:~$ mv newfile newdir/newfile
However, it's much more common to type
darkstar:~$ mv newfile newdir
Here, because you have typed a directory name for the destination,
Linux assumes that you want the file to be placed in the specified
directory.
You could also use cd to change to the directory you want to move the
file to:
darkstar:~$ cd newdir
darkstar:~newdir$ copy ../newfile .
This example is a bit less intuitive than the first two! You specify
that the source is ../newfile, which means "the file newfile in the
current directory's parent directory." The destination you simply
specify as ".", which is short for "the current directory." In other
words, you're telling mv to "go up one level, grab newfile, and move
it to right here." Because this is less intuitive, you might find
yourself automatically pushing a file from your current directory to
another directory rather than pulling a file from another directory
into your current directory.
You can also change the name of the file while moving or copying it to
another directory. The following is just one possible way:
darkstar:~$ cp newfile newdir/anothername
This would create a copy of newfile in the directory newdir and name
the copied file anothername.
______________________________________________________________
NOTE: When moving or copying files between directories, you should
always double-check that the file's destination directory exists
and verify the directory's name. Otherwise, the results of your
command can be unexpected, as the following two examples show.
If in the example just shown you mistyped newdir—for instance,
as mv newfile mewdir—you would wind up with a file called
mewdir in your current directory and no file newfile in the newdir
subdirectory!
Another way you would get an unexpected result would be to type cp
newfile newdir if you didn't realize that the directory newdir
existed. In this case, you would be expecting to create an
identical file called newdir in your current directory. What you
would actually do is create a copy of newfile, called newfile, in
the subdirectory newdir.
______________________________________________________________
The mv command is much more efficient than the cp command. When you
use mv, the file's contents are not moved at all; rather, Linux makes
a note that the file is to be found elsewhere within the file system's
structure of directories.
When you use cp, you are actually making a second physical copy of
your file and placing it on your disk. This can be slower (although
for small files, you won't notice any difference), and it causes a bit
more wear and tear on your computer. Don't make copies of files when
all you really want to do is move them!
Moving and Copying with Wildcards
If you have 20 files in a directory, and you want to copy them to
another directory, it would be very tedious to use the cp command on
each one. Fortunately, you can use the wildcards * and ? to copy more
than one file at a time.
If you want to move or copy all files in a directory, use the wildcard
*:
darkstar:~$ cp * /tmp
This command copies every file in your current directory to the
directory /tmp.
You can use *, along with other characters, to match only certain
files. For instance, suppose you have a directory that contains the
files book1, book_idea, book-chapter-1, and poem.book. To copy just
the first three files, you could type cp book* /tmp. When you type
book*, you are asking Linux to match all files whose names start with
book. In this case, poem.book does not start with book, so there is no
way book* can match it. (Note that if your filename were book.poem,
book* would match it.)
______________________________________________________________
NOTE: As you saw at the outset, mv and cp are very simple commands.
It's specifying the files that's the complicated part! If things
still seem confusing, don't worry. Even experts sometimes mess up
"simple" moves and copies. Follow the examples and try any
different ways you think of. There is a definite logic as to how
the files to be moved and copied should be specified. It takes a
while to become familiar with this logic, and you will have to
practice a while before these things become intuitive.
______________________________________________________________
Moving Directories
To move a directory, use the mv command. The syntax is mv <directory>
<destination>. In the following example, you would move the newdir
subdirectory found in your current directory to the /tmp directory:
darkstar:~$ mv newdir /tmp
darkstar:~$ cd /tmp
darkstar:/tmp$ ls
/newdir
The directory newdir is now a subdirectory of /tmp.
______________________________________________________________
NOTE: When you move a directory, all its files and subdirectories
go with it.
______________________________________________________________
Removing Files and Directories
Now that you know how to create files and directories, it's time to
learn how to undo your handiwork.
To remove (or delete) a file, use the rm command (rm is a very terse
spelling of remove). The syntax is rm <filename>. For instance:
darkstar:~$ rm dead_duck
removes the file dead_duck from your current directory.
darkstar:~$ rm /tmp/dead_duck
removes the file dead_duck from the /tmp directory.
darkstar:~$ rm *
removes all files from your current directory. (Be careful when using
wildcards!)
darkstar:~$ rm /tmp/*duck
removes all files ending in duck from the /tmp directory.
______________________________________________________________
NOTE: As soon as a file is removed, it is gone! Always think about
what you're doing before you remove a file. You can use one of the
following techniques to keep out of trouble when using wildcards.
1. Run ls using the same file specification you use with the rm
command. For instance:
darkstar:~$ ls *duck
dead_duck guiduck lame-duck
:~$ rm *duck
NOTE: In this case, you thought you wanted to remove all files that
matched *duck. To verify that this indeed was the case, you listed
all the *duck files (wildcards work the same way with all
commands). The listing looked okay, so you went ahead and removed
the files.
2. Use the i (interactive) option with rm:
______________________________________________________________
darkstar:~$ rm -i *duck
rm: remove 'dead_duck'? y
rm: remove 'guiduck'? n
rm: remove 'lame-duck'? y
darkstar:~$
______________________________________________________________
NOTE: When you use rm -i, the command goes through the list of
files to be deleted one by one, prompting you for the OK to remove
the file. If you type y or Y, rm removes the file. If you type any
other character, rm does not remove it. The only disadvantage of
using this interactive mode is that it can be very tedious when the
list of files to be removed is long.
______________________________________________________________
Removing Directories
The command normally used to remove (delete) directories is rmdir. The
syntax is rmdir <directory>.
Before you can remove a directory, it must be empty (the directory
can't hold any files or subdirectories). Otherwise, you see
rmdir: <directory>: Directory not empty
This is as close to a safety feature as you will see in Linux!
______________________________________________________________
NOTE: This one might mystify you:
darkstar:/home$ ls
fido/ root/ zippy/
darkstar:/home$ ls zippy
core kazoo stuff
darkstar:/home$ rm zippy/*
darkstar:/home/zippy$ ls zippy
darkstar:/home$ rmdir zippy
rmdir: zippy: Directory not empty
darkstar:~$
The reason for the Directory not empty message is that files
starting with . usually are special system files and are usually
hidden from the user. To list files whose names start with ., you
have to use ls -a. To delete these files, use rm .*:
darkstar:/home$ ls -a zippy
./ ../ .bashrc .profile
darkstar:/home$ rm zippy/.*
rm: cannot remove '.' or '..'
darkstar:/home$ ls -a zippy
./ ../
darkstar:/home$ rmdir zippy
darkstar:/home$ ls
fido/ root/
darkstar:~$
You will most often come across this situation in a system
administrator role.
______________________________________________________________
Sometimes you want to remove a directory with many layers of
subdirectories. Emptying and then deleting all the subdirectories one
by one would be very tedious. Linux offers a way to remove a directory
and all the files and subdirectories it contains in one easy step.
This is the r (recursive) option of the rm command. The syntax is rm
-r <directory>. The directory and all its contents are removed.
______________________________________________________________
NOTE: You should use rm -r only when you really have to. To
paraphrase an old saying, "It's only a shortcut until you make a
mistake." For instance, if you're logged in as root, the following
command removes all files from your hard disk, and then it's
"Hello, installation procedure" time (do not type the following
command!):
rm -rf /
Believe it or not, people do this all too often. Don't join the
club!
______________________________________________________________
File Permissions and Ownership
All Linux files and directories have ownership and permissions. You
can change permissions, and sometimes ownership, to provide greater or
lesser access to your files and directories. File permissions also
determine whether a file can be executed as a command.
If you type ls -l or dir, you see entries that look like this:
-rw-r—r— 1 fido users 163 Dec 7 14:31 myfile
The -rw-r—r— represents the permissions for the file myfile.
The file's ownership includes fido as the owner and users as the
group.
File and Directory Ownership
When you create a file, you are that file's owner. Being the file's
owner gives you the privilege of changing the file's permissions or
ownership. Of course, once you change the ownership to another user,
you can't change the ownership or permissions anymore!
File owners are set up by the system during installation. Linux system
files are owned by IDs such as root, uucp, and bin. Do not change the
ownership of these files.
Use the chown (change ownership) command to change ownership of a
file. The syntax is chown <owner> <filename>. In the following
example, you change the ownership of the file myfile to root:
darkstar:~$ ls -l myfile
-rw-r—r— 1 fido users 114 Dec 7 14:31 myfile
darkstar:~$ chown root myfile
darkstar:~$ ls -l myfile
-rw-r—r— 1 root users 114 Dec 7 14:31 myfile
To make any further changes to the file myfile, or to chown it back to
fido, you must use su or log in as root.
Files (and users) also belong to groups. Groups are a convenient way
of providing access to files for more than one user but not to every
user on the system. For instance, users working on a special project
could all belong to the group project. Files used by the whole group
would also belong to the group project, giving those users special
access. Groups normally are used in larger installations. You may
never need to worry about groups.
The chgrp command is used to change the group the file belongs to. It
works just like chown.
File Permissions
Linux lets you specify read, write, and execute permissions for each
of the following: the owner, the group, and "others" (everyone else).
read permission enables you to look at the file. In the case of a
directory, it lets you list the directory's contents using ls.
write permission enables you to modify (or delete!) the file. In the
case of a directory, you must have write permission in order to
create, move, or delete files in that directory.
execute permission enables you to execute the file by typing its name.
With directories, execute permission enables you to cd into them.
For a concrete example, let's look at myfile again:
-rw-r—r— 1 fido users 163 Dec 7 14:31 myfile
The first character of the permissions is -, which indicates that it's
an ordinary file. If this were a directory, the first character would
be d. There are also some other, more exotic classes. These are beyond
the scope of this chapter.
The next nine characters are broken into three groups of three, giving
permissions for owner, group, and other. Each triplet gives read,
write, and execute permissions, always in that order. Permission to
read is signified by an r in the first position, permission to write
is shown by a w in the second position, and permission to execute is
shown by an x in the third position. If the particular permission is
absent, its space is filled by -.
In the case of myfile, the owner has rw-, which means read and write
permissions. This file can't be executed by typing myfile at the Linux
prompt.
The group permissions are r—, which means that members of the
group "users" (by default, all ordinary users on the system) can read
the file but not change it or execute it.
Likewise, the permissions for all others are r—: read-only.
File permissions are often given as a three-digit number—for
instance, 751. It's important to understand how the numbering system
works, because these numbers are used to change a file's permissions.
Also, error messages that involve permissions use these numbers.
The first digit codes permissions for the owner, the second digit
codes permissions for the group, and the third digit codes permissions
for other (everyone else).
The individual digits are encoded by summing up all the "allowed"
permissions for that particular user as follows:
read permission 4
write permission 2
execute permission 1
Therefore, a file permission of 751 means that the owner has read,
write, and execute permission (4+2+1=7), the group has read and
execute permission (4+1=5), and others have execute permission (1).
If you play with the numbers, you quickly see that the permission
digits can range between 0 and 7, and that for each digit in that
range there's only one possible combination of read, write, and
execute permissions.
______________________________________________________________
NOTE: If you're familiar with the binary system, think of rwx as a
three-digit binary number. If permission is allowed, the
corresponding digit is 1. If permission is denied, the digit is 0.
So r-x would be the binary number 101, which is 4+0+1, or 5.
—x would be 001, which is 0+0+1, which is 1, and so on.
______________________________________________________________
The following combinations are possible:
0 or —-: No permissions at all
4 or r—: read-only
2 or -w-: write-only (rare)
1 or —x: execute
6 or rw-: read and write
5 or r-x: read and execute
3 or -wx: write and execute (rare)
7 or rwx: read, write, and execute
______________________________________________________________
NOTE: Anyone who has permission to read a file can then copy that
file. When a file is copied, the copy is owned by the person doing
the copying. He or she can then change ownership and permissions,
edit the file, and so on.
______________________________________________________________
______________________________________________________________
NOTE: Removing write permission from a file doesn't prevent the
file from being deleted! It does prevent it from being deleted
accidentally, since Linux asks you whether you want to override the
file permissions. You have to answer y, or the file will not be
deleted.
______________________________________________________________
Changing File Permissions
To change file permissions, use the chmod (change [file] mode)
command. The syntax is chmod <specification> file.
There are two ways to write the permission specification. One is by
using the numeric coding system for permissions:
darkstar:~$ ls -l myfile
-rw-r—r— 1 fido users 114 Dec 7 14:31 myfile
darkstar:~$ chmod 345 myfile
darkstar:~$ ls -l myfile
—wxr—r-x 1 fido users 114 Dec 7 14:31 myfile
darkstar:~$ chmod 701 myfile
darkstar:~$ ls -l myfile
-rwx——x 1 root users 114 Dec 7 14:31 myfile
This method has the advantage of specifying the permissions in an
absolute, rather than relative, fashion. Also, it's easier to tell
someone "Change permissions on the file to seven-five-five" than to
say "Change permissions on the file to read-write-execute,
read-execute, read-execute."
You can also use letter codes to change the existing permissions. To
specify which of the permissions to change, type u (user), g (group),
o (other), or a (all). This is followed by a + to add permissions or a
- to remove them. This in turn is followed by the permissions to be
added or removed. For example, to add execute permissions for the
group and others, you would type
darkstar:~$ chmod go+r myfile
Other ways of using the symbolic file permissions are described in the
chmod man page.
Changing Directory Permissions
You change directory permissions with chmod, exactly the same way as
with files. Remember that if a directory doesn't have execute
permissions, you can't cd to it.
______________________________________________________________
NOTE: Any user who has write permission in a directory can delete
files in that directory, whether or not that user owns or has write
privileges to those files.
Most directories, therefore, have permissions set to drwxr-xr-x.
This ensures that only the directory's owner can create or delete
files in that directory.
It is especially dangerous to give write permission to all users
for directories!
______________________________________________________________
Miscellaneous File Commands
There are many Linux commands to manipulate files, directories, and
the entire file system. Many of these commands are used only by system
administrators. You will touch on a few that are also used by ordinary
users. These and other important system administrator commands are
further detailed in Chapter 37, "System Administration Basics."
Fear of Compression The Zipless File
Most Linux files are stored on the installation CD-ROM in compressed
form. This allows more information to be stored.
When you installed Linux, the installation program uncompressed many
of the files it transferred to your hard drive. However, if you look,
you will be able to find compressed files!
Any file ending in .gz—for example, squashed.gz—is a
compressed file. To uncompress this particular type of file, type
gunzip <file>. For this example, you would type gunzip squashed.gz.
The gunzip program creates an uncompressed file and removes the .gz
extension. Therefore, you would wind up with a normal file called
squashed.
To compress a file, use the gzip command. Typing gzip squashed would
compress squashed and rename it squashed.gz.
Another type of compressed file you might see ends with the extension
.zip. Use unzip to uncompress these files. To create files of this
type, use zip.
How to tar Without Feathering
In almost any location with several Linux or UNIX systems, sooner or
later you will hear someone say, "Put that in a tar file and send it
over."
They are referring to the output created by the tar program. Although
tar stands for tape archive, it can copy files to floppy disk or to
any filename you specify in the Linux file system. The tar command is
used because it can archive files and directories into a single file
and then recreate the files and even the directory structures later.
It's also the easiest way to place Linux files on a floppy disk.
To create a tar file, you typically type tar cvf <destination>
<files/directories>, where files/directories specifies the files and
directories to be archived, and destination is where you want the tar
file to be created. If you want the destination to be a floppy disk,
you usually type /dev/fd0 as the destination. This specifies your
primary floppy drive (A: in MS-DOS). You can use a floppy disk that's
been formatted under MS-DOS.
______________________________________________________________
NOTE: When tar archives to a floppy disk, all the data already on
the disk is destroyed. You have to reformat it to use it with
MS-DOS again.
______________________________________________________________
To extract a tar file, you typically type tar xvf <tar file>. For
instance, to pull files from a floppy disk, you would type tar xvf
/dev/fd0.
______________________________________________________________
NOTE: Unlike gzip, tar doesn't remove, delete, or rename files it
puts into the archive. However, when tar extracts archived files,
it overwrites existing files with files of the same name from the
archive.
______________________________________________________________
Important Directories in the Linux File System
Most of the directories that hold Linux system files are "standard."
Other UNIX systems will have identical directories with similar
contents. This section summarizes some of the more important
directories on your Linux system.
/
This is the root directory. It holds the actual Linux program, as well
as subdirectories. Do not clutter this directory with your files!
/home
This directory holds users' home directories. In other UNIX systems,
this can be the /usr or /u directory.
/bin
This directory holds many of the basic Linux programs. bin stands for
binaries, files that are executable and that hold text only computers
could understand.
/usr
This directory holds many other user-oriented directories. Some of the
most important are described in the following sections. Other
directories found in /usr include
docs Various documents, including useful Linux information
man The man pages accessed by typing man <command>
games The fun stuff!
/usr/bin
This directory holds user-oriented Linux programs.
/var/spool
This directory has several subdirectories. mail holds mail files,
spool holds files to be printed, and uucp holds files copied between
Linux machines.
/dev
Linux treats everything as a file! The /dev directory holds devices.
These are special files that serve as gateways to physical computer
components. For instance, if you copy to /dev/fd0, you're actually
sending data to the system's floppy disk. Your terminal is one of the
/dev/tty files. Partitions on the hard drive are of the form /dev/hd0.
Even the system's memory is a device!
A famous device is /dev/null. This is sometimes called the bit bucket.
All information sent to /dev/null vanishes—it's thrown into the
trash.
/usr/sbin
This directory holds system administration files. If you do an ls -l,
you see that you must be the owner, root, to run these commands.
/sbin
This directory holds system files that are usually run automatically
by the Linux system.
/etc
This directory and its subdirectories hold many of the Linux
configuration files. These files are usually text, and they can be
edited to change the system's configuration (if you know what you're
doing!).
Summary
You should now feel more comfortable working in Linux. Understanding
and being able to navigate the Linux file system is very important,
since Linux really does consist simply of some files organized in a
fairly standard way.
You still might find yourself stumped by certain file or directory
problems. Remember that the online man pages can assist you. Linux
gives you a lot of flexibility in creating files, specifying absolute
or relative names, and setting permissions. Don't be afraid to
experiment (as an ordinary user, in your home directory). There are
too many different ways to perform tasks to list or exhaustively
describe here. Don't cling to rigid recipes written on a piece of
paper. You learn by trying!
You should go on to Chapters 9, "Introduction to the GNU Project
Utilities," through 12, "Using tcsh," especially if you want to create
programs or macros from system command files or learn more about the
built-in user interface features in Linux shells. Chapter 16 has some
very useful information about editing text files.
Once you are familiar with shells and have had some practice
manipulating files and directories, you can move on to the advanced
topics in Part III, "Editing, Typesetting, and More," through Part
VIII, "Advanced Programming Topics," of this book.
--
Enjoy Linux!
-----It's FREE!-----
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