| Here we will see how
Unix/Linux File system is organized and what
commands can be used to
search for files and programs.
The first noticeable difference between Linux/Unix and a
DOS
or
Windows system is the filesystem. For starters, we do not use drive
letters to denote different partitions. Under Linux, there is one main
directory. You can relate this to the C: drive under DOS. Each
partition
on your system is mounted to a directory on the main directory. It's
kind
of like an ever-expanding hard disk.
We call the main directory the root directory, and it's
denoted with a
single slash (/). This concept may seem
strange, but it actually makes life easy for you when you want to add
more space. For example, let's say you run out of space on the drive
that has /home on it. Most people install
Linux and make one big root drive. Well, since a partition can be
mounted to any directory, you can simply go to the store and pick up a
new
hard drive and mount it to /home. You've now
“grafted” on some more space to your system. And all without having to
move many things around.
The files are organized into a tree structure
with a
root
named by the character '/'. The first few levels of the tree look
like this:
/
|
-------------------------------
/ | | | \
etc bin usr tmp dev
| | |
---- ---- ----
/ \ / \ / \
ls . csh bin lib ucb. lib
Below, you will find descriptions of the major top level
directories
under Linux.
- /bin
-
Essential user programs are stored here. These
represent
the bare
minimum set of programs required for a user to use the system.
Things like the shell and the filesystem commands (ls,
cp, and so on) are stored
here. The /bin directory usually doesn't
receive modification after installation. If it does, it's usually in
the form of package upgrades that we provide.
- /boot
-
Files that are used by the Linux Loader (LILO). This directory also receives little
modification after an installation.
- /cdrom
-
Remember that all drives have to be mounted to a
directory
on the
main root directory? Well, /cdrom
is provided as a mount point for your CD-ROM drive.
- /dev
-
Everything in Linux is treated as a file, even
hardware
devices like
serial ports, hard disks, and scanners. In order to access these
devices, a special file called a device node has to be present. All
device nodes are stored in the /dev
directory. You will find
this to be true across many UNIX-like operating systems.
- /etc
-
This directory holds system configuration files.
Everything from
the X Window configuration file, the user database, to the system
startup scripts. The system administrator will become quite
familiar with this directory over time.
- /home
-
Linux is a multiuser operating system. Each user on
the
system is
given an account and a unique directory for personal files. This
directory is called the user's “home” directory. The /home directory is provided as the default
location for user home directories.
- /lib
-
System libraries that are required for basic
operation are
stored here.
The C library, the dynamic loader, the ncurses library, and kernel
modules are among the things stored here.
- /lost+found
-
When the system boots, the filesystems are checked
for any
errors.
If errors are detected, the fsck program is run
to see if any can be corrected. The corrected parts of the filesystem
are written to the /lost+found directory.
- /mnt
-
This directory is provided as a temporary mount
point for
working
on hard disks or removable drives.
- /opt
-
Optional software packages. The idea behind /opt is that each software package installs to /opt/<software package>, which makes it
easy to remove later. Linux distributes some things in /opt (such as KDE in /opt/kde),
but you are free to add
anything you want to /opt.
- /proc
-
This is a unique directory. It's not really part of
the
filesystem, but a virtual filesystem that provides access to kernel
information. Various pieces of information that the kernel wants you to
know are conveyed to you through “files” in the /proc
directory. You can also send information to the kernel through some of
these “files”. Try doing cat /proc/cpuinfo.
- /root
-
The system administrator is known as “root” on the
system.
root's home directory is kept in /root
instead of /home/root.
The reason is simple. What if /home was a
different
partition from / and it could not be
mounted? root would naturally want to log in and repair the problem. If
his home directory was on the damaged filesystem, it would make it
difficult for him to log in.
- /sbin
-
Essential programs that are run by root and during
the
system bootup
process are kept here. Normal users will not run programs in this
directory.
- /tmp
-
The temporary storage location. All users have read
and
write access to this directory.
- /usrIntroduction to
Linux/Unix
-
This is the big directory on a Linux system.
Everything
else pretty
much goes here, programs, documentation, the kernel source code, and
the X Window system. This is the directory to which you will most
likely be installing programs.
- /var
-
System log files, cache data, and program lock files
are
stored
here. This is the directory for frequently-changing data.unix
2. File Types
There are four types of files in the Unix file system.
2.1 Ordinary Files
An ordinary file may contain text, a program, or other
data.
It can be either an ASCII file, with each of its bytes being in the
numerical range 0 to 127, i.e. in the 7-bit range, or a binary file,
whose bytes can be of all possible values 0 to 255, in the 8-bit range.
2.2 Directory Files
Suppose that in the directory x I have a, b and c, and
that b
is a
directory, containing files u and v. Then b can be viewed not only
as a directory, containing further files, but also as a file itself.
The file b consists of information about the directory
b; i.e. the file b has information stating
that the directory b has files u and v, how large they are, when
they were last modified, etc.1
2.3 Device Files
In Unix, physical devices (printers, terminals etc.) are
represented as ``files.'' This seems odd at first, but it really makes
sense: This way,
the same read() and write() functions used to read and
write real files can also be used to read from and write to these
devices.
2.4 Link Files
Suppose we have a file X, and type
ln X Y
If we then run ls, it will appear that a new file, Y, has been
created, as a copy of X, as if we had typed
cp X YIntroduction to Linux/Unix
However, the difference is the cp does create a new file, while
ln merely gives an alternate name to an old file. If
we
make Y
using ln, then Y is merely a new name for the same physical
file
X.
3. Obtaining Information
About
the Files in a Given Directory
The `a' (``all'') and `l' (``long'') options of the ls
command
will give us a lot of information about files in a specified directory
(if we don't specify a directory, then the current directory is
assumed). Here is a sample output from typing
root@gaurav:/ # ls -la
total 60
drwxr-xr-x 24 root root 640
2005-04-14 12:00 .
drwxr-xr-x 24 root root 640
2005-04-14 12:00 ..
drwxr-xr-x 2 root root 2320
2005-04-16 01:19 bin
drwxr-xr-x 3 root root
696 2005-04-16 10:05 boot
drwxr-xr-x 12 root root 14280 2005-04-16 10:08 dev
drwxr-xr-x 11 root root 39136 2005-04-14 13:05
.dev
drwxr-xr-x 116 root root 6432 2005-04-16 10:15 etc
drwxr-xr-x 3 root root
72 2005-04-14 04:59 home
drwxr-xr-x 15 root root 4376 2005-04-14
16:11 lib
drwxr-xr-x 10 root root 240
2005-04-15 23:18 mnt
drwxr-xr-x 2 root root
48 2005-04-14 04:50 opt
dr-xr-xr-x 118 root root
0 2005-04-16 15:37 proc
drwxr-xr-x 27 root root 1544 2005-04-16
10:14 root
drwx------ 2 root
root 48 2005-04-14 04:48 .reiserfs_priv
drwxr-xr-x 2 root root 6752
2005-04-16 01:19 sbin
drwxr-xr-x 2 root root
48 2005-04-14 04:50 srv
drwxr-xr-x 10 root root
0 2005-04-16 15:37 sys
drwxrwxrwt 11 root root 416 2005-04-16 10:35 tmp
drwxr-xr-x 12 root root 312
2005-04-14 04:51 usr
drwxr-xr-x 16 root root 384
2005-04-14 13:09 var
Suppose you want to get a listing of the hidden files
in the
current directory. This command will do just that:
root@gaurav:/ # ls -a
. boot .dev
plugins root sys var
.. cdrom etc mnt proc
sbin tmp
bin dev home
lib
opt .reiserfs_priv srv usr
The output is separated into six columns:
1st column - access permissions
2nd column - number of file entries (in the case of directory files)
3rd column - owner
4th column - size in bytes
5th column - date and time of last modification
6th column - name
Files beginning with a period (called “dot files”) are
“hidden” when you run ls. You will only see
them if you pass the -a option.
There are many more options that can be found in the online manual
page. Don't forget that you can combine options that you pass to ls
You now know what each directory holds, but it still
doesn't
really help
you find things. I mean, you could go looking through directories, but
there are quicker ways. There are four main file search commands
available in Slackware.
The first is the which(1)
command. which is usually used to locate a
program quickly.
It just searches your PATH and returns the
first instance it finds and the directory path to it. Take this example:
$ which bash
/bin/bash
From that you see that bash
is in
the /bin directory. This is a very
limited command for searching, since it only searches your PATH.
The whereis(1) command works
similar
to which, but can also search for man pages and
source files. A whereis search for bash should
return this:
$ whereis bash
bash: /bin/bash /usr/bin/bash /usr/man/man1/bash.1.gz
This command not only told us where the actual program,
but
also where the
online documentation is stored. Still, this command is limited. What if
you wanted to search for a specific configuration file? You can't use which or whereis
for that.
The find(1) command will search
for
anything. I want to search the entire system for the default xinitrc file on the system.
$ find / -name xinitrc
./var/X11R6/lib/xinit/xinitrc
find will take a while to run,
since it
has to traverse the entire root directory tree. And if you run this
command as a normal user, you will probably get permission denied error
messages for directories that only
root can see. But find found our file, so
that's good. If only it could be a bit faster...
The locate(1) command searches
the
entire filesystem, just like the find command can do, but it searches a
database instead of the actual filesystem. The database is set to
automatically update at 4:40AM, so you have a somewhat fresh listing of
files on your system. You can manually run updatedb(1)
to update the locate database (before running updatedb
by hand, you must first su to
the nobody user). Here's an example
of locate in action:
$ locate xinitrc
/var/X11R6/lib/xinit/xinitrc
/var/X11R6/lib/xinit/xinitrc.fvwm2
/var/X11R6/lib/xinit/xinitrc.openwin
/var/X11R6/lib/xinit/xinitrc.twm
We got more than what we were looking for, and quickly
too.
With these
commands, you should be able to find whatever you're looking for on
your
Linux system.
References:
http://slackware.com
http://heather.cs.ucdavis.edu/~matloff/UnixAndC/Unix/FileSyst.html
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