This page is provided as a convenient way to download. It should also be helpful for people on dial-up to have one big page to read. A list of the other “AllOfs” can be found at http://linuxbasics.org/course/book/allallofs.

Please take into considerations that once you save this page, it cannot keep up with the dynamics of the wiki. If you find something to be corrected or improved, you need to check or edit the actual current wiki-page.

The current version of this document can be found at http://linuxbasics.org/course/book/index.

This book is licensed and may be distributed under the terms of the GNU Free Documentation License. A copy of the license can be found at http://linuxbasics.org/course/book/allof_appendix

Usually, these snapshots are updated daily between 05:00 and 05:15 UTC

This snapshot was created: Sun Jul 20 06:08:07 CEST 2008

• 3.1. General overview of the Linux file system
  • 3.1.1. Files
  • 3.1.2. About partitioning
  • 3.1.3. More file system layout
• 3.2. Orientation in the file system
  • 3.2.1. The path
  • 3.2.2. Absolute and relative paths
  • 3.2.3. The most important files and directories
  • 3.2.4. The most important configuration files
  • 3.2.5. The most common devices
  • 3.2.6. The most common variable files
• 3.3. Manipulating files
  • 3.3.1. Viewing file properties
  • 3.3.2. Creating and deleting files and directories
  • 3.3.3. Finding files
  • 3.3.4. More ways to view file content
  • 3.3.5. Linking files
• 3.4. File security
  • 3.4.1. Access rights - Linux's first line of defense
  • 3.4.2. The tools
• 3.5. Summary
• 3.6. Exercises
  • 3.6.1. Partitions
  • 3.6.2. Paths
  • 3.6.3. Tour of the system
  • 3.6.4. Manipulating files
  • 3.6.5. File permissions
  • Answer Key

After the initial exploration in Chapter 2, we are ready to discuss the files and directories on a Linux system in more detail. Many users have difficulties with Linux because they lack an overview of what kind of data is kept in which locations. We will try to shine some light on the organization of files in the file system.

We will also list the most important files and directories and use different methods of viewing the content of those files, and learn how files and directories can be created, moved and deleted.

After completion of the exercises in this chapter, you will be able to:

• Describe the layout of a Linux file system
• Display and set paths
• Describe the most important files, including kernel and shell
• Find lost and hidden files
• Create, move and delete files and directories
• Display contents of files
• Understand and use different link types
• Find out about file properties and change file permissions

---

Prev: Exercises
Home
Next: General overview of the Linux file system

A simple description of the UNIX system, also applicable to Linux, is this:

“On a UNIX system, everything is a file; if something is not a file, it is a process.”

This statement is true because there are special files that are more than just files (named pipes and sockets, for instance), but to keep things simple, saying that everything is a file is an acceptable generalization. A Linux system, just like UNIX, makes no difference between a file and a directory, since a directory is just a file containing names of other files. Programs, services, texts, images, and so forth, are all files. Input and output devices, and generally all devices, are considered to be files, according to the system.

In order to manage all those files in an orderly fashion, one may think of them in an ordered tree-like structure on the hard disk, as we know from MS-DOS (Disk Operating System) for instance. The large branches contain more branches, and the branches at the end contain the tree’s leaves or normal files. For now we will use this image of the tree, but we will find out later why this is not a fully accurate image.

Most files are just files, called regular files; they contain normal data, for example text files, executable files or programs, input for or output from a program and so on.

While it is reasonably safe to suppose that everything you encounter on a Linux system is a file, there are some exceptions.

• Directories: files that are lists of other files.
• Special files: the mechanism used for input and output. Most special files are in /dev, we will discuss them later.
• Links: a system to make a file or directory visible in multiple parts of the system’s file tree. We will talk about links in detail.
• (Domain) sockets: a special file type, similar to TCP/IP sockets, providing inter-process networking protected by the file system’s access control.
• Named pipes: act more or less like sockets and form a way for processes to communicate with each other, without using network socket semantics.

The -l option to ls displays the file type, using the first character of each input line:

jaime:~/Documents> ls -l
total 80
-rw-rw-r--   1 jaime   jaime   31744 Feb 21 17:56 intro Linux.doc
-rw-rw-r--   1 jaime   jaime   41472 Feb 21 17:56 Linux.doc
drwxrwxr-x   2 jaime   jaime    4096 Feb 25 11:50 course

This table gives an overview of the characters determining the file type:

Table 3-1. File types in a long list

Symbol	Meaning
-	Regular file
d	Directory
l	Link
c	Special file
s	Socket
p	Named pipe (FIFO)
b	Block device

In order not to always have to perform a long listing for seeing the file type, a lot of systems by default don’t issue just ls, but ls -F, which suffixes file names with one of the characters / = * | @ to indicate the file type. (See Table 3.6 for suffix meaning.) To make it extra easy on the beginning user, both the -F and --color options are usually combined, see Section 3.3.1.1.
We will use ls -F throughout this document for better readability.

As a user, you only need to deal directly with plain files, executable files, directories and links. The special file types are there for making your system do what you demand from it and are dealt with by system administrators and programmers.

Now, before we look at the important files and directories, we need to know more about partitions.

Most people have a vague knowledge of what partitions are, since every operating system has the ability to create or remove them. It may seem strange that Linux uses more than one partition on the same disk, even when using the standard installation procedure, so some explanation is called for.

One of the goals of having different partitions is to achieve higher data security in case of disaster. By dividing the hard disk in partitions, data can be grouped and separated. When an accident occurs, only the data in the partition that got the hit will be damaged, while the data on the other partitions will most likely survive.

This principle dates from the days when Linux didn’t have journaled file systems and power failures might have led to disaster. The use of partitions remains for security and robustness reasons, so a breach on one part of the system doesn’t automatically mean that the whole computer is in danger. This is currently the most important reason for partitioning. A simple example: a user creates a script, a program or a web application that starts filling up the disk. If the disk contains only one big partition, the entire system will stop functioning if the disk is full. If the user stores the data on a separate partition, then only that (data) partition will be affected, while the system partitions and possibly other data partitions keep functioning.

Mind that having a journaled file system only provides data security in case of power failure and sudden disconnection of storage devices. This does not protect your data against bad blocks and logical errors in the file system. In those cases, you should use a RAID (Redundant Array of Inexpensive Disks) solution.

There are two major kinds of partitions on a Linux system:

• data partition: normal Linux system data, including the root partition containing all the data to start up and run the system; and
• swap partition: expansion of the computer’s physical memory, extra memory on hard disk.

Most systems contain a root partition, one or more data partitions and one or more swap partitions. Systems in mixed environments may contain partitions for other system data, such as a partition with a FAT or VFAT file system for MS Windows data.

Most Linux systems use fdisk at installation time to set the partition type. As you may have noticed during the exercise from Chapter 1, this usually happens automatically. On some occasions, however, you may not be so lucky. In such cases, you will need to select the partition type manually and even manually do the actual partitioning. The standard Linux partitions have number 82 for swap and 83 for data, which can be journaled (ext3) or normal (ext2, on older systems). The fdisk utility has built-in help, should you forget these values.

Apart from these two, Linux supports a variety of other file system types, such as the relatively new Reiser file system, JFS, NFS, FATxx and many other file systems natively available on other (proprietary) operating systems. When choosing one, be aware of their respective strengths and weaknesses (for instance, a maximum file size of 2 GB for FAT32, or 4.2 GB for some others).

The standard root partition (indicated with a single forward slash, /) is about 100-500 MB, and contains the system configuration files, most basic commands and server programs, system libraries, some temporary space and the home directory of the administrative user (/root). A standard installation requires about 250 MB for the root partition. Note that this assumes that non-system critical data resides on separate partitions. Many, if not most, Linux users today will have a much larger root partition, because they will not have many separate partitions. Each distribution will have different space requirements depending on its type and purpose. You should always consult the distribution’s web site or documentation when planning partitions for an install.

Swap space (indicated with swap) is only accessible for the system itself, and is hidden from view during normal operation. Swap is the system that ensures, like on normal UNIX systems, that you can keep on working, whatever happens. On Linux, you will virtually never see irritating messages like “Out of memory, please close some applications first and try again”, because of this extra memory. The swap or virtual memory procedure has long been adopted by operating systems outside the UNIX world.

Using memory on a hard disk is naturally slower than using the real memory chips of a computer, but having this little extra is a great comfort. We will learn more about swap when we discuss Processes in Chapter 4.

Linux generally counts on having twice the amount of physical memory in the form of swap space on the hard disk. When installing a system, you have to know how you are going to do this. An example on a system with 512 MB of RAM:

• 1st possibility: one swap partition of 1 GB
• 2nd possibility: two swap partitions of 512 MB
• 3rd possibility: with two hard disks: 1 partition of 512 MB on each disk.

The last option will give the best results when a lot of I/O is to be expected.

Read the software documentation for specific guidelines. Some applications, such as databases, might require more swap space. Others, such as some handheld systems, might not have any swap at all for lack of a hard disk. Swap space may also depend on your kernel version.

The kernel is on a separate partition as well in many distributions, because it is the most important file of your system. If this is the case, you will find that you also have a /boot partition, holding your kernel(s) and accompanying data files.

The rest of the hard disk(s) is generally divided in data partitions, although it may be that all of the non-system critical data resides on one partition, for example when you perform a standard workstation installation. When non-critical data is separated on different partitions, it usually happens following a set pattern:

• a partition for user programs (/usr)
• a partition containing the users’ personal data (/home)
• a partition to store temporary data like print- and mail-queues (/var)
• a partition for third party and extra software (/opt)

Once the partitions are made, you can only add more. Changing sizes or properties of existing partitions is possible but not advisable.

The division of hard disks into partitions is determined by the system administrator. On larger systems, he or she may even spread one partition over several hard disks, using the appropriate software. Most distributions allow for standard setups optimized for workstations (average users) and for general server purposes, but also accept customized partitions. During the installation process you can define your own partition layout using either your distribution specific tool, which is usually a straightforward graphical interface, or fdisk, a text-based tool for creating partitions and setting their properties.

A workstation or client installation is for use by mainly one and the same person. The selected software for installation reflects this and the stress is on common user packages, such as nice desktop themes, development tools, client programs for E-mail, multimedia software, web and other services. Everything is put together on one large partition, swap space twice the amount of RAM is added and your generic workstation is complete, providing the largest amount of disk space possible for personal use, but with the disadvantage of possible data integrity loss during problem situations.

On a server, system data tends to be separate from user data. Programs that offer services are kept in a different place than the data handled by this service. Different partitions will be created on such systems:

• a partition with all data necessary to boot the machine
• a partition with configuration data and server programs
• one or more partitions containing the server data such as database tables, user mails, an ftp archive etc.
• a partition with user programs and applications
• one or more partitions for the user specific files (home directories)
• one or more swap partitions (virtual memory)

Servers usually have more memory and thus more swap space. Certain server processes, such as databases, may require more swap space than usual; see the specific documentation for detailed information. For better performance, swap is often divided into different swap partitions.

All partitions are attached to the system via a mount point. The mount point defines the place of a particular data set in the file system. Usually, all partitions are connected through the root partition. On this partition, which is indicated with the slash (/), directories are created. These empty directories will be the starting point of the partitions that are attached to them. An example: given a partition that holds the following directories:

  videos/		cd-images/	pictures/ 

We want to attach this partition in the filesystem in a directory called /opt/media. In order to do this, the system administrator has to make sure that the directory /opt/media exists on the system. Preferably, it should be an empty directory. How this is done is explained later on in this chapter. Then, using the mount command, the administrator can attach the partition to the system. When you look at the content of the formerly empty directory /opt/media, it will contain the files and directories that are on the mounted medium (hard disk or partition of a hard disk, CD, DVD, flash card, USB or other storage device).

During system startup, all the partitions are thus mounted, as described in the file /etc/fstab. Some partitions are not mounted by default, for instance if they are not constantly connected to the system, such as the storage used by your digital camera. If well configured, the device will be mounted as soon as the system notices that it is connected, or it can be user-mountable, i.e. you don’t need to be system administrator to attach and detach the device to and from the system. There is an example in Section 9.3.

On a running system, information about the partitions and their mount points can be displayed using the df command (which stands for disk full or disk free). In Linux, df is the GNU version, and supports the -h or human readable option which greatly improves readability. Note that commercial UNIX machines commonly have their own versions of df and many other commands. Their behavior is usually the same, though GNU versions of common tools often have more and better features.

The df command only displays information about active non-swap partitions. These can include partitions from other networked systems, like in the example below where the home directories are mounted from a file server on the network, a situation often encountered in corporate environments.

freddy:~> df -h
Filesystem          Size  Used Avail Use% Mounted on
/dev/hda8           496M  183M  288M  39% /
/dev/hda1           124M  8.4M  109M   8% /boot
/dev/hda5            19G   15G  2.7G  85% /opt
/dev/hda6           7.0G  5.4G  1.2G  81% /usr
/dev/hda7           3.7G  2.7G  867M  77% /var
fs1:/home           8.9G  3.7G  4.7G  44% /.automount/fs1/root/home

For convenience, the Linux file system is usually thought of as a tree structure. On a standard Linux system you will find the layout generally follows the scheme presented below.

Figure 3-1. Linux file system layout

/─┬─bin
  ├─boot─┬─grub
  │      └─[...]
  ├─cdrom (*)
  ├─dev─┬─bus
  │     ├─input
  │     ├─disk
  │     └─[...]
  ├─etc─┬─default
  │     ├─rc?.d                                         
  │     ├─init.d
  │     ├─X11
  │     └─[...]
  ├─home─┬─anita    
  │      ├─jisao    
  │      ├─sam      
  │      └─stw
  ├─lib      
  ├─lost+found
  ├─media─┬─cdrom
  │       ├─dvdrecorder
  │       └─usbstick
  ├─mnt (*)
  ├─opt
  ├─proc─┬─1920
  │      ├─loadavg
  │      └─[...]
  ├─root
  ├─sbin
  ├─srv─┬─www
  │     ├─samba
  ├─tmp
  ├─usr─┬─bin
  │     ├─sbin
  │     ├─local
  │     └─share
  └─[...]

This is a layout from a Ubuntu system. Depending on the system admin, the operating system and the mission of the UNIX machine, the structure may vary, and directories may be left out or added at will. The names are not even required; they are only a convention.

Directories marked with (*) are not used commonly anymore. Mounts for removable devices (like /cdrom) are moved to /media. /mnt is used as a convenient mountpoint for “quick manual mounts”, not as location for devices.

The tree of the file system starts at the trunk or slash, indicated by a forward slash (/). This directory, containing all underlying directories and files, is also called the root directory or the root of the file system.

Directories that are only one level below the root directory are often preceded by a slash, to indicate their position and prevent confusion with other directories that could have the same name. When starting with a new system, it is always a good idea to take a look in the root directory. Let’s see what you could run into:

emmy:~> cd /
emmy:/> ls
bin/   dev/  home/    lib/         misc/  opt/     root/  tmp/  var/
boot/  etc/  initrd/  lost+found/  mnt/   proc/    sbin/  usr/

Table 3-2. Subdirectories of the root directory

Directory	Content
/bin	Common programs, shared by the system, the system administrator and the users.
/boot	The startup files and the kernel, vmlinuz. In some recent distributions also grub data. Grub is the GRand Unified Boot loader and is an attempt to get rid of the many different boot-loaders we know today.
/dev	Contains references to all the CPU peripheral hardware, which are represented as files with special properties.
/etc	Most important system configuration files are in /etc, this directory contains data similar to those in the Control Panel in Windows. While it is easy to remember that /etc contains Editable Text Configuration, Dennis Ritchie - one of the designers of the UNIX-system - says that it was short for "et cetera". And he should know.
/home	Home directories of the common users.
/initrd	(on some distributions) Information for booting. Do not remove!
/lib	Library files, includes files for all kinds of programs needed by the system and the users.
/lost+found	Every partition has a lost+found in its upper directory. Files that were saved during failures are here.
/misc	For miscellaneous purposes.
/media	Standard mount point for external file systems, e.g. a CD-ROM or a digital camera.
/opt	Typically contains extra and third party software.
/proc	A virtual file system containing information about system resources. More information about the meaning of the files in proc is obtained by entering the command man proc in a terminal window. The file proc.txt discusses the virtual file system in detail.
/root	The administrative user’s home directory. Mind the difference between /, the root directory and /root, the home directory of the root user.
/sbin	Programs for use by the system and the system administrator.
/tmp	Temporary space for use by the system, cleaned upon reboot, so don’t use this for saving any work!
/usr	Programs, libraries, documentation etc. for all user-related programs.
/var	Storage for all variable files and temporary files created by users, such as log files, the mail queue, the print spooler area, space for temporary storage of files downloaded from the Internet, or to keep an image of a CD before burning it.

How can you find out which partition a directory is on? Using the df command with a dot (.) as an option shows the partition the current directory belongs to, and informs about the amount of space used on this partition:

sandra:/lib> df -h .
Filesystem            Size  Used Avail Use% Mounted on
/dev/hda7             980M  163M  767M  18% /

As a general rule, every directory under the root directory is on the root partition, unless it has a separate entry in the full listing from df (or df -h with no other options).

Read more in man hier.

For most users and for most common system administration tasks, it is enough to accept that files and directories are ordered in a tree-like structure. The computer, however, doesn’t understand a thing about trees or tree-structures.

Every partition has its own file system. By imagining all those file systems together, we can form an idea of the tree-structure of the entire system, but it is not as simple as that. In a file system, a file is represented by an inode, a kind of serial number containing information about the actual data that makes up the file: to whom this file belongs, and where is it located on the hard disk.

Every partition has its own set of inodes; throughout a system with multiple partitions, files with the same inode number can exist.

Each inode describes a data structure on the hard disk, storing the properties of a file, including the physical location of the file data. When a hard disk is initialized to accept data storage, usually during the initial system installation process or when adding extra disks to an existing system, a fixed number of inodes per partition is created. This number will be the maximum amount of files, of all types (including directories, special files, links etc.) that can exist at the same time on the partition. We typically count on having 1 inode per 2 to 8 kilobytes of storage.

At the time a new file is created, it gets a free inode. In that inode is the following information:

• Owner and group owner of the file.
• File type (regular, directory, …)
• Permissions on the file Section 3.4.1
• Date and time of creation, last read and change.
• Date and time this information has been changed in the inode.
• Number of links to this file (see later in this chapter).
• File size
• An address defining the actual location of the file data.

The only information not included in an inode, is the file name and directory. These are stored in the special directory files. By comparing file names and inode numbers, the system can make up a tree-structure that the user understands. Users can display inode numbers using the -i option to ls. The inodes have their own separate space on the disk.

---

Prev: About files and the file system
Home
Next: Orientation in the file system

The kernel is the heart of the system. It manages the communication between the underlying hardware and the peripherals. The kernel also makes sure that processes and daemons (server processes) are started and stopped at the exact right times. The kernel has a lot of other important tasks, so many that there is a special kernel-development mailing list on this subject only, where huge amounts of information are shared. It would lead us too far to discuss the kernel in detail. For now it suffices to know that the kernel is the most important file on the system.

When I was looking for an appropriate explanation on the concept of a shell, it gave me more trouble than I expected. All kinds of definitions are available, ranging from the simple comparison that “the shell is the steering wheel of the car”, to the vague definition in the Bash manual which says that “bash is an sh-compatible command language interpreter,” or an even more obscure expression, “a shell manages the interaction between the system and its users”. A shell is much more than that.

A shell can best be compared with a way of talking to the computer, a language. Most users do know that other language, the point-and-click language of the desktop. But in that language the computer is leading the conversation, while the user has the passive role of picking tasks from the ones presented. It is very difficult for a programmer to include all options and possible uses of a command in the GUI-format. Thus, GUIs are almost always less capable than the command or commands that form the backend.

The shell, on the other hand, is an advanced way of communicating with the system, because it allows for two-way conversation and taking initiative. Both partners in the communication are equal, so new ideas can be tested. The shell allows the user to handle a system in a very flexible way. An additional asset is that the shell allows for task automation.

Just like people know different languages and dialects, the computer knows different shell types:

• sh or Bourne Shell: the original shell still used on UNIX systems and in UNIX related environments. This is the basic shell, a small program with few features. When in POSIX-compatible mode, bash will emulate this shell.
• bash or Bourne Again shell: the standard GNU shell, intuitive and flexible. Probably most advisable for beginning users while being at the same time a powerful tool for the advanced and professional user. On Linux, bash is the standard shell for common users. This shell is a so-called superset of the Bourne shell, a set of add-ons and plug-ins. This means that the Bourne Again shell is compatible with the Bourne shell: commands that work in sh, also work in bash. However, the reverse is not always the case. All examples and exercises in this book use bash.
• csh or C shell: the syntax of this shell resembles that of the C programming language. Sometimes asked for by programmers.
• tcsh or Turbo C shell: a superset of the common C shell, enhancing user-friendliness and speed.
• ksh or the Korn shell: sometimes appreciated by people with a UNIX background. A superset of the Bourne shell; with standard configuration a nightmare for beginning users.

The file /etc/shells gives an overview of known shells on a Linux system:

mia:~> cat /etc/shells
/bin/bash
/bin/sh
/bin/tcsh
/bin/csh

Fake Bourne shell

• Note that /bin/sh is usually a link to Bash, which will execute in Bourne shell compatible mode when called on this way.

Your default shell is set in the /etc/passwd file, like this line for user mia:

mia:L2NOfqdlPrHwE:504:504:Mia Maya:/home/mia:/bin/bash

To switch from one shell to another, just enter the name of the new shell in the active terminal. The system finds the directory where the name occurs using the PATH settings, and since a shell is an executable file (program), the current shell activates it and it gets executed. A new prompt is usually shown, because each shell has its typical appearance:

mia:~> tcsh
[mia@post21 ~]$

If you don’t know which shell you are using, either check the line for your account in /etc/passwd or type the command

echo $SHELL

Your home directory is your default destination when connecting to the system. In most cases it is a subdirectory of /home, though this may vary. Your home directory may be located on the hard disk of a remote file server; in that case your home directory may be found in /nethome/your_user_name. In another case the system administrator may have opted for a less comprehensible layout and your home directory may be on /disk6/HU/07/jgillard.

Whatever the path to your home directory, you don’t have to worry too much about it. The correct path to your home directory is stored in the HOME environment variable, in case some program needs it. With the echo command you can display the content of this variable:

orlando:~> echo $HOME
/nethome/orlando

You can do whatever you like in your home directory. You can put as many files in as many directories as you want, although the total amount of data and files is naturally limited because of the hardware and size of the partitions, and sometimes because the system administrator has applied a quota system. Limiting disk usage was common practice when hard disk space was still expensive. Nowadays, limits are almost exclusively applied in large environments. You can see for yourself if a limit is set using the quota command:

pierre@lamaison:/> quota -v
Diskquotas for user pierre (uid 501): none

In case quotas have been set, you get a list of the limited partitions and their specific limitations. Exceeding the limits may be tolerated during a grace period with fewer or no restrictions at all. Detailed information can be found using the info quota or man quota commands.

No Quota?

• If your system can not find the quota, then no limitation of file system usage is being applied.

Your home directory is indicated by a tilde (~), shorthand for /path_to_home/user_name. This same path is stored in the HOME variable, so you don’t have to do anything to activate it. A simple application: switch from /var/music/albums/arno/2001 to images in your home directory using one elegant command:

rom:/var/music/albums/arno/2001> cd ~/images

rom:~/images> pwd
/home/rom/images

Later in this chapter we will talk about the commands for managing files and directories in order to keep your home directory tidy.

As we mentioned before, most configuration files are stored in the /etc directory. Content can be viewed using the cat command, which sends text files to the standard output (usually your monitor). The syntax is straight forward:

cat file1 file2 … fileN

In this section we try to give an overview of the most common configuration files. This is certainly not a complete list. Adding extra packages may also add extra configuration files in /etc. When reading the configuration files, you will find that they are usually quite well commented and self-explanatory. Some files also have man pages which contain extra documentation, such as man group.

Table 3-3. Most common configuration files

File	Information/service
aliases	Mail aliases file for use with the Sendmail and Postfix mail server. Running a mail server on each and every system has long been common use in the UNIX world, and almost every Linux distribution still comes with a Sendmail package. In this file local user names are matched with real names as they occur in E-mail addresses, or with other local addresses.
apache	Config files for the Apache web server.
bashrc	The system-wide configuration file for the Bourne Again SHell. Defines functions and aliases for all users. Other shells may have their own system-wide config files, like cshrc.
crontab and the cron.* directories	Configuration of tasks that need to be executed periodically - backups, updates of the system databases, cleaning of the system, rotating logs etc.
default	Default options for certain commands, such as useradd.
filesystems	Known file systems: ext3, vfat, iso9660 etc.
fstab	Lists partitions and their mount points.
ftp*	Configuration of the ftp-server: who can connect, what parts of the system are accessible etc.
group	Configuration file for user groups. Use the shadow utilities groupadd, groupmod and groupdel to edit this file. Edit manually only if you really know what you are doing.
hosts	A list of machines that can be contacted using the network, but without the need for a domain name service. This has nothing to do with the system’s network configuration, which is done in /etc/sysconfig.
inittab	Information for booting: mode, number of text consoles etc.
issue	Information about the distribution (release version and/or kernel info).
ld.so.conf	Locations of library files.
lilo.conf, silo.conf, aboot.conf etc.	Boot information for the LInux LOader, the system for booting that is now gradually being replaced with GRUB.
logrotate.*	Rotation of the logs, a system preventing the collection of huge amounts of log files.
mail	Directory containing instructions for the behavior of the mail server.
modules.conf	Configuration of modules that enable special features (drivers).
motd	Message Of The Day: Shown to everyone who connects to the system (in text mode), may be used by the system admin to announce system services/maintenance etc.
mtab	Currently mounted file systems. It is advised to never edit this file.
nsswitch.conf	Order in which to contact the name resolvers when a process demands resolving of a host name.
pam.d	Configuration of authentication modules.
passwd	Lists local users. Use the shadow utilities useradd, usermod and userdel to edit this file. Edit manually only when you really know what you are doing.
printcap	Outdated but still frequently used printer configuration file. Don’t edit this manually unless you really know what you are doing.
profile	System wide configuration of the shell environment: variables, default properties of new files, limitation of resources etc.
rc*	Directories defining active services for each run level.
resolv.conf	Order in which to contact DNS servers (Domain Name Servers only).
sendmail.cf	Main config file for the Sendmail server.
services	Connections accepted by this machine (open ports).
sndconfig or sound	Configuration of the sound card and sound events.
ssh	Directory containing the config files for secure shell client and server.
sysconfig	Directory containing the system configuration files: mouse, keyboard, network, desktop, system clock, power management etc. (specific to RedHat)
X11	Settings for the graphical server, X. RedHat uses XFree, which is reflected in the name of the main configuration file, XFree86Config. Also contains the general directions for the window managers available on the system, for example gdm, fvwm, twm, etc.
xinetd.* or inetd.conf	Configuration files for Internet services that are run from the system’s (extended) Internet services daemon (servers that don’t run an independent daemon).

Throughout this guide we will learn more about these files and study some of them in detail.

Prev: Orientation in the file system: The path
Home
Next: Orientation in the file system:The most common devices

Devices, generally every peripheral attachment of a PC that is not the CPU itself, are presented to the system as an entry in the /dev directory. One of the advantages of this UNIX-way of handling devices is that neither the user nor the system has to worry much about the specification of devices.

Users that are new to Linux or UNIX in general are often overwhelmed by the amount of new names and concepts they have to learn. That is why a list of common devices is included in this introduction.

Table 3-4. Common devices

Name	Device
cdrom	CD drive
console	Special entry for the currently used console.
ttyS* (was cua*)	Serial ports
dsp*	Devices for sampling and recording
fd*	Entries for most kinds of floppy drives, the default is /dev/fd0, a floppy drive for 1.44 MB floppies.
hd[a-t][1-16]	Standard support for IDE drives with maximum amount of partitions each.
ir*	Infrared devices
isdn*	Management of ISDN connections
js*	Joystick(s)
lp*	Printers
mem	Memory
midi*	midi player
mixer* and music	Idealized model of a mixer (combines or adds signals)
modem	Modem
mouse (also msmouse, logimouse, psmouse, input/mice, psaux)	All kinds of mouses
null	Bottomless garbage can
par*	Entries for parallel port support
pty*	Pseudo terminals
radio*	For Radio Amateurs (HAMs).
ram*	boot device
sd*	SCSI and Serial-ATA disks with their partitions
sequencer	For audio applications using the synthesizer features of the sound card (MIDI-device controller)
tty*	Virtual consoles simulating vt100 terminals.
usb*	USB card and scanner
video*	For use with a graphics card supporting video.

In the /var directory we find a set of directories for storing specific non-constant data (as opposed to the ls program or the system configuration files, which change relatively infrequently or never at all). All files that change frequently, such as log files, mailboxes, lock files, spoolers etc. are kept in a subdirectory of /var.

As a security measure these files are usually kept in separate parts from the main system files, so we can keep a close eye on them and set stricter permissions where necessary. A lot of these files also need more permissions than usual, like /var/tmp, which needs to be writable for everyone. A lot of user activity might be expected here, which might even be generated by anonymous Internet users connected to your system. This is one reason why the /var directory, including all its subdirectories, is usually on a separate partition. This way, there is no risk that a mail bomb, for instance, fills up the rest of the file system, containing more important data such as your programs and configuration files.

/var/tmp and /tmp

• Files in /tmp can be deleted without notice by regular system tasks or because of a system reboot. On some (customized) systems, also /var/tmp might behave unpredictably. Nevertheless, since this is not the case by default, we advise using the /var/tmp directory for saving temporary files. When in doubt, check with your system administrator. If you manage your own system, you can be reasonably sure that this is a safe place if you did not consciously change settings on /var/tmp (as root, a normal user can not do this). Whatever you do, try to stick to the privileges granted to a normal user–don’t go saving files directly under the root (/) of the file system, don’t put them in /usr or some subdirectory or in another reserved place. This pretty much limits your access to safe file systems.

One of the main security systems on a UNIX system, which is naturally implemented on every Linux machine as well, is the log-keeping facility, which logs all user actions, processes, system events etc. The configuration file of the so-called syslogdaemon determines which, and how long, logged information will be kept. The default location of all logs is /var/log, containing different files for access log, server logs, system messages etc.

In /var we typically find server data, which is kept here to separate it from critical data such as the server program itself and its configuration files. A typical example on Linux systems is /var/www, which contains the actual HTML pages, scripts and images that a web server offers. The FTP-tree of an FTP server (data that can be downloaded by a remote client) is also best kept in one of /var’s subdirectories. Because this data is publicly accessible and often changeable by anonymous users, it is safer to keep it here, away from partitions or directories with sensitive data.

On most workstation installations, /var/spool will at least contain an at and a cron directory, containing scheduled tasks. In office environments this directory usually contains lpd as well, which holds the print queue(s) and further printer configuration files, as well as the printer log files.

On server systems we will generally find /var/spool/mail, containing incoming mails for local users, sorted in one file per user, the user’s inbox. A related directory is mqueue, the spooler area for unsent mail messages. These parts of the system can be very busy on mail servers with a lot of users. News servers also use the /var/spool area because of the enormous amounts of messages they have to process.

The /var/lib/rpm directory is specific to RPM-based (RedHat Package Manager) distributions; it is where RPM package information is stored.

---

Prev: Orientation in the file system: The most important files and directories
Home
Next: Manipulating files

When you want the system to execute a command, you almost never have to give the full path to that command. For example, we know that the ls command is in the /bin directory (check with which -a ls), yet we don’t have to enter the command /bin/ls for the computer to list the content of the current directory.

The PATH environment variable takes care of this. This variable lists those directories in the system where executable files can be found, and thus saves the user a lot of typing and memorizing locations of commands. So the path naturally contains a lot of directories containing bin somewhere in their names, as the user below demonstrates. The echo command is used to display the content ($) of the variable PATH:

rogier:> echo $PATH  
 /opt/local/bin:/usr/X11R6/bin:/usr/bin:/usr/sbin/:/bin 

In this example, the directories /opt/local/bin, /usr/X11R6/bin, /usr/bin, /usr/sbin and /bin are subsequently searched for the required program. As soon as a match is found, the search is stopped, even if not every directory in the path has been searched. This can lead to strange situations. In the first example below, the user knows there is a program called sendsms to send an SMS message, and another user on the same system can use it, but she can’t. The difference is in the configuration of the PATH variable:

[jenny@blob jenny]$ sendsms
bash: sendsms: command not found
[jenny@blob jenny]$ echo $PATH
/bin:/usr/bin:/usr/bin/X11:/usr/X11R6/bin:/home/jenny/bin
[jenny@blob jenny]$ su - tony
Password:
tony:~>which sendsms
sendsms is /usr/local/bin/sendsms

tony:~>echo $PATH
/home/tony/bin.Linux:/home/tony/bin:/usr/local/bin:/usr/local/sbin:\
/usr/X11R6/bin:/usr/bin:/usr/sbin:/bin:/sbin

Note the use of the su (switch user) facility, which allows you to run a shell in the environment of another user, on the condition that you know the user’s password.

A backslash indicates the continuation of a line on the next, without an Enter separating one line from the other.

In the next example, a user wants to call on the wc (word count) command to check the number of lines in a file, but nothing happens and he has to break off his action using the Ctrl+C combination:

jumper:~> wc -l test

(Ctrl-C)
jumper:~> which wc
wc is hashed (/home/jumper/bin/wc)

jumper:~> echo $PATH
/home/jumper/bin:/usr/local/bin:/usr/local/sbin:/usr/X11R6/bin:\
/usr/bin:/usr/sbin:/bin:/sbin

The use of the which command shows us that this user has a bin-directory in his home directory, containing a program that is also called wc. Since the program in his home directory is found first when searching the paths upon a call for wc, this “home-made” program is executed, with input it probably doesn’t understand, so we have to stop it. To resolve this problem there are several ways (there are always several ways to solve a problem in UNIX/Linux): one answer could be to rename the user’s wc program, or the user can give the full path to the exact command he wants, which can be found by using the -a to the which command:

jumper:~> which -a wc
/home/jumper/bin/wc
/usr/bin/wc
jumper:~> /usr/bin/wc -l test
       10 test

If the user uses programs in the other directories more frequently, he can change his path to look in his own directories last:

jumper:~> export PATH=/usr/local/bin:/usr/local/sbin:/usr/X11R6/bin:\
/usr/bin:/usr/sbin:/bin:/sbin:/home/jumper/bin

• Changes are not permanent!
  • Note that when using the export command in a shell, the changes are temporary and only valid for this session (until you log out). Opening new sessions, even while the current one is still running, will not result in a new path in the new session. We will see in Section 7.2 how we can make these kinds of changes to the environment permanent, adding these lines to the shell configuration files.

A path, which is the way you need to follow in the tree structure to reach a given file, can be described as starting from the trunk of the tree (the / or root directory). In that case, the path starts with a slash and is called an absolute path, since there can be no mistake: only one file on the system can comply.

In the other case, the path doesn’t start with a slash and confusion is possible between ~/bin/wc (in the user’s home directory) and bin/wc in /usr, from the previous example. Paths that don’t start with a slash are always relative.

In relative paths we also use the . and .. indications for the current and the parent directory. A couple of practical examples:

• When you want to compile source code, the installation documentation often instructs you to run the command ./configure, which runs the configure program located in the current directory (that came with the new code), as opposed to running another configure program elsewhere on the system.
• In HTML files, relative paths are often used to make a set of pages easily movable to another place:

<img alt="Garden with trees" src="../images/garden.jpg">

• Notice the difference one more time:

theo:~> ls /mp3
ls: /mp3: No such file or directory
theo:~>ls mp3/
oriental/  pop/  sixties/

Prev: General overview of the Linux file system
Home
Next: Orientation in the file system: The most important files and directories

In the example on moving files we already saw how the shell can manipulate multiple files at once. In that example, the shell finds out automatically what the user means by the requirements between the square braces, [ and ]. The shell can substitute ranges of numbers and upper or lower case characters alike. It also substitutes as many characters as you want with an asterisk, and only one character with a question mark.

All sorts of substitutions can be used simultaneously; the shell is very logical about it. The Bash shell, for instance, has no problem with expressions like ls dirname/*/*/*[2-3].

In other shells, the asterisk is commonly used to minimize the efforts of typing: people would enter cd dir* instead of cd directory. In Bash however, this is not necessary because the GNU shell has a feature called file name completion. It means that you can type the first few characters of a command (anywhere) or a file (in the current directory) and if no confusion is possible, the shell will find out what you mean. For example in a directory containing many files, you can check if there are any files beginning with the letter A just by typing ls A and pressing the Tab key twice, rather than pressing Enter. If there is only one file starting with A, this file will be shown as the argument to ls (or any shell command, for that matter) immediately.

A very simple way of looking up executable files is using the which command to look in the directories listed in the user’s search path. which will not find ordinary files or files located outside the user’s search path. The which command is useful when troubleshooting Command not Found problems. In the example below, user tina can’t use the acroread program, while her colleague has no troubles whatsoever on the same system. The problem is similar to the PATH problem in the previous part: Tina’s colleague tells her that he can see the required program in /opt/acroread/bin, but this directory is not in her path:

tina:~> which acroread
/usr/bin/which: no acroread in (/bin:/usr/bin:/usr/bin/X11)

The problem can be solved by giving the full path to the command to run, or by re-exporting the content of the PATH variable:

tina:~> export PATH=$PATH:/opt/acroread/bin

tina:~> echo $PATH
/bin:/usr/bin:/usr/bin/X11:/opt/acroread/bin

Using the which command with the -a switch will find all instances of the command:

tina:~> which -a make
/usr/bin/make
/usr/bin/X11/make

These are the real tools, used when searching other paths beside those listed in the search path. The UNIX find tool, very powerful, uses a somewhat complex syntax. GNU find, however, deals better with the difficult syntax. This command not only allows you to search file names, it can also accept file size, date of last change and other file properties as criteria for a search. The most common use is for finding file names:

find path -name searchstring

This can be interpreted as “Look in all files and subdirectories contained in a given path, and print the names of the files containing the search string in their name” (not in their content).

peter:~> find /boot -name menu.lst
/boot/grub/menu.lst

Another application of find is for searching files of a certain size, as in the example below, where user peter wants to find all files in the current directory or one of its subdirectories, that are bigger than 5 MB:

peter:~> find . -size +5M
psychotic_chaos.mp3

If you dig in the man pages, you will see that find can also perform operations on the found files. A common example is removing files. It is best to first test without the -exec option that the correct files are selected, after that the command can be rerun to delete the selected files. Below, we search for files ending in .tmp:

peter:~>  find . -name "*.tmp" -exec rm {} \;

peter:~>

Optimize!

• This command will call on rm as many times as a file answering the requirements is found. In the worst case, this might be thousands or millions of times. This is quite a load on your system. A more realistic way of working would be the use of a pipe (|) and the xargs tool with rm as an argument. This way, the rm command is only called when the command line is full, instead of for every file. See Chapter 5 for more on using I/O redirection to ease everyday tasks.

Later on (in 1999 according to the man pages, after 20 years of find), locate was developed. This program is easier to use, but more restricted than find, since its output is based on a file index database that is updated only once every day. On the other hand, a search in the locate database uses less resources than find and therefore shows the results nearly instantly.

Most Linux distributions use slocate these days, security enhanced locate, the modern version of locate that prevents users from getting output they have no right to read. The files in root’s home directory are such an example, these are not normally accessible to the public. A user who wants to find someone who knows about the C-shell may issue the command locate .cshrc, to display all users who have a customized configuration file for the C shell. Supposing the users root and jenny are running C shell, then only the file /home/jenny/.cshrc will be displayed, and not the one in root’s home directory. On most systems, locate is a symbolic link to the slocate program:

billy:~> ls -l /usr/bin/locate
lrwxrwxrwx 1 root slocate  7 Oct 28 14:18 /usr/bin/locate -> slocate*

User tina could have used locate (instead of her previous attempt using which) to find the application she wanted:

tina:~> locate acroread
/usr/share/icons/hicolor/16x16/apps/acroread.png
/usr/share/icons/hicolor/32x32/apps/acroread.png
/usr/share/icons/locolor/16x16/apps/acroread.png
/usr/share/icons/locolor/32x32/apps/acroread.png
/usr/local/bin/acroread
/usr/local/Acrobat4/Reader/intellinux/bin/acroread
/usr/local/Acrobat4/bin/acroread

Directories that don’t contain the name bin can’t contain the program - they don’t contain executable files. There are three possibilities left. The file in /usr/local/bin is the one tina would have wanted: it is a link to the shell script that starts the actual program:

tina:~> file /usr/local/bin/acroread
/usr/local/bin/acroread: symbolic link to ../Acrobat4/bin/acroread

tina:~> file /usr/local/Acrobat4/bin/acroread
/usr/local/Acrobat4/bin/acroread: Bourne shell script text executable

tina:~> file /usr/local/Acrobat4/Reader/intellinux/bin/acroread
/usr/local/Acrobat4/Reader/intellinux/bin/acroread: ELF 32-bit LSB 
executable, Intel 80386, version 1, dynamically linked (uses 
shared libs), not stripped

In order to keep the path as short as possible, so the system doesn’t have to search too long every time a user wants to execute a command, we add /usr/local/bin to the path and not the other directories, which only contain the binary files of one specific program, while /usr/local/bin contains other useful programs as well. To allow the binaries in the other /usr/local directories to run, symbolic links to them are placed in /usr/local/bin.

Again, a description of the full features of find and locate can be found in the Info pages.

A simple but powerful program, grep is used for filtering input lines and returning certain patterns to the output. There are literally thousands of applications for the grep program. In the example below, jerry uses grep to see how he did the thing with find:

jerry:~> grep find .bash_history
find . -name userinfo
man find
find ../ -name common.cfg

• Search history
  • Also useful in these cases is the search function in bash, activated by pressing Ctrl+R at once, such as in the example where we want to check how we did that last find again:

  • thomas ~> ^R 
    (reverse-i-search)`find': find `/home/thomas` -name *.xml
    

  • Type your search string at the search prompt. The more characters you type, the more restricted the search gets. This reads the command history for this shell session (which is written to .bash_history in your home directory when you quit that session). The most recent occurrence of your search string is shown. If you want to see previous commands containing the same string, type Ctrl+R again.
  • See the Info pages on bash for more.

All UNIXes with just a little bit of decency have an online dictionary. So does Linux. The dictionary is a list of known words in a file named words, located in /usr/share/dict. To quickly check the correct spelling of a word, no graphical application is needed:

william:~> grep pinguin /usr/share/dict/words

william:~> grep penguin /usr/share/dict/words
penguin
penguins

Who is the owner of that home directory next to mine? Hey, there’s his telephone number!

lisa:~> grep gdbruyne /etc/passwd
gdbruyne:x:981:981:Guy Debruyne, tel 203234:/home/gdbruyne:/bin/bash

And what was the E-mail address of Arno again?

serge:~/mail> grep -i arno *
sent-mail: To: <Arno.Hintjens@celeb.com>
sent-mail: On Mon, 24 Dec 2001, Arno.Hintjens@celeb.com wrote:

find and locate are often used in combination with grep to define some serious queries. For more information, see Chapter 5 on I/O redirection.

Characters that have a special meaning to the shell have to be escaped. The escape character in Bash is backslash, as in most shells; this takes away the special meaning of the following character. The shell knows about several special characters, among the most common “/”, “.”, “?” and “*”. A full list can be found in the Info pages and documentation for your shell.

For instance, say that you want to display the file * instead of all the files in a directory, you would have to use

less \*

The same goes for filenames containing a space:

cat This\ File

Apart from cat, which really doesn’t do much more than sending files to the standard output, there are other tools to view file content.

The easiest way of course would be to use graphical tools instead of command line tools. In the introduction we already saw a glimpse of an office application, OpenOffice. Other examples are the GIMP (start up with gimp from the command line), the GNU Image Manipulation Program; xpdf to view Portable Document Format files (PDF); GhostView (gv) for viewing PostScript files; Mozilla/FireFox, lynx and links (two text mode browsers), Konqueror, Opera and many others for web content; XMMS, CDplay and others for multimedia file content; AbiWord, Gnumeric, KOffice etc. for all kinds of office applications and so on. There are thousands of Linux applications; to list them all would take days.

Instead we keep concentrating on shell- or text-mode applications, which form the basics for all other applications. These commands work best in a text environment on files containing text. When in doubt, check first using the file command.

So let’s see what text tools we have that are useful to look inside files.

• Font problems
  • Plain text tools such as the ones we will now be discussing, often have problems with plain text files because of the font encoding used in those files. Special characters, such as accented alphabetical characters, Chinese characters and other characters from languages using different character sets than the default en_US encoding and so on, are then displayed the wrong way or replaced by unreadable rubbish. These problems are discussed in Section 7.4.

Undoubtedly you will hear someone say this phrase sooner or later when working in a UNIX environment. A little bit of the UNIX history of pagers explains this:

• First there was cat. Output was streamed in an uncontrollable way.
• Then there was pg, which may still be found on older UNIXes. This command puts text to the output one page at a time.
• The more program was a revised version of pg. This command is still available on every Linux system.
• less is the GNU version of more and has extra features allowing highlighting of search strings, scrolling back etc. The syntax is very simple: less file

More information is located in the Info pages.

You already know about pagers by now, because they are used for viewing the man pages.

These two commands display the n first/last lines of a file respectively. To see the last ten commands entered:

tony:~> tail -10 .bash_history 
locate configure | grep bin
man bash
cd
xawtv &
grep usable /usr/share/dict/words 
grep advisable /usr/share/dict/words 
info quota
man quota
echo $PATH
frm

head works similarly showing the first lines in a file. The tail command has a handy feature to continuously show the last lines of a file as the file is changing. This -f option is often used by system administrators to check on log files. For example, you use “tail -f /var/log/syslog” in a terminal to watch what happens as you plug in a usb device. More information is located in the system documentation files.

Since we know more about files and their representation in the file system, understanding links (or shortcuts) is a piece of cake. A link is nothing more than a way of matching two or more file names to the same set of file data. There are two ways to achieve this:

• Hard link: Associate two or more filenames with the same inode. Hard links share the same data blocks on the hard disk, while they continue to behave as independent files.
  There is an immediate disadvantage: hard links can’t span partitions, because inode numbers are unique only within a given partition.
• Soft link or symbolic link (or for short: symlink): a small file that is a pointer to another file. A symbolic link contains the path to the target file instead of a physical location on the hard disk. Since inodes are not used in this system, soft links can span across partitions.

The two link types behave similarly, but are not the same, as illustrated in the scheme below:

Figure 3-2. Hard and soft link mechanism

Here’s another good reference: Q & A: The difference between hard and soft links

Note that removing the target file for a symbolic link makes the link useless.

Each regular file is in principle a hard link. Hard links cannot span across partitions, since they refer to inodes, and inode numbers are unique only within a given partition. The number of hard links that exist for a file is displayed by the ls command. The rm command actually removes the hard link, not the file itself. Thus if one hard link to a file is deleted, the others continue to work. Only when the hard link count drops to zero, will the inode itself will be freed.

stw@laptop:~/LBook$ ln -s file slink
stw@laptop:~/LBook$ ls -l
lrwxrwxrwx 1 stw stw 4 2006-11-08 20:08 slink -> file 
stw@laptop:~/LBook$ ln file hlink
ln: accessing `file': No such file or directory

• A soft link (“slink”) to “file” is created. Note that this target does not exist yet. Broken “slink” may be red.
• Attempt to make a hard link (“hlink”) fails since no target (“file”) exists yet.

stw@laptop:~/LBook$ echo > file "Hello Linuxbasics.org"
stw@laptop:~/LBook$ ls -l
-rw-r--r-- 1 stw stw 22 2006-11-08 20:12 file
lrwxrwxrwx 1 stw stw  4 2006-11-08 20:08 slink -> file
stw@laptop:~/LBook$ ln file hlink
stw@laptop:~/LBook$ ls -l
-rw-r--r-- 2 stw stw 22 2006-11-08 20:13 file
-rw-r--r-- 2 stw stw 22 2006-11-08 20:13 hlink
lrwxrwxrwx 1 stw stw  4 2006-11-08 20:08 slink -> file

• The actual data is written to a file named “file”.
• A hard link named “hlink” is created, pointing to the same data as “file”.
• The link count displayed by “ls” (the number directly after the permissions) is increased.
• The color of “slink” may change from red to cyan, showing that it is no longer broken.

stw@laptop:~/LBook$ cat file
Hello Linuxbasics.org
stw@laptop:~/LBook$ cat hlink
Hello Linuxbasics.org
stw@laptop:~/LBook$ cat slink
Hello Linuxbasics.org

• All three “locations” act on the same data.

stw@laptop:~/LBook$ rm file
stw@laptop:~/LBook$ ls -l
-rw-r--r-- 1 stw stw 22 2006-11-08 20:13 hlink
lrwxrwxrwx 1 stw stw  4 2006-11-08 20:08 slink -> file
stw@laptop:~/LBook$ cat slink
cat: slink: No such file or directory
stw@laptop:~/LBook$ cat hlink
Hello Linuxbasics.org

• Deleting the link-target breaks the soft link. Again the link may show as red or blinking red.
• The hard link is still good.

It may be argued that there is a third kind of link, the user-space link, which is similar to a shortcut in MS Windows. These are files containing meta-data which can only be interpreted by the graphical file manager. To the kernel and the shell these are just normal files. They may end in a .desktop or .lnk suffix; an example can be found in ~/.gnome-desktop:

[dupont@boulot .gnome-desktop]$ cat La\ Maison\ Dupont
[Desktop Entry]
Encoding=Legacy-Mixed
Name=La Maison Dupont
Type=X-nautilus-home
X-Nautilus-Icon=temp-home
URL=file:///home/dupont

This example is from a KDE desktop:

[lena@venus Desktop]$ cat camera
[Desktop Entry]
Dev=/dev/sda1
FSType=auto
Icon=memory
MountPoint=/mnt/camera
Type=FSDevice
X-KDE-Dynamic-Device=true

Creating this kind of link is easy enough using the features of your graphical environment. Should you need help, your system documentation should be your first resort.

In the next section, we will study the creation of UNIX-style symbolic links using the command line.

The symbolic link is particularly interesting for beginning users: they are fairly obvious to see and you don’t need to worry about partitions.

The command to make links is ln. In order to create symlinks, you need to use the -s option:

ln -s targetfile linkname

In the example below, user freddy creates a link in a subdirectory of his home directory to a directory on another part of the system:

freddy:~/music> ln -s /opt/mp3/Queen/ Queen

freddy:~/music> ls -l
lrwxrwxrwx  1 freddy  freddy  17 Jan 22 11:07 Queen -> /opt/mp3/Queen

Symbolic links are always very small files, while hard links have the same size as the original file.

The application of symbolic links is widespread. They are often used to save disk space, to make a copy of a file in order to satisfy installation requirements of a new program that expects the file to be in another location, they are used to fix scripts that suddenly have to run in a new environment and can generally save a lot of work. A system admin may decide to move the home directories of the users to a new location, disk2 for instance, but if he wants everything to work like before, like the /etc/passwd file, with a minimum of effort he will create a symlink from /home to the new location /disk2/home.

---

Prev: Manipulating files
Home
Next: File security

Besides the name of the file, ls can give a lot of other information, such as the file type, as we already discussed. It can also show permissions on a file, file size, inode number, creation date and time, owners and number of links to the file. With the -a option to ls, files that are normally hidden from view can be displayed as well. These are files that have a name that start with a dot. Typical examples include the configuration files in your home directory which you will begin to notice after you’ve worked with a system for a while. Every directory also contains a file named with a single dot (.) and one with two dots (..), which are used in combination with their inode number to determine the directory’s position in the file system’s tree structure.

You should really read the Info pages about ls, since it is a very common command with a lot of useful options. Options can be combined, as is the case with most UNIX commands and their options. A common combination is ls -al; it shows a long list of files and their properties as well as the destinations that any symbolic links point to. ls -latr displays the same files, only now in reversed order of the last change, so that the file changed most recently occurs at the bottom of the list. Here are a couple of examples:

krissie:~/mp3> ls
Albums/  Radio/  Singles/  gene/  index.html

krissie:~/mp3> ls -a
./   .thumbs  Radio/     gene/
../  Albums/  Singles/  index.html

krissie:~/mp3> ls -l Radio/
total 8
drwxr-xr-x    2 krissie krissie  4096 Oct 30  1999 Carolina/
drwxr-xr-x    2 krissie krissie  4096 Sep 24  1999 Slashdot/

krissie:~/mp3> ls -ld Radio/
drwxr-xr-x    4 krissie krissie  4096 Oct 30  1999 Radio/

krissie:~/mp3> ls -ltr
total 20
drwxr-xr-x    4 krissie krissie  4096 Oct 30  1999 Radio/
-rw-r--r--    1 krissie krissie   453 Jan  7  2001 index.html
drwxrwxr-x   30 krissie krissie  4096 Oct 20 17:32 Singles/
drwxr-xr-x    2 krissie krissie  4096 Dec  4 23:22 gene/
drwxrwxr-x   13 krissie krissie  4096 Dec 21 11:40 Albums/

On most Linux versions ls is aliased to color-ls by default. Every file type has its own color which allows one to see the file type without using any options to ls. The standard scheme is in /etc/DIR_COLORS:

Table 3-5. Color-ls default color scheme

Color	File type
blue	directories
red	compressed archives
white	text files
pink	images
cyan	links
yellow	devices
green	executables
flashing red	broken links

More information is in the man page. The same information was in earlier days displayed using suffixes to every non-standard file name. For mono-color use (like printing a directory listing) and for general readability, this scheme is still in use (ls -F):

Table 3-6. Default suffix scheme for ls

Character	File type
nothing	regular file
/	directory
*	executable file
@	link
=	socket
|	named pipe

A description of the full functionality and features of the ls command can be read with info coreutils ls.

To find out more about the kind of data we are dealing with, we use the file command. By applying certain tests that check properties of a file in the file system, magic numbers and language tests, file tries to make an educated guess about the format of a file. Some examples:

mike:~> file Documents/
Documents/: directory

mike:~> file high-tech-stats.pdf
high-tech-stats.pdf: PDF document, version 1.2

mike:~> file Nari-288.rm
Nari-288.rm: RealMedia file

mike:~> file bijlage10.sdw
bijlage10.sdw: Microsoft Office Document

mike:~> file logo.xcf
logo.xcf: GIMP XCF image data, version 0, 150 x 38, RGB Color

mike:~> file cv.txt
cv.txt: ISO-8859 text

mike:~> file image.png
image.png: PNG image data, 616 x 862, 8-bit grayscale, non-interlaced

mike:~> file figure
figure: ASCII text

mike:~> file me+tux.jpg
me+tux.jpg: JPEG image data, JFIF standard 1.01, resolution (DPI),
            "28 Jun 1999", 144 x 144

mike:~> file 42.zip.gz
42.zip.gz: gzip compressed data, deflated, original filename,
         `42.zip', last modified: Thu Nov  1 23:45:39 2001, os: Unix

mike:~> file vi.gif
vi.gif: GIF image data, version 89a, 88 x 31

mike:~> file slide1
slide1: HTML document text

mike:~> file template.xls
template.xls: Microsoft Office Document

mike:~> file abook.ps
abook.ps: PostScript document text conforming at level 2.0

mike:~> file /dev/log
/dev/log: socket

mike:~> file /dev/hda
/dev/hda: block special (3/0)

The file command has a series of options, among others the -z option to look into compressed files. See info file for a detailed description. Keep in mind that the results of file are not absolute, it is only a guess. In other words, file can be tricked.

Why all the fuss about file types and formats?

• Shortly, we will discuss a couple of command-line tools for looking at plain text files. These tools will not work when used on the wrong type of files. In the worst case, they will crash your terminal and/or make a lot of beeping noises. If this happens to you, just close the terminal session and start a new one. But try to avoid it, because it is usually very disturbing for other people.

… Is not a difficult thing to do. Today almost every system is networked, so naturally files get copied from one machine to another. And especially when working in a graphical environment, creating new files is a piece of cake and is often done without the approval of the user. To illustrate the problem, here’s the full content of a new user’s directory, created on a standard RedHat system:

[newuser@blob user]$ ls -al
total 32
drwx------   3 user 	user        4096 Jan 16 13:32 .
drwxr-xr-x   6 root     root        4096 Jan 16 13:32 ..
-rw-r--r--   1 user 	user      24 Jan 16 13:32 .bash_logout
-rw-r--r--   1 user 	user     191 Jan 16 13:32 .bash_profile
-rw-r--r--   1 user 	user     124 Jan 16 13:32 .bashrc
drwxr-xr-x   3 user 	user    4096 Jan 16 13:32 .kde
-rw-r--r--   1 user 	user    3511 Jan 16 13:32 .screenrc
-rw-------   1 user 	user      61 Jan 16 13:32 .xauthDqztLr

On first sight, the content of a used home directory doesn’t look that bad either:

olduser:~> ls
app-defaults/ crossover/   Fvwm@     mp3/      OpenOffice.org638/
articles/     Desktop/     GNUstep/  Nautilus/ staroffice6.0/
bin/          Desktop1/    images/   nqc/      training/
brol/         desktoptest/ Machines@ ns_imap/  webstart/
C/            Documents/   mail/     nsmail/   xml/
closed/       Emacs@       Mail/     office52/ Xrootenv.0

But when all the directories and files starting with a dot are included, there are 185 items in this directory. This is because most applications have their own directories and/or files, containing user-specific settings, in the home directory of that user. Usually these files are created the first time you start an application. In some cases you will be notified when a non-existent directory needs to be created, but most of the time everything is done automatically.

Furthermore, new files are created seemingly continuously because users want to save files, keep different versions of their work, use Internet applications, and download files and attachments to their local machine. It doesn’t stop. It is clear that one definitely needs a scheme to keep an overview on things.

In the next section, we will discuss our means of keeping order. We only discuss text tools available to the shell, since the graphical tools are very intuitive and have the same look and feel as the well known point-and-click MS Windows-style file managers, including graphical help functions and other features you expect from this kind of application. The following list is an overview of the most popular file managers for GNU/Linux. Most file managers can be started from the menu of your desktop manager, or by clicking your home directory icon, or from the command line, issuing these commands:

• nautilus: The default file manager in Gnome, the GNU desktop. Excellent documentation about working with this tool can be found at http://www.gnome.org.
• konqueror: The file manager typically used on a KDE desktop. The handbook is at http://docs.kde.org.
• mc: Midnight Commander, the Unix file manager after the fashion of Norton Commander. Gnu mirror page on MC: note the Frequently Asked Questions and some helpful screenshots. Article on Midnight Commander in Linux Gazette. Shorter description of MC use.
• thunar: Thunar is a lightweight, but nice GUI file manager. It uses icons for various file types including small thumbnails of images. Default file manager for the latest xfce4 desktop.

These applications are certainly worth giving a try and usually impress newcomers to Linux, if only because there is such a wide variety: these are only the most popular tools for managing directories and