Chapter 1. GNU/Linux tutorials

Table of Contents

1.1. Console basics
1.1.1. The shell prompt
1.1.2. The shell prompt under X
1.1.3. The root account
1.1.4. The root shell prompt
1.1.5. GUI system administration tools
1.1.6. Virtual consoles
1.1.7. How to leave the command prompt
1.1.8. How to shutdown the system
1.1.9. Recovering a sane console
1.1.10. Additional package suggestions for the newbie
1.1.11. An extra user account
1.1.12. sudo configuration
1.1.13. Play time
1.2. Unix-like filesystem
1.2.1. Unix file basics
1.2.2. Filesystem internals
1.2.3. Filesystem permissions
1.2.4. Control of permissions for newly created files: umask
1.2.5. Permissions for groups of users (group)
1.2.6. Timestamps
1.2.7. Links
1.2.8. Named pipes (FIFOs)
1.2.9. Sockets
1.2.10. Device files
1.2.11. Special device files
1.2.12. procfs and sysfs
1.2.13. tmpfs
1.3. Midnight Commander (MC)
1.3.1. Customization of MC
1.3.2. Starting MC
1.3.3. File manager in MC
1.3.4. Command-line tricks in MC
1.3.5. The internal editor in MC
1.3.6. The internal viewer in MC
1.3.7. Auto-start features of MC
1.3.8. FTP virtual filesystem of MC
1.4. The basic Unix-like work environment
1.4.1. The login shell
1.4.2. Customizing bash
1.4.3. Special key strokes
1.4.4. Unix style mouse operations
1.4.5. The pager
1.4.6. The text editor
1.4.7. Setting a default text editor
1.4.8. Customizing vim
1.4.9. Recording the shell activities
1.4.10. Basic Unix commands
1.5. The simple shell command
1.5.1. Command execution and environment variable
1.5.2. "$LANG" variable
1.5.3. "$PATH" variable
1.5.4. "$HOME" variable
1.5.5. Command line options
1.5.6. Shell glob
1.5.7. Return value of the command
1.5.8. Typical command sequences and shell redirection
1.5.9. Command alias
1.6. Unix-like text processing
1.6.1. Unix text tools
1.6.2. Regular expressions
1.6.3. Replacement expressions
1.6.4. Global substitution with regular expressions
1.6.5. Extracting data from text file table
1.6.6. Script snippets for piping commands

I think learning a computer system is like learning a new foreign language. Although tutorial books and documentation are helpful, you have to practice it yourself. In order to help you get started smoothly, I elaborate a few basic points.

The powerful design of Debian GNU/Linux comes from the Unix operating system, i.e., a multiuser, multitasking operating system. You must learn to take advantage of the power of these features and similarities between Unix and GNU/Linux.

Don't shy away from Unix oriented texts and don't rely solely on GNU/Linux texts, as this robs you of much useful information.

[Note] Note

If you have been using any Unix-like system for a while with command line tools, you probably know everything I explain here. Please use this as a reality check and refresher.

1.1. Console basics

1.1.1. The shell prompt

Upon starting the system, you are presented with the character based login screen if you did not install X Window System with the display manager such as gdm3. Suppose your hostname is foo, the login prompt looks as follows.

foo login:

If you did install a GUI environment such as GNOME or KDE, then you can get to a login prompt by Ctrl-Alt-F1, and you can return to the GUI environment via Alt-F7 (see Section 1.1.6, “Virtual consoles” below for more).

At the login prompt, you type your username, e.g. penguin, and press the Enter-key, then type your password and press the Enter-key again.

[Note] Note

Following the Unix tradition, the username and password of the Debian system are case sensitive. The username is usually chosen only from the lowercase. The first user account is usually created during the installation. Additional user accounts can be created with adduser(8) by root.

The system starts with the greeting message stored in "/etc/motd" (Message Of The Day) and presents a command prompt.

Debian GNU/Linux lenny/sid foo tty1
foo login: penguin
Last login: Sun Apr 22 09:29:34 2007 on tty1
Linux snoopy 2.6.20-1-amd64 #1 SMP Sun Apr 15 20:25:49 UTC 2007 x86_64

The programs included with the Debian GNU/Linux system are free software;
the exact distribution terms for each program are described in the
individual files in /usr/share/doc/*/copyright.

Debian GNU/Linux comes with ABSOLUTELY NO WARRANTY, to the extent
permitted by applicable law.

Here, the main part of the greeting message can be customized by editing the "/etc/motd.tail" file. The first line is generated from the system information using "uname -snrvm".

Now you are in the shell. The shell interprets your commands.

1.1.2. The shell prompt under X

If you installed X Window System with a display manager such as GNOME's gdm3 by selecting "Desktop environment" task during the installation, you are presented with the graphical login screen upon starting your system. You type your username and your password to login to the non-privileged user account. Use tab to navigate between username and password, or use the mouse and primary click.

You can gain the shell prompt under X by starting a x-terminal-emulator program such as gnome-terminal(1), rxvt(1) or xterm(1). Under the GNOME Desktop environment, clicking "Applications" → "Accessories" → "Terminal" does the trick.

You can also see the section below Section 1.1.6, “Virtual consoles”.

Under some other Desktop systems (like fluxbox), there may be no obvious starting point for the menu. If this happens, just try (right) clicking the center of the screen and hope for a menu to pop-up.

1.1.3. The root account

The root account is also called superuser or privileged user. From this account, you can perform the following system administration tasks.

  • Read, write, and remove any files on the system irrespective of their file permissions

  • Set file ownership and permissions of any files on the system

  • Set the password of any non-privileged users on the system

  • Login to any accounts without their passwords

This unlimited power of root account requires you to be considerate and responsible when using it.

[Warning] Warning

Never share the root password with others.

[Note] Note

File permissions of a file (including hardware devices such as CD-ROM etc. which are just another file for the Debian system) may render it unusable or inaccessible by non-root users. Although the use of root account is a quick way to test this kind of situation, its resolution should be done through proper setting of file permissions and user's group membership (see Section 1.2.3, “Filesystem permissions”).

1.1.4. The root shell prompt

Here are a few basic methods to gain the root shell prompt by using the root password.

  • Type root at the character based login prompt.

  • Click "Applications" → "Accessories" → "Root Terminal", under the GNOME Desktop environment.

  • Type "su -l" from any user shell prompt.

    • This does not preserve the environment of the current user.

  • Type "su" from any user shell prompt.

    • This preserves some of the environment of the current user.

1.1.5. GUI system administration tools

When your desktop menu does not start GUI system administration tools automatically with the appropriate privilege, you can start them from the root shell prompt of the X terminal emulator, such as gnome-terminal(1), rxvt(1), or xterm(1). See Section 1.1.4, “The root shell prompt” and Section 7.8.4, “Running X clients as root”.

[Warning] Warning

Never start the X display/session manager under the root account by typing in root to the prompt of the display manager such as gdm3(1).

[Warning] Warning

Never run untrusted remote GUI program under X Window when critical information is displayed since it may eavesdrop your X screen.

1.1.6. Virtual consoles

In the default Debian system, there are six switchable VT100-like character consoles available to start the command shell directly on the Linux host. Unless you are in a GUI environment, you can switch between the virtual consoles by pressing the Left-Alt-key and one of the F1F6 keys simultaneously. Each character console allows independent login to the account and offers the multiuser environment. This multiuser environment is a great Unix feature, and very addictive.

If you are under the X Window System, you gain access to the character console 1 by pressing Ctrl-Alt-F1 key, i.e., the left-Ctrl-key, the left-Alt-key, and the F1-key are pressed together. You can get back to the X Window System, normally running on the virtual console 7, by pressing Alt-F7.

You can alternatively change to another virtual console, e.g. to the console 1, from the commandline.

# chvt 1

1.1.7. How to leave the command prompt

You type Ctrl-D, i.e., the left-Ctrl-key and the d-key pressed together, at the command prompt to close the shell activity. If you are at the character console, you return to the login prompt with this. Even though these control characters are referred as "control D" with the upper case, you do not need to press the Shift-key. The short hand expression, ^D, is also used for Ctrl-D. Alternately, you can type "exit".

If you are at x-terminal-emulator(1), you can close x-terminal-emulator window with this.

1.1.8. How to shutdown the system

Just like any other modern OS where the file operation involves caching data in memory for improved performance, the Debian system needs the proper shutdown procedure before power can safely be turned off. This is to maintain the integrity of files, by forcing all changes in memory to be written to disk. If the software power control is available, the shutdown procedure automatically turns off power of the system. (Otherwise, you may have to press power button for few seconds after the shutdown procedure.)

You can shutdown the system under the normal multiuser mode from the commandline.

# shutdown -h now

You can shutdown the system under the single-user mode from the commandline.

# poweroff -i -f

Alternatively, you may type Ctrl-Alt-Delete (The left-Ctrl-key, the left-Alt-Key, and the Delete are pressed together) to shutdown if "/etc/inittab" contains "ca:12345:ctrlaltdel:/sbin/shutdown -t1 -a -h now" in it. See inittab(5) for details.

See Section 6.9.6, “How to shutdown the remote system on SSH”.

1.1.9. Recovering a sane console

When the screen goes berserk after doing some funny things such as "cat <some-binary-file>", type "reset" at the command prompt. You may not be able to see the command echoed as you type. You may also issue "clear" to clean up the screen.

1.1.10. Additional package suggestions for the newbie

Although even the minimal installation of the Debian system without any desktop environment tasks provides the basic Unix functionality, it is a good idea to install few additional commandline and curses based character terminal packages such as mc and vim with apt-get(8) for beginners to get started by the following.

# apt-get update
# apt-get install mc vim sudo

If you already had these packages installed, no new packages are installed.

Table 1.1. List of interesting text-mode program packages

package popcon size description
mc 1303 A text-mode full-screen file manager
sudo 1397 A program to allow limited root privileges to users
vim 1873 Unix text editor Vi IMproved, a programmers text editor (standard version)
vim-tiny 830 Unix text editor Vi IMproved, a programmers text editor (compact version)
emacs23 13038 GNU project Emacs, the Lisp based extensible text editor (version 23)
w3m 1825 Text-mode WWW browsers
gpm 584 The Unix style cut-and-paste on the text console (daemon)

It may be a good idea to read some informative documentations.

You can install some of these packages by the following.

# apt-get install package_name

1.1.11. An extra user account

If you do not want to use your main user account for the following training activities, you can create a training user account, e.g. fish by the following.

# adduser fish

Answer all questions.

This creates a new account named as fish. After your practice, you can remove this user account and its home directory by the following.

# deluser --remove-home fish

1.1.12. sudo configuration

For the typical single user workstation such as the desktop Debian system on the laptop PC, it is common to deploy simple configuration of sudo(8) as follows to let the non-privileged user, e.g. penguin, to gain administrative privilege just with his user password but without the root password.

# echo "penguin  ALL=(ALL) ALL" >> /etc/sudoers

Alternatively, it is also common to do as follows to let the non-privileged user, e.g. penguin, to gain administrative privilege without any password.

# echo "penguin  ALL=(ALL) NOPASSWD:ALL" >> /etc/sudoers

This trick should only be used for the single user workstation which you administer and where you are the only user.

[Warning] Warning

Do not set up accounts of regular users on multiuser workstation like this because it would be very bad for system security.

[Caution] Caution

The password and the account of the penguin in the above example requires as much protection as the root password and the root account.

[Caution] Caution

Administrative privilege in this context belongs to someone authorized to perform the system administration task on the workstation. Never give some manager in the Admin department of your company or your boss such privilege unless they are authorized and capable.

[Note] Note

For providing access privilege to limited devices and limited files, you should consider to use group to provide limited access instead of using the root privilege via sudo(8).

[Note] Note

With more thoughtful and careful configuration, sudo(8) can grant limited administrative privileges to other users on a shared system without sharing the root password. This can help with accountability with hosts with multiple administrators so you can tell who did what. On the other hand, you might not want anyone else to have such privileges.

1.1.13. Play time

Now you are ready to play with the Debian system without risks as long as you use the non-privileged user account.

This is because the Debian system is, even after the default installation, configured with proper file permissions which prevent non-privileged users from damaging the system. Of course, there may still be some holes which can be exploited but those who worry about these issues should not be reading this section but should be reading Securing Debian Manual.

We learn the Debian system as a Unix-like system with the following.

1.2. Unix-like filesystem

In GNU/Linux and other Unix-like operating systems, files are organized into directories. All files and directories are arranged in one big tree rooted at "/". It's called a tree because if you draw the filesystem, it looks like a tree but it is upside down.

These files and directories can be spread out over several devices. mount(8) serves to attach the filesystem found on some device to the big file tree. Conversely, umount(8) detaches it again. On recent Linux kernels, mount(8) with some options can bind part of a file tree somewhere else or can mount filesystem as shared, private, slave, or unbindable. Supported mount options for each filesystem are available in "/share/doc/linux-doc-2.6.*/Documentation/filesystems/".

Directories on Unix systems are called folders on some other systems. Please also note that there is no concept for drive such as "A:" on any Unix system. There is one filesystem, and everything is included. This is a huge advantage compared to Windows.

1.2.1. Unix file basics

Here are some Unix file basics.

  • Filenames are case sensitive. That is, "MYFILE" and "MyFile" are different files.

  • The root directory means root of the filesystem referred as simply "/". Don't confuse this with the home directory for the root user: "/root".

  • Every directory has a name which can contain any letters or symbols except "/". The root directory is an exception; its name is "/" (pronounced "slash" or "the root directory") and it cannot be renamed.

  • Each file or directory is designated by a fully-qualified filename, absolute filename, or path, giving the sequence of directories which must be passed through to reach it. The three terms are synonymous.

  • All fully-qualified filenames begin with the "/" directory, and there's a "/" between each directory or file in the filename. The first "/" is the top level directory, and the other "/"'s separate successive subdirectories, until we reach the last entry which is the name of the actual file. The words used here can be confusing. Take the following fully-qualified filename as an example: "/usr/share/keytables/". However, people also refers to its basename "" alone as a filename.

  • The root directory has a number of branches, such as "/etc/" and "/usr/". These subdirectories in turn branch into still more subdirectories, such as "/etc/init.d/" and "/usr/local/". The whole thing viewed collectively is called the directory tree. You can think of an absolute filename as a route from the base of the tree ("/") to the end of some branch (a file). You also hear people talk about the directory tree as if it were a family tree encompassing all direct descendants of a single figure called the root directory ("/"): thus subdirectories have parents, and a path shows the complete ancestry of a file. There are also relative paths that begin somewhere other than the root directory. You should remember that the directory "../" refers to the parent directory. This terminology also applies to other directory like structures, such as hierarchical data structures.

  • There's no special directory path name component that corresponds to a physical device, such as your hard disk. This differs from RT-11, CP/M, OpenVMS, MS-DOS, AmigaOS, and Microsoft Windows, where the path contains a device name such as "C:\". (However, directory entries do exist that refer to physical devices as a part of the normal filesystem. See Section 1.2.2, “Filesystem internals”.)

[Note] Note

While you can use almost any letters or symbols in a file name, in practice it is a bad idea to do so. It is better to avoid any characters that often have special meanings on the command line, including spaces, tabs, newlines, and other special characters: { } ( ) [ ] ' ` " \ / > < | ; ! # & ^ * % @ $ . If you want to separate words in a name, good choices are the period, hyphen, and underscore. You could also capitalize each word, "LikeThis". Experienced Linux users tend to avoid spaces in filenames.

[Note] Note

The word "root" can mean either "root user" or "root directory". The context of their usage should make it clear.

[Note] Note

The word path is used not only for fully-qualified filename as above but also for the command search path. The intended meaning is usually clear from the context.

The detailed best practices for the file hierarchy are described in the Filesystem Hierarchy Standard ("/usr/share/doc/debian-policy/fhs/fhs-2.3.txt.gz" and hier(7)). You should remember the following facts as the starter.

Table 1.3. List of usage of key directories

directory usage of the directory
/ the root directory
/etc/ system wide configuration files
/var/log/ system log files
/home/ all the home directories for all non-privileged users

1.2.2. Filesystem internals

Following the Unix tradition, the Debian GNU/Linux system provides the filesystem under which physical data on hard disks and other storage devices reside, and the interaction with the hardware devices such as console screens and remote serial consoles are represented in an unified manner under "/dev/".

Each file, directory, named pipe (a way two programs can share data), or physical device on a Debian GNU/Linux system has a data structure called an inode which describes its associated attributes such as the user who owns it (owner), the group that it belongs to, the time last accessed, etc. If you are really interested, see "/usr/include/linux/fs.h" for the exact definition of "struct inode" in the Debian GNU/Linux system. The idea of representing just about everything in the filesystem was a Unix innovation, and modern Linux kernels have developed this idea ever further. Now, even information about processes running in the computer can be found in the filesystem.

This abstract and unified representation of physical entities and internal processes is very powerful since this allows us to use the same command for the same kind of operation on many totally different devices. It is even possible to change the way the kernel works by writing data to special files that are linked to running processes.

[Tip] Tip

If you need to identify the correspondence between the file tree and the physical entity, execute mount(8) with no arguments.

1.2.3. Filesystem permissions

Filesystem permissions of Unix-like system are defined for three categories of affected users.

  • The user who owns the file (u)

  • Other users in the group which the file belongs to (g)

  • All other users (o) also referred to as "world" and "everyone"

For the file, each corresponding permission allows following actions.

  • The read (r) permission allows owner to examine contents of the file.

  • The write (w) permission allows owner to modify the file.

  • The execute (x) permission allows owner to run the file as a command.

For the directory, each corresponding permission allows following actions.

  • The read (r) permission allows owner to list contents of the directory.

  • The write (w) permission allows owner to add or remove files in the directory.

  • The execute (x) permission allows owner to access files in the directory.

Here, the execute permission on a directory means not only to allow reading of files in that directory but also to allow viewing their attributes, such as the size and the modification time.

ls(1) is used to display permission information (and more) for files and directories. When it is invoked with the "-l" option, it displays the following information in the order given.

  • Type of file (first character)

  • Access permission of the file (nine characters, consisting of three characters each for user, group, and other in this order)

  • Number of hard links to the file

  • Name of the user who owns the file

  • Name of the group which the file belongs to

  • Size of the file in characters (bytes)

  • Date and time of the file (mtime)

  • Name of the file

Table 1.4. List of the first character of "ls -l" output

character meaning
- normal file
d directory
l symlink
c character device node
b block device node
p named pipe
s socket

chown(1) is used from the root account to change the owner of the file. chgrp(1) is used from the file's owner or root account to change the group of the file. chmod(1) is used from the file's owner or root account to change file and directory access permissions. Basic syntax to manipulate a foo file is the following.

# chown <newowner> foo
# chgrp <newgroup> foo
# chmod  [ugoa][+-=][rwxXst][,...] foo

For example, you can make a directory tree to be owned by a user foo and shared by a group bar by the following.

# cd /some/location/
# chown -R foo:bar .
# chmod -R ug+rwX,o=rX .

There are three more special permission bits.

  • The set user ID bit (s or S instead of user's x)

  • The set group ID bit (s or S instead of group's x)

  • The sticky bit (t or T instead of other's x)

Here the output of "ls -l" for these bits is capitalized if execution bits hidden by these outputs are unset.

Setting set user ID on an executable file allows a user to execute the executable file with the owner ID of the file (for example root). Similarly, setting set group ID on an executable file allows a user to execute the executable file with the group ID of the file (for example root). Because these settings can cause security risks, enabling them requires extra caution.

Setting set group ID on a directory enables the BSD-like file creation scheme where all files created in the directory belong to the group of the directory.

Setting the sticky bit on a directory prevents a file in the directory from being removed by a user who is not the owner of the file. In order to secure contents of a file in world-writable directories such as "/tmp" or in group-writable directories, one must not only reset the write permission for the file but also set the sticky bit on the directory. Otherwise, the file can be removed and a new file can be created with the same name by any user who has write access to the directory.

Here are a few interesting examples of file permissions.

$ ls -l /etc/passwd /etc/shadow /dev/ppp /usr/sbin/exim4
crw------- 1 root root   108, 0 2007-04-29 07:00 /dev/ppp
-rw-r--r-- 1 root root     1427 2007-04-16 00:19 /etc/passwd
-rw-r----- 1 root shadow    943 2007-04-16 00:19 /etc/shadow
-rwsr-xr-x 1 root root   700056 2007-04-22 05:29 /usr/sbin/exim4
$ ls -ld /tmp /var/tmp /usr/local /var/mail /usr/src
drwxrwxrwt 10 root root  4096 2007-04-29 07:59 /tmp
drwxrwsr-x 10 root staff 4096 2007-03-24 18:48 /usr/local
drwxrwsr-x  4 root src   4096 2007-04-27 00:31 /usr/src
drwxrwsr-x  2 root mail  4096 2007-03-28 23:33 /var/mail
drwxrwxrwt  2 root root  4096 2007-04-29 07:11 /var/tmp

There is an alternative numeric mode to describe file permissions with chmod(1). This numeric mode uses 3 to 4 digit wide octal (radix=8) numbers.

Table 1.5. The numeric mode for file permissions in chmod(1) commands

digit meaning
1st optional digit sum of set user ID (=4), set group ID (=2), and sticky bit (=1)
2nd digit sum of read (=4), write (=2), and execute (=1) permissions for user
3rd digit ditto for group
4th digit ditto for other

This sounds complicated but it is actually quite simple. If you look at the first few (2-10) columns from "ls -l" command output and read it as a binary (radix=2) representation of file permissions ("-" being "0" and "rwx" being "1"), the last 3 digit of the numeric mode value should make sense as an octal (radix=8) representation of file permissions to you.

For example, try the following

$ touch foo bar
$ chmod u=rw,go=r foo
$ chmod 644 bar
$ ls -l foo bar
-rw-r--r-- 1 penguin penguin 17 2007-04-29 08:22 bar
-rw-r--r-- 1 penguin penguin 12 2007-04-29 08:22 foo
[Tip] Tip

If you need to access information displayed by "ls -l" in shell script, you should use pertinent commands such as test(1), stat(1) and readlink(1). The shell builtin such as "[" or "test" may be used too.

1.2.4. Control of permissions for newly created files: umask

What permissions are applied to a newly created file or directory is restricted by the umask shell builtin command. See dash(1), bash(1), and builtins(7).

 (file permissions) = (requested file permissions) & ~(umask value)

Table 1.6. The umask value examples

umask file permissions created directory permissions created usage
0022 -rw-r--r-- -rwxr-xr-x writable only by the user
0002 -rw-rw-r-- -rwxrwxr-x writable by the group

The Debian system uses a user private group (UPG) scheme as its default. A UPG is created whenever a new user is added to the system. A UPG has the same name as the user for which it was created and that user is the only member of the UPG. UPG scheme makes it is safe to set umask to 0002 since every user has their own private group. (In some Unix variants, it is quite common to setup all normal users belonging to a single users group and is good idea to set umask to 0022 for security in such cases.)

1.2.5. Permissions for groups of users (group)

In order to make group permissions to be applied to a particular user, that user needs to be made a member of the group using "sudo vigr".

[Note] Note

Alternatively, you may dynamically add users to groups during the authentication process by adding "auth optional" line to "/etc/pam.d/common-auth" and setting "/etc/security/group.conf". (See Chapter 4, Authentication.)

The hardware devices are just another kind of file on the Debian system. If you have problems accessing devices such as CD-ROM and USB memory stick from a user account, you should make that user a member of the relevant group.

Some notable system-provided groups allow their members to access particular files and devices without root privilege.

Table 1.7. List of notable system-provided groups for file access

group description for accessible files and devices
dialout full and direct access to serial ports ("/dev/ttyS[0-3]")
dip limited access to serial ports for Dialup IP connection to trusted peers
cdrom CD-ROM, DVD+/-RW drives
audio audio device
video video device
scanner scanner(s)
adm system monitoring logs
staff some directories for junior administrative work: "/usr/local", "/home"

[Tip] Tip

You need to belong to the dialout group to reconfigure modem, dial anywhere, etc. But if root creates pre-defined configuration files for trusted peers in "/etc/ppp/peers/", you only need to belong to the dip group to create Dialup IP connection to those trusted peers using pppd(8), pon(1), and poff(1) commands.

Some notable system-provided groups allow their members to execute particular commands without root privilege.

Table 1.8. List of notable system provided groups for particular command executions

group accessible commands
sudo execute sudo without their password
lpadmin execute commands to add, modify, and remove printers from printer databases

For the full listing of the system provided users and groups, see the recent version of the "Users and Groups" document in "/usr/share/doc/base-passwd/users-and-groups.html" provided by the base-passwd package.

See passwd(5), group(5), shadow(5), newgrp(1), vipw(8), vigr(8), and pam_group(8) for management commands of the user and group system.

1.2.6. Timestamps

There are three types of timestamps for a GNU/Linux file.

Table 1.9. List of types of timestamps

type meaning
mtime the file modification time (ls -l)
ctime the file status change time (ls -lc)
atime the last file access time (ls -lu)

[Note] Note

ctime is not file creation time.

  • Overwriting a file changes all of the mtime, ctime, and atime attributes of the file.

  • Changing ownership or permission of a file changes the ctime and atime attributes of the file.

  • Reading a file changes the atime of the file.

[Note] Note

Even simply reading a file on the Debian system normally causes a file write operation to update atime information in the inode. Mounting a filesystem with "noatime" or "relatime" option makes the system skip this operation and results in faster file access for the read. This is often recommended for laptops, because it reduces hard drive activity and saves power. See mount(8).

Use touch(1) command to change timestamps of existing files.

For timestamps, the ls command outputs different strings under the modern English locale ("en_US.UTF-8") from under the old one ("C").

$ LANG=en_US.UTF-8  ls -l foo
-rw-r--r-- 1 penguin penguin 3 2008-03-05 00:47 foo
$ LANG=C  ls -l foo
-rw-r--r-- 1 penguin penguin 3 Mar  5 00:47 foo
[Tip] Tip

See Section 9.2.5, “Customized display of time and date” to customize "ls -l" output.

1.2.7. Links

There are two methods of associating a file "foo" with a different filename "bar".

See the following example for changes in link counts and the subtle differences in the result of the rm command.

$ echo "Original Content" > foo
$ ls -li foo
2398521 -rw-r--r-- 1 penguin penguin 17 2007-04-29 08:15 foo
$ ln foo bar     # hard link
$ ln -s foo baz  # symlink
$ ls -li foo bar baz
2398521 -rw-r--r-- 2 penguin penguin 17 2007-04-29 08:15 bar
2398538 lrwxrwxrwx 1 penguin penguin  3 2007-04-29 08:16 baz -> foo
2398521 -rw-r--r-- 2 penguin penguin 17 2007-04-29 08:15 foo
$ rm foo
$ echo "New Content" > foo
$ ls -li foo bar baz
2398521 -rw-r--r-- 1 penguin penguin 17 2007-04-29 08:15 bar
2398538 lrwxrwxrwx 1 penguin penguin  3 2007-04-29 08:16 baz -> foo
2398540 -rw-r--r-- 1 penguin penguin 12 2007-04-29 08:17 foo
$ cat bar
Original Content
$ cat baz
New Content

The hardlink can be made within the same filesystem and shares the same inode number which the "-i" option with ls(1) reveals.

The symlink always has nominal file access permissions of "rwxrwxrwx", as shown in the above example, with the effective access permissions dictated by permissions of the file that it points to.

[Caution] Caution

It is generally good idea not to create complicated symbolic links or hardlinks at all unless you have a very good reason. It may cause nightmares where the logical combination of the symbolic links results in loops in the filesystem.

[Note] Note

It is generally preferable to use symbolic links rather than hardlinks unless you have a good reason for using a hardlink.

The "." directory links to the directory that it appears in, thus the link count of any new directory starts at 2. The ".." directory links to the parent directory, thus the link count of the directory increases with the addition of new subdirectories.

If you are just moving to Linux from Windows, it soon becomes clear how well-designed the filename linking of Unix is, compared with the nearest Windows equivalent of "shortcuts". Because it is implemented in the filesystem, applications can't see any difference between a linked file and the original. In the case of hardlinks, there really is no difference.

1.2.8. Named pipes (FIFOs)

A named pipe is a file that acts like a pipe. You put something into the file, and it comes out the other end. Thus it's called a FIFO, or First-In-First-Out: the first thing you put in the pipe is the first thing to come out the other end.

If you write to a named pipe, the process which is writing to the pipe doesn't terminate until the information being written is read from the pipe. If you read from a named pipe, the reading process waits until there is nothing to read before terminating. The size of the pipe is always zero --- it does not store data, it just links two processes like the shell "|". However, since this pipe has a name, the two processes don't have to be on the same command line or even be run by the same user. Pipes were a very influential innovation of Unix.

For example, try the following

$ cd; mkfifo mypipe
$ echo "hello" >mypipe & # put into background
[1] 8022
$ ls -l mypipe
prw-r--r-- 1 penguin penguin 0 2007-04-29 08:25 mypipe
$ cat mypipe
[1]+  Done                    echo "hello" >mypipe
$ ls mypipe
$ rm mypipe

1.2.9. Sockets

Sockets are used extensively by all the Internet communication, databases, and the operating system itself. It is similar to the named pipe (FIFO) and allows processes to exchange information even between different computers. For the socket, those processes do not need to be running at the same time nor to be running as the children of the same ancestor process. This is the endpoint for the inter process communication (IPC). The exchange of information may occur over the network between different hosts. The two most common ones are the Internet socket and the Unix domain socket.

[Tip] Tip

"netstat -an" provides a very useful overview of sockets that are open on a given system.

1.2.10. Device files

Device files refer to physical or virtual devices on your system, such as your hard disk, video card, screen, or keyboard. An example of a virtual device is the console, represented by "/dev/console".

There are 2 types of device files.

  • Character device

    • Accessed one character at a time

    • 1 character = 1 byte

    • E.g. keyboard device, serial port, …

  • Block device

    • accessed in larger units called blocks

    • 1 block > 1 byte

    • E.g. hard disk, …

You can read and write device files, though the file may well contain binary data which may be an incomprehensible-to-humans gibberish. Writing data directly to these files is sometimes useful for the troubleshooting of hardware connections. For example, you can dump a text file to the printer device "/dev/lp0" or send modem commands to the appropriate serial port "/dev/ttyS0". But, unless this is done carefully, it may cause a major disaster. So be cautious.

[Note] Note

For the normal access to a printer, use lp(1).

The device node number are displayed by executing ls(1) as the following.

$ ls -l /dev/hda /dev/ttyS0 /dev/zero
brw-rw---- 1 root cdrom   3,  0 2007-04-29 07:00 /dev/hda
crw-rw---- 1 root dialout 4, 64 2007-04-29 07:00 /dev/ttyS0
crw-rw-rw- 1 root root    1,  5 2007-04-29 07:00 /dev/zero
  • "/dev/hda" has the major device number 3 and the minor device number 0. This is read/write accessible by the user who belongs to cdrom group.

  • "/dev/ttyS0" has the major device number 4 and the minor device number 64. This is read/write accessible by the user who belongs to dialout group.

  • "/dev/zero" has the major device number 1 and the minor device number 5. This is read/write accessible by anyone.

In the Linux 2.6 system, the filesystem under "/dev/" is automatically populated by the udev(7) mechanism.

1.2.11. Special device files

There are some special device files.

Table 1.10. List of special device files

device file action description of response
/dev/null read return "end-of-file (EOF) character"
/dev/null write return nothing (a bottomless data dump pit)
/dev/zero read return "the \0 (NUL) character" (not the same as the number zero ASCII)
/dev/random read return random characters from a true random number generator, delivering real entropy (slow)
/dev/urandom read return random characters from a cryptographically secure pseudorandom number generator
/dev/full write return the disk-full (ENOSPC) error

These are frequently used in conjunction with the shell redirection (see Section 1.5.8, “Typical command sequences and shell redirection”).

1.2.12. procfs and sysfs

The procfs and sysfs mounted on "/proc" and "/sys" are the pseudo-filesystem and expose internal data structures of the kernel to the userspace. In other word, these entries are virtual, meaning that they act as a convenient window into the operation of the operating system.

The directory "/proc" contains (among other things) one subdirectory for each process running on the system, which is named after the process ID (PID). System utilities that access process information, such as ps(1), get their information from this directory structure.

The directories under "/proc/sys/" contain interface to change certain kernel parameters at run time. (You may do the same through specialized sysctl(8) command or its preload/configuration file "/etc/sysctrl.conf".)

[Note] Note

The Linux kernel may complain "Too many open files". You can fix this by increasing "file-max" value to a larger value from the root shell, e.g., "echo "65536" > /proc/sys/fs/file-max" (This was needed on older kernels).

People frequently panic when they notice one file in particular - "/proc/kcore" - which is generally huge. This is (more or less) a copy of the content of your computer's memory. It's used to debug the kernel. It is a virtual file that points to computer memory, so don't worry about its size.

The directory under "/sys" contains exported kernel data structures, their attributes, and their linkages between them. It also contains interface to change certain kernel parameters at run time.

See "proc.txt(.gz)", "sysfs.txt(.gz)" and other related documents in the Linux kernel documentation ("/usr/share/doc/linux-doc-2.6.*/Documentation/filesystems/*") provided by the linux-doc-2.6.* package.

1.2.13. tmpfs

The tmpfs is a temporary filesystem which keeps all files in the virtual memory. The data of the tmpfs in the page cache on memory may be swapped out to the swap space on disk as needed.

The directory "/run" is mounted as the tmpfs in the early boot process. This enables writing to it even when the directory "/" is mounted as read-only. This is the new location for the storage of transient state files and replaces several locations described in the Filesystem Hierarchy Standard version 2.3:

  • "/var/run" → "/run"

  • "/var/lock" → "/run/lock"

  • "/dev/shm" → "/run/shm"

See "tmpfs.txt(.gz)" in the Linux kernel documentation ("/usr/share/doc/linux-doc-*/Documentation/filesystems/*") provided by the linux-doc-* package.

1.3. Midnight Commander (MC)

Midnight Commander (MC) is a GNU "Swiss army knife" for the Linux console and other terminal environments. This gives newbie a menu driven console experience which is much easier to learn than standard Unix commands.

You may need to install the Midnight Commander package which is titled "mc" by the following.

$ sudo apt-get install mc

Use the mc(1) command to explore the Debian system. This is the best way to learn. Please explore few interesting locations just using the cursor keys and Enter key.

  • "/etc" and its subdirectories

  • "/var/log" and its subdirectories

  • "/usr/share/doc" and its subdirectories

  • "/sbin" and "/bin"

1.3.1. Customization of MC

In order to make MC to change working directory upon exit and cd to the directory, I suggest to modify "~/.bashrc" to include a script provided by the mc package.

. /usr/lib/mc/

See mc(1) (under the "-P" option) for the reason. (If you do not understand what exactly I am talking here, you can do this later.)

1.3.2. Starting MC

MC can be started by the following.

$ mc

MC takes care of all file operations through its menu, requiring minimal user effort. Just press F1 to get the help screen. You can play with MC just by pressing cursor-keys and function-keys.

[Note] Note

In some consoles such as gnome-terminal(1), key strokes of function-keys may be stolen by the console program. You can disable these features by "Edit" → "Keyboard Shortcuts" for gnome-terminal.

If you encounter character encoding problem which displays garbage characters, adding "-a" to MC's command line may help prevent problems.

If this doesn't clear up your display problems with MC, see Section 9.6.6, “The terminal configuration”.

1.3.3. File manager in MC

The default is two directory panels containing file lists. Another useful mode is to set the right window to "information" to see file access privilege information, etc. Following are some essential keystrokes. With the gpm(8) daemon running, one can use a mouse on Linux character consoles, too. (Make sure to press the shift-key to obtain the normal behavior of cut and paste in MC.)

Table 1.11. The key bindings of MC

key key binding
F1 help menu
F3 internal file viewer
F4 internal editor
F9 activate pull down menu
F10 exit Midnight Commander
Tab move between two windows
Insert or Ctrl-T mark file for a multiple-file operation such as copy
Del delete file (be careful---set MC to safe delete mode)
Cursor keys self-explanatory

1.3.4. Command-line tricks in MC

  • cd command changes the directory shown on the selected screen.

  • Ctrl-Enter or Alt-Enter copies a filename to the command line. Use this with cp(1) and mv(1) commands together with command-line editing.

  • Alt-Tab shows shell filename expansion choices.

  • One can specify the starting directory for both windows as arguments to MC; for example, "mc /etc /root".

  • Esc + n-keyFn (i.e., Esc + 1F1, etc.; Esc + 0F10)

  • Pressing Esc before the key has the same effect as pressing the Alt and the key together.; i.e., type Esc + c for Alt-C. Esc is called meta-key and sometimes noted as "M-".

1.3.5. The internal editor in MC

The internal editor has an interesting cut-and-paste scheme. Pressing F3 marks the start of a selection, a second F3 marks the end of selection and highlights the selection. Then you can move your cursor. If you press F6, the selected area is moved to the cursor location. If you press F5, the selected area is copied and inserted at the cursor location. F2 saves the file. F10 gets you out. Most cursor keys work intuitively.

This editor can be directly started on a file using one of the following commands.

$ mc -e filename_to_edit
$ mcedit filename_to_edit

This is not a multi-window editor, but one can use multiple Linux consoles to achieve the same effect. To copy between windows, use Alt-F<n> keys to switch virtual consoles and use "File→Insert file" or "File→Copy to file" to move a portion of a file to another file.

This internal editor can be replaced with any external editor of choice.

Also, many programs use the environment variables "$EDITOR" or "$VISUAL" to decide which editor to use. If you are uncomfortable with vim(1) or nano(1) initially, you may set these to "mcedit" by adding the following lines to "~/.bashrc".

export EDITOR=mcedit
export VISUAL=mcedit

I do recommend setting these to "vim" if possible.

If you are uncomfortable with vim(1), you can keep using mcedit(1) for most system maintenance tasks.

1.3.6. The internal viewer in MC

MC is a very smart viewer. This is a great tool for searching words in documents. I always use this for files in the "/usr/share/doc" directory. This is the fastest way to browse through masses of Linux information. This viewer can be directly started using one of the following commands.

$ mc -v path/to/filename_to_view
$ mcview path/to/filename_to_view

1.3.7. Auto-start features of MC

Press Enter on a file, and the appropriate program handles the content of the file (see Section 9.5.11, “Customizing program to be started”). This is a very convenient MC feature.

Table 1.12. The reaction to the enter key in MC

file type reaction to enter key
executable file execute command
man file pipe content to viewer software
html file pipe content to web browser
"*.tar.gz" and "*.deb" file browse its contents as if subdirectory

In order to allow these viewer and virtual file features to function, viewable files should not be set as executable. Change their status using chmod(1) or via the MC file menu.

1.3.8. FTP virtual filesystem of MC

MC can be used to access files over the Internet using FTP. Go to the menu by pressing F9, then type "p" to activate the FTP virtual filesystem. Enter a URL in the form "username:passwd@hostname.domainname", which retrieves a remote directory that appears like a local one.

Try "[]" as the URL and browse the Debian archive.

1.4. The basic Unix-like work environment

Although MC enables you to do almost everything, it is very important for you to learn how to use the command line tools invoked from the shell prompt and become familiar with the Unix-like work environment.

1.4.1. The login shell

You can select your login shell with chsh(1).

Table 1.13. List of shell programs

package popcon size POSIX shell description
bash 3935 Yes Bash: the GNU Bourne Again SHell (de facto standard)
tcsh 1213 No TENEX C Shell: an enhanced version of Berkeley csh
dash 212 Yes Debian Almquist Shell, good for shell script
zsh 11462 Yes Z shell: the standard shell with many enhancements
pdksh 388 Yes public domain version of the Korn shell
csh 340 No OpenBSD C Shell, a version of Berkeley csh
sash 946 Yes Stand-alone shell with builtin commands (Not meant for standard "/bin/sh")
ksh 3105 Yes the real, AT&T version of the Korn shell
rc 169 No implementation of the AT&T Plan 9 rc shell
posh 201 Yes Policy-compliant Ordinary SHell (pdksh derivative)

In this tutorial chapter, the interactive shell always means bash.

1.4.2. Customizing bash

You can customize bash(1) behavior by "~/.bashrc".

For example, try the following.

# CD upon exiting MC
. /usr/lib/mc/

# set CDPATH to good one
export CDPATH

# set PATH so it includes user's private bin if it exists
if [ -d ~/bin ] ; then
export PATH

export EDITOR
[Tip] Tip

You can find more bash customization tips, such as Section 9.2.7, “Colorized commands”, in Chapter 9, System tips.

1.4.3. Special key strokes

In the Unix-like environment, there are few key strokes which have special meanings. Please note that on a normal Linux character console, only the left-hand Ctrl and Alt keys work as expected. Here are few notable key strokes to remember.

Table 1.14. List of key bindings for bash

key description of key binding
Ctrl-U erase line before cursor
Ctrl-H erase a character before cursor
Ctrl-D terminate input (exit shell if you are using shell)
Ctrl-C terminate a running program
Ctrl-Z temporarily stop program by moving it to the background job
Ctrl-S halt output to screen
Ctrl-Q reactivate output to screen
Ctrl-Alt-Del reboot/halt the system, see inittab(5)
Left-Alt-key (optionally, Windows-key) meta-key for Emacs and the similar UI
Up-arrow start command history search under bash
Ctrl-R start incremental command history search under bash
Tab complete input of the filename to the command line under bash
Ctrl-V Tab input Tab without expansion to the command line under bash

[Tip] Tip

The terminal feature of Ctrl-S can be disabled using stty(1).

1.4.4. Unix style mouse operations

Unix style mouse operations are based on the 3 button mouse system.

Table 1.15. List of Unix style mouse operations

action response
Left-click-and-drag mouse select and copy to the clipboard
Left-click select the start of selection
Right-click select the end of selection and copy to the clipboard
Middle-click paste clipboard at the cursor

The center wheel on the modern wheel mouse is considered middle mouse button and can be used for middle-click. Clicking left and right mouse buttons together serves as the middle-click under the 2 button mouse system situation. In order to use a mouse in Linux character consoles, you need to have gpm(8) running as daemon.

1.4.5. The pager

less(1) is the enhanced pager (file content browser). Hit "h" for help. It can do much more than more(1) and can be supercharged by executing "eval $(lesspipe)" or "eval $(lessfile)" in the shell startup script. See more in "/usr/share/doc/lessf/LESSOPEN". The "-R" option allows raw character output and enables ANSI color escape sequences. See less(1).

1.4.6. The text editor

You should become proficient in one of variants of Vim or Emacs programs which are popular in the Unix-like system.

I think getting used to Vim commands is the right thing to do, since Vi-editor is always there in the Linux/Unix world. (Actually, original vi or new nvi are programs you find everywhere. I chose Vim instead for newbie since it offers you help through F1 key while it is similar enough and more powerful.)

If you chose either Emacs or XEmacs instead as your choice of the editor, that is another good choice indeed, particularly for programming. Emacs has a plethora of other features as well, including functioning as a newsreader, directory editor, mail program, etc. When used for programming or editing shell scripts, it intelligently recognizes the format of what you are working on, and tries to provide assistance. Some people maintain that the only program they need on Linux is Emacs. Ten minutes learning Emacs now can save hours later. Having the GNU Emacs manual for reference when learning Emacs is highly recommended.

All these programs usually come with tutoring program for you to learn them by practice. Start Vim by typing "vim" and press F1-key. You should at least read the first 35 lines. Then do the online training course by moving cursor to "|tutor|" and pressing Ctrl-].

[Note] Note

Good editors, such as Vim and Emacs, can be used to handle UTF-8 and other exotic encoding texts correctly with proper option in the x-terminal-emulator on X under UTF-8 locale with proper font settings. Please refer to their documentation on multibyte text.

1.4.7. Setting a default text editor

Debian comes with a number of different editors. We recommend to install the vim package, as mentioned above.

Debian provides unified access to the system default editor via command "/usr/bin/editor" so other programs (e.g., reportbug(1)) can invoke it. You can change it by the following.

$ sudo update-alternatives --config editor

The choice "/usr/bin/vim.basic" over "/usr/bin/vim.tiny" is my recommendation for newbies since it supports syntax highlighting.

[Tip] Tip

Many programs use the environment variables "$EDITOR" or "$VISUAL" to decide which editor to use (see Section 1.3.5, “The internal editor in MC” and Section 9.5.11, “Customizing program to be started”). For the consistency on Debian system, set these to "/usr/bin/editor". (Historically, "$EDITOR" was "ed" and "$VISUAL" was "vi".)

1.4.8. Customizing vim

You can customize vim(1) behavior by "~/.vimrc".

For example, try the following

" -------------------------------
" Local configuration
set nocompatible
set nopaste
set pastetoggle=<f2>
syn on
if $USER == "root"
 set nomodeline
 set noswapfile
 set modeline
 set swapfile
" filler to avoid the line above being recognized as a modeline
" filler
" filler

1.4.9. Recording the shell activities

The output of the shell command may roll off your screen and may be lost forever. It is good practice to log shell activities into the file for you to review them later. This kind of record is essential when you perform any system administration tasks.

The basic method of recording the shell activity is to run it under script(1).

For example, try the following

$ script
Script started, file is typescript

Do whatever shell commands under script.

Press Ctrl-D to exit script.

$ vim typescript

See Section 9.2.3, “Recording the shell activities cleanly” .

1.4.10. Basic Unix commands

Let's learn basic Unix commands. Here I use "Unix" in its generic sense. Any Unix clone OSs usually offer equivalent commands. The Debian system is no exception. Do not worry if some commands do not work as you wish now. If alias is used in the shell, its corresponding command outputs are different. These examples are not meant to be executed in this order.

Try all following commands from the non-privileged user account.

Table 1.16. List of basic Unix commands

command description
pwd display name of current/working directory
whoami display current user name
id display current user identity (name, uid, gid, and associated groups)
file <foo> display a type of file for the file "<foo>"
type -p <commandname> display a file location of command "<commandname>"
which <commandname> , ,
type <commandname> display information on command "<commandname>"
apropos <key-word> find commands related to "<key-word>"
man -k <key-word> , ,
whatis <commandname> display one line explanation on command "<commandname>"
man -a <commandname> display explanation on command "<commandname>" (Unix style)
info <commandname> display rather long explanation on command "<commandname>" (GNU style)
ls list contents of directory (non-dot files and directories)
ls -a list contents of directory (all files and directories)
ls -A list contents of directory (almost all files and directories, i.e., skip ".." and ".")
ls -la list all contents of directory with detail information
ls -lai list all contents of directory with inode number and detail information
ls -d list all directories under the current directory
tree display file tree contents
lsof <foo> list open status of file "<foo>"
lsof -p <pid> list files opened by the process ID: "<pid>"
mkdir <foo> make a new directory "<foo>" in the current directory
rmdir <foo> remove a directory "<foo>" in the current directory
cd <foo> change directory to the directory "<foo>" in the current directory or in the directory listed in the variable "$CDPATH"
cd / change directory to the root directory
cd change directory to the current user's home directory
cd /<foo> change directory to the absolute path directory "/<foo>"
cd .. change directory to the parent directory
cd ~<foo> change directory to the home directory of the user "<foo>"
cd - change directory to the previous directory
</etc/motd pager display contents of "/etc/motd" using the default pager
touch <junkfile> create a empty file "<junkfile>"
cp <foo> <bar> copy a existing file "<foo>" to a new file "<bar>"
rm <junkfile> remove a file "<junkfile>"
mv <foo> <bar> rename an existing file "<foo>" to a new name "<bar>" ("<bar>" must not exist)
mv <foo> <bar> move an existing file "<foo>" to a new location "<bar>/<foo>" (the directory "<bar>" must exist)
mv <foo> <bar>/<baz> move an existing file "<foo>" to a new location with a new name "<bar>/<baz>" (the directory "<bar>" must exist but the directory "<bar>/<baz>" must not exist)
chmod 600 <foo> make an existing file "<foo>" to be non-readable and non-writable by the other people (non-executable for all)
chmod 644 <foo> make an existing file "<foo>" to be readable but non-writable by the other people (non-executable for all)
chmod 755 <foo> make an existing file "<foo>" to be readable but non-writable by the other people (executable for all)
find . -name <pattern> find matching filenames using shell "<pattern>" (slower)
locate -d . <pattern> find matching filenames using shell "<pattern>" (quicker using regularly generated database)
grep -e "<pattern>" *.html find a "<pattern>" in all files ending with ".html" in current directory and display them all
top display process information using full screen, type "q" to quit
ps aux | pager display information on all the running processes using BSD style output
ps -ef | pager display information on all the running processes using Unix system-V style output
ps aux | grep -e "[e]xim4*" display all processes running "exim" and "exim4"
ps axf | pager display information on all the running processes with ASCII art output
kill <1234> kill a process identified by the process ID: "<1234>"
gzip <foo> compress "<foo>" to create "<foo>.gz" using the Lempel-Ziv coding (LZ77)
gunzip <foo>.gz decompress "<foo>.gz" to create "<foo>"
bzip2 <foo> compress "<foo>" to create "<foo>.bz2" using the Burrows-Wheeler block sorting text compression algorithm, and Huffman coding (better compression than gzip)
bunzip2 <foo>.bz2 decompress "<foo>.bz2" to create "<foo>"
xz <foo> compress "<foo>" to create "<foo>.xz" using the Lempel–Ziv–Markov chain algorithm (better compression than bzip2)
unxz <foo>.xz decompress "<foo>.xz" to create "<foo>"
tar -xvf <foo>.tar extract files from "<foo>.tar" archive
tar -xvzf <foo>.tar.gz extract files from gzipped "<foo>.tar.gz" archive
tar -xvjf <foo>.tar.bz2 extract files from "<foo>.tar.bz2" archive
tar -xvJf <foo>.tar.xz extract files from "<foo>.tar.xz" archive
tar -cvf <foo>.tar <bar>/ archive contents of folder "<bar>/" in "<foo>.tar" archive
tar -cvzf <foo>.tar.gz <bar>/ archive contents of folder "<bar>/" in compressed "<foo>.tar.gz" archive
tar -cvjf <foo>.tar.bz2 <bar>/ archive contents of folder "<bar>/" in "<foo>.tar.bz2" archive
tar -cvJf <foo>.tar.xz <bar>/ archive contents of folder "<bar>/" in "<foo>.tar.xz" archive
zcat README.gz | pager display contents of compressed "README.gz" using the default pager
zcat README.gz > foo create a file "foo" with the decompressed content of "README.gz"
zcat README.gz >> foo append the decompressed content of "README.gz" to the end of the file "foo" (if it does not exist, create it first)

[Note] Note

Unix has a tradition to hide filenames which start with ".". They are traditionally files that contain configuration information and user preferences.

[Note] Note

For cd command, see builtins(7).

[Note] Note

The default pager of the bare bone Debian system is more(1) which cannot scroll back. By installing the less package using command line "apt-get install less", less(1) becomes default pager and you can scroll back with cursor keys.

[Note] Note

The "[" and "]" in the regular expression of the "ps aux | grep -e "[e]xim4*"" command above enable grep to avoid matching itself. The "4*" in the regular expression means 0 or more repeats of character "4" thus enables grep to match both "exim" and "exim4". Although "*" is used in the shell filename glob and the regular expression, their meanings are different. Learn the regular expression from grep(1).

Please traverse directories and peek into the system using the above commands as training. If you have questions on any of console commands, please make sure to read the manual page.

For example, try the following

$ man man
$ man bash
$ man builtins
$ man grep
$ man ls

The style of man pages may be a little hard to get used to, because they are rather terse, particularly the older, very traditional ones. But once you get used to it, you come to appreciate their succinctness.

Please note that many Unix-like commands including ones from GNU and BSD display brief help information if you invoke them in one of the following ways (or without any arguments in some cases).

$ <commandname> --help
$ <commandname> -h

1.5. The simple shell command

Now you have some feel on how to use the Debian system. Let's look deep into the mechanism of the command execution in the Debian system. Here, I have simplified reality for the newbie. See bash(1) for the exact explanation.

A simple command is a sequence of components.

  1. Variable assignments (optional)

  2. Command name

  3. Arguments (optional)

  4. Redirections (optional: > , >> , < , << , etc.)

  5. Control operator (optional: && , || , <newline> , ; , & , ( , ) )

1.5.1. Command execution and environment variable

Values of some environment variables change the behavior of some Unix commands.

Default values of environment variables are initially set by the PAM system and then some of them may be reset by some application programs.

  • The display manager such as gdm3 resets environment variables.

  • The shell in its start up codes resets environment variables in "~/bash_profile" and "~/.bashrc".

1.5.2. "$LANG" variable

The full locale value given to "$LANG" variable consists of 3 parts: "xx_YY.ZZZZ".

For language codes and country codes, see pertinent description in the "info gettext".

For the codeset on the modern Debian system, you should always set it to UTF-8 unless you specifically want to use the historic one with good reason and background knowledge.

For fine details of the locale configuration, see Section 8.3, “The locale”.

[Note] Note

The "LANG=en_US" is not "LANG=C" nor "LANG=en_US.UTF-8". It is "LANG=en_US.ISO-8859-1" (see Section 8.3.1, “Basics of encoding”).

Table 1.18. List of locale recommendations

locale recommendation Language (area)
en_US.UTF-8 English(USA)
en_GB.UTF-8 English(Great_Britain)
fr_FR.UTF-8 French(France)
de_DE.UTF-8 German(Germany)
it_IT.UTF-8 Italian(Italy)
es_ES.UTF-8 Spanish(Spain)
ca_ES.UTF-8 Catalan(Spain)
sv_SE.UTF-8 Swedish(Sweden)
pt_BR.UTF-8 Portuguese(Brazil)
ru_RU.UTF-8 Russian(Russia)
zh_CN.UTF-8 Chinese(P.R._of_China)
zh_TW.UTF-8 Chinese(Taiwan_R.O.C.)
ja_JP.UTF-8 Japanese(Japan)
ko_KR.UTF-8 Korean(Republic_of_Korea)
vi_VN.UTF-8 Vietnamese(Vietnam)

Typical command execution uses a shell line sequence as the following.

$ date
Sun Jun  3 10:27:39 JST 2007
$ LANG=fr_FR.UTF-8 date
dimanche 3 juin 2007, 10:27:33 (UTC+0900)

Here, the program date(1) is executed with different values of the environment variable "$LANG".

  • For the first command, "$LANG" is set to the system default locale value "en_US.UTF-8".

  • For the second command, "$LANG" is set to the French UTF-8 locale value "fr_FR.UTF-8".

Most command executions usually do not have preceding environment variable definition. For the above example, you can alternatively execute as the following.

$ LANG=fr_FR.UTF-8
$ date
dimanche 3 juin 2007, 10:27:33 (UTC+0900)

As you can see here, the output of command is affected by the environment variable to produce French output. If you want the environment variable to be inherited to subprocesses (e.g., when calling shell script), you need to export it instead by the following.

$ export LANG
[Tip] Tip

When filing a bug report, running and checking the command under "LANG=en_US.UTF-8" is good idea if you use non-English environment.

See locale(5) and locale(7) for "$LANG" and related environment variables.

[Note] Note

I recommend you to configure the system environment just by the "$LANG" variable and to stay away from "$LC_*" variables unless it is absolutely needed.

1.5.3. "$PATH" variable

When you type a command into the shell, the shell searches the command in the list of directories contained in the "$PATH" environment variable. The value of the "$PATH" environment variable is also called the shell's search path.

In the default Debian installation, the "$PATH" environment variable of user accounts may not include "/sbin" and "/usr/sbin". For example, the ifconfig command needs to be issued with full path as "/sbin/ifconfig". (Similar ip command is located in "/bin".)

You can change the "$PATH" environment variable of Bash shell by "~/.bash_profile" or "~/.bashrc" files.

1.5.4. "$HOME" variable

Many commands stores user specific configuration in the home directory and changes their behavior by their contents. The home directory is identified by the environment variable "$HOME".

Table 1.19. List of "$HOME" values

value of "$HOME" program execution situation
/ program run by the init process (daemon)
/root program run from the normal root shell
/home/<normal_user> program run from the normal user shell
/home/<normal_user> program run from the normal user GUI desktop menu
/home/<normal_user> program run as root with "sudo program"
/root program run as root with "sudo -H program"

[Tip] Tip

Shell expands "~/" to current user's home directory, i.e., "$HOME/". Shell expands "~foo/" to foo's home directory, i.e., "/home/foo/".

1.5.5. Command line options

Some commands take arguments. Arguments starting with "-" or "--" are called options and control the behavior of the command.

$ date
Mon Oct 27 23:02:09 CET 2003
$ date -R
Mon, 27 Oct 2003 23:02:40 +0100

Here the command-line argument "-R" changes date(1) behavior to output RFC2822 compliant date string.

1.5.6. Shell glob

Often you want a command to work with a group of files without typing all of them. The filename expansion pattern using the shell glob, (sometimes referred as wildcards), facilitate this need.

Table 1.20. Shell glob patterns

shell glob pattern description of match rule
* filename (segment) not started with "."
.* filename (segment) started with "."
? exactly one character
[…] exactly one character with any character enclosed in brackets
[a-z] exactly one character with any character between "a" and "z"
[^…] exactly one character other than any character enclosed in brackets (excluding "^")

For example, try the following

$ mkdir junk; cd junk; touch 1.txt 2.txt 3.c 4.h .5.txt ..6.txt
$ echo *.txt
1.txt 2.txt
$ echo *
1.txt 2.txt 3.c 4.h
$ echo *.[hc]
3.c 4.h
$ echo .*
. .. .5.txt ..6.txt
$ echo .*[^.]*
.5.txt ..6.txt
$ echo [^1-3]*
$ cd ..; rm -rf junk

See glob(7).

[Note] Note

Unlike normal filename expansion by the shell, the shell pattern "*" tested in find(1) with "-name" test etc., matches the initial "." of the filename. (New POSIX feature)

[Note] Note

BASH can be tweaked to change its glob behavior with its shopt builtin options such as "dotglob", "noglob", "nocaseglob", "nullglob", "nocaseglob", "extglob", etc. See bash(1).

1.5.7. Return value of the command

Each command returns its exit status (variable: "$?") as the return value.

Table 1.21. Command exit codes

command exit status numeric return value logical return value
success zero, 0 TRUE
error non-zero, -1 FALSE

For example, try the following.

$ [ 1 = 1 ] ; echo $?
$ [ 1 = 2 ] ; echo $?
[Note] Note

Please note that, in the logical context for the shell, success is treated as the logical TRUE which has 0 (zero) as its value. This is somewhat non-intuitive and needs to be reminded here.

1.5.8. Typical command sequences and shell redirection

Let's try to remember following shell command idioms typed in one line as a part of shell command.

Table 1.22. Shell command idioms

command idiom description
command & background execution of command in the subshell
command1 | command2 pipe the standard output of command1 to the standard input of command2 (concurrent execution)
command1 2>&1 | command2 pipe both standard output and standard error of command1 to the standard input of command2 (concurrent execution)
command1 ; command2 execute command1 and command2 sequentially
command1 && command2 execute command1; if successful, execute command2 sequentially (return success if both command1 and command2 are successful)
command1 || command2 execute command1; if not successful, execute command2 sequentially (return success if command1 or command2 are successful)
command > foo redirect standard output of command to a file foo (overwrite)
command 2> foo redirect standard error of command to a file foo (overwrite)
command >> foo redirect standard output of command to a file foo (append)
command 2>> foo redirect standard error of command to a file foo (append)
command > foo 2>&1 redirect both standard output and standard error of command to a file "foo"
command < foo redirect standard input of command to a file foo
command << delimiter redirect standard input of command to the following lines until "delimiter" is met (here document)
command <<- delimiter redirect standard input of command to the following lines until "delimiter" is met (here document, the leading tab characters are stripped from input lines)

The Debian system is a multi-tasking system. Background jobs allow users to run multiple programs in a single shell. The management of the background process involves the shell builtins: jobs, fg, bg, and kill. Please read sections of bash(1) under "SIGNALS", and "JOB CONTROL", and builtins(1).

For example, try the following

$ </etc/motd pager
$ pager </etc/motd
$ pager /etc/motd
$ cat /etc/motd | pager

Although all 4 examples of shell redirections display the same thing, the last example runs an extra cat command and wastes resources with no reason.

The shell allows you to open files using the exec builtin with an arbitrary file descriptor.

$ echo Hello >foo
$ exec 3<foo 4>bar  # open files
$ cat <&3 >&4       # redirect stdin to 3, stdout to 4
$ exec 3<&- 4>&-    # close files
$ cat bar

Here, "n<&-" and "n>&-" mean to close the file descriptor "n".

The file descriptor 0-2 are predefined.

Table 1.23. Predefined file descriptors

device description file descriptor
stdin standard input 0
stdout standard output 1
stderr standard error 2

1.5.9. Command alias

You can set an alias for the frequently used command.

For example, try the following

$ alias la='ls -la'

Now, "la" works as a short hand for "ls -la" which lists all files in the long listing format.

You can list any existing aliases by alias (see bash(1) under "SHELL BUILTIN COMMANDS").

$ alias
alias la='ls -la'

You can identity exact path or identity of the command by type (see bash(1) under "SHELL BUILTIN COMMANDS").

For example, try the following

$ type ls
ls is hashed (/bin/ls)
$ type la
la is aliased to ls -la
$ type echo
echo is a shell builtin
$ type file
file is /usr/bin/file

Here ls was recently searched while "file" was not, thus "ls" is "hashed", i.e., the shell has an internal record for the quick access to the location of the "ls" command.

1.6. Unix-like text processing

In Unix-like work environment, text processing is done by piping text through chains of standard text processing tools. This was another crucial Unix innovation.

1.6.1. Unix text tools

There are few standard text processing tools which are used very often on the Unix-like system.

  • No regular expression is used:

    • cat(1) concatenates files and outputs the whole content.

    • tac(1) concatenates files and outputs in reverse.

    • cut(1) selects parts of lines and outputs.

    • head(1) outputs the first part of files.

    • tail(1) outputs the last part of files.

    • sort(1) sorts lines of text files.

    • uniq(1) removes duplicate lines from a sorted file.

    • tr(1) translates or deletes characters.

    • diff(1) compares files line by line.

  • Basic regular expression (BRE) is used:

    • grep(1) matches text with patterns.

    • ed(1) is a primitive line editor.

    • sed(1) is a stream editor.

    • vim(1) is a screen editor.

    • emacs(1) is a screen editor. (somewhat extended BRE)

  • Extended regular expression (ERE) is used:

    • egrep(1) matches text with patterns.

    • awk(1) does simple text processing.

    • tcl(3tcl) can do every conceivable text processing: re_syntax(3). Often used with tk(3tk).

    • perl(1) can do every conceivable text processing. perlre(1).

    • pcregrep(1) from the pcregrep package matches text with Perl Compatible Regular Expressions (PCRE) pattern.

    • python(1) with the re module can do every conceivable text processing. See "/usr/share/doc/python/html/index.html".

If you are not sure what exactly these commands do, please use "man command" to figure it out by yourself.

[Note] Note

Sort order and range expression are locale dependent. If you wish to obtain traditional behavior for a command, use C locale instead of UTF-8 ones by prepnding command with "LANG=C" (see Section 1.5.2, “"$LANG" variable” and Section 8.3, “The locale”).

[Note] Note

Perl regular expressions (perlre(1)), Perl Compatible Regular Expressions (PCRE), and Python regular expressions offered by the re module have many common extensions to the normal ERE.

1.6.2. Regular expressions

Regular expressions are used in many text processing tools. They are analogous to the shell globs, but they are more complicated and powerful.

The regular expression describes the matching pattern and is made up of text characters and metacharacters.

The metacharacter is just a character with a special meaning. There are 2 major styles, BRE and ERE, depending on the text tools as described above.

Table 1.24. Metacharacters for BRE and ERE

BRE ERE description of the regular expression
\ . [ ] ^ $ * \ . [ ] ^ $ * common metacharacters
\+ \? \( \) \{ \} \|   BRE only "\" escaped metacharacters
  + ? ( ) { } | ERE only non-"\" escaped metacharacters
c c match non-metacharacter "c"
\c \c match a literal character "c" even if "c" is metacharacter by itself
. . match any character including newline
^ ^ position at the beginning of a string
$ $ position at the end of a string
\< \< position at the beginning of a word
\> \> position at the end of a word
\[abc…\] [abc…] match any characters in "abc…"
\[^abc…\] [^abc…] match any characters except in "abc…"
r* r* match zero or more regular expressions identified by "r"
r\+ r+ match one or more regular expressions identified by "r"
r\? r? match zero or one regular expressions identified by "r"
r1\|r2 r1|r2 match one of the regular expressions identified by "r1" or "r2"
\(r1\|r2\) (r1|r2) match one of the regular expressions identified by "r1" or "r2" and treat it as a bracketed regular expression

The regular expression of emacs is basically BRE but has been extended to treat "+"and "?" as the metacharacters as in ERE. Thus, there are no needs to escape them with "\" in the regular expression of emacs.

grep(1) can be used to perform the text search using the regular expression.

For example, try the following

$ egrep 'GNU.*LICENSE|Yoyodyne' /usr/share/common-licenses/GPL
Yoyodyne, Inc., hereby disclaims all copyright interest in the program

1.6.3. Replacement expressions

For the replacement expression, some characters have special meanings.

Table 1.25. The replacement expression

replacement expression description of the text to replace the replacement expression
& what the regular expression matched (use \& in emacs)
\n what the n-th bracketed regular expression matched ("n" being number)

For Perl replacement string, "$n" is used instead of "\n" and "&" has no special meaning.

For example, try the following

$ echo zzz1abc2efg3hij4 | \
sed -e 's/\(1[a-z]*\)[0-9]*\(.*\)$/=&=/'
$ echo zzz1abc2efg3hij4 | \
sed -e 's/\(1[a-z]*\)[0-9]*\(.*\)$/\2===\1/'
$ echo zzz1abc2efg3hij4 | \
perl -pe 's/(1[a-z]*)[0-9]*(.*)$/$2===$1/'
$ echo zzz1abc2efg3hij4 | \
perl -pe 's/(1[a-z]*)[0-9]*(.*)$/=&=/'

Here please pay extra attention to the style of the bracketed regular expression and how the matched strings are used in the text replacement process on different tools.

These regular expressions can be used for cursor movements and text replacement actions in some editors too.

The back slash "\" at the end of line in the shell commandline escapes newline as a white space character and continues shell command line input to the next line.

Please read all the related manual pages to learn these commands.

1.6.4. Global substitution with regular expressions

The ed(1) command can replace all instances of "FROM_REGEX" with "TO_TEXT" in "file".

$ ed file <<EOF

The sed(1) command can replace all instances of "FROM_REGEX" with "TO_TEXT" in "file".

$ sed -ie 's/FROM_REGEX/TO_TEXT/g' file

The vim(1) command can replace all instances of "FROM_REGEX" with "TO_TEXT" in "file" by using ex(1) commands.

$ vim '+%s/FROM_REGEX/TO_TEXT/gc' '+w' '+q' file
[Tip] Tip

The "c" flag in the above ensures interactive confirmation for each substitution.

Multiple files ("file1", "file2", and "file3") can be processed with regular expressions similarly with vim(1) or perl(1).

$ vim '+argdo %s/FROM_REGEX/TO_TEXT/ge|update' '+q' file1 file2 file3
[Tip] Tip

The "e" flag in the above prevents the "No match" error from breaking a mapping.

$ perl -i -p -e 's/FROM_REGEX/TO_TEXT/g;' file1 file2 file3

In the perl(1) example, "-i" is for in-place editing, "-p" is for implicit loop over files.

[Tip] Tip

Use of argument "-i.bak" instead of "-i" keeps each original file by adding ".bak" to its filename. This makes recovery from errors easier for complex substitutions.

[Note] Note

ed(1) and vim(1) are BRE; perl(1) is ERE.

1.6.5. Extracting data from text file table

Let's consider a text file called "DPL" in which some pre-2004 Debian project leader's names and their initiation days are listed in a space-separated format.

Ian     Murdock   August  1993
Bruce   Perens    April   1996
Ian     Jackson   January 1998
Wichert Akkerman  January 1999
Ben     Collins   April   2001
Bdale   Garbee    April   2002
Martin  Michlmayr March   2003

Awk is frequently used to extract data from these types of files.

For example, try the following

$ awk '{ print $3 }' <DPL                   # month started
$ awk '($1=="Ian") { print }' <DPL          # DPL called Ian
Ian     Murdock   August  1993
Ian     Jackson   January 1998
$ awk '($2=="Perens") { print $3,$4 }' <DPL # When Perens started
April 1996

Shells such as Bash can be also used to parse this kind of file.

For example, try the following

$ while read first last month year; do
    echo $month
  done <DPL
... same output as the first Awk example

Here, the read builtin command uses characters in "$IFS" (internal field separators) to split lines into words.

If you change "$IFS" to ":", you can parse "/etc/passwd" with shell nicely.

$ oldIFS="$IFS"   # save old value
$ IFS=':'
$ while read user password uid gid rest_of_line; do
    if [ "$user" = "bozo" ]; then
      echo "$user's ID is $uid"
  done < /etc/passwd
bozo's ID is 1000
$ IFS="$oldIFS"   # restore old value

(If Awk is used to do the equivalent, use "FS=':'" to set the field separator.)

IFS is also used by the shell to split results of parameter expansion, command substitution, and arithmetic expansion. These do not occur within double or single quoted words. The default value of IFS is <space>, <tab>, and <newline> combined.

Be careful about using this shell IFS tricks. Strange things may happen, when shell interprets some parts of the script as its input.

$ IFS=":,"                        # use ":" and "," as IFS
$ echo IFS=$IFS,   IFS="$IFS"     # echo is a Bash builtin
IFS=  , IFS=:,
$ date -R                         # just a command output
Sat, 23 Aug 2003 08:30:15 +0200
$ echo $(date -R)                 # sub shell --> input to main shell
Sat  23 Aug 2003 08 30 36 +0200
$ unset IFS                       # reset IFS to the default
$ echo $(date -R)
Sat, 23 Aug 2003 08:30:50 +0200

1.6.6. Script snippets for piping commands

The following scripts do nice things as a part of a pipe.

Table 1.26. List of script snippets for piping commands

script snippet (type in one line) effect of command
find /usr -print find all files under "/usr"
seq 1 100 print 1 to 100
| xargs -n 1 <command> run command repeatedly with each item from pipe as its argument
| xargs -n 1 echo split white-space-separated items from pipe into lines
| xargs echo merge all lines from pipe into a line
| grep -e <regex_pattern> extract lines from pipe containing <regex_pattern>
| grep -v -e <regex_pattern> extract lines from pipe not containing <regex_pattern>
| cut -d: -f3 - extract third field from pipe separated by ":" (passwd file etc.)
| awk '{ print $3 }' extract third field from pipe separated by whitespaces
| awk -F'\t' '{ print $3 }' extract third field from pipe separated by tab
| col -bx remove backspace and expand tabs to spaces
| expand - expand tabs
| sort| uniq sort and remove duplicates
| tr 'A-Z' 'a-z' convert uppercase to lowercase
| tr -d '\n' concatenate lines into one line
| tr -d '\r' remove CR
| sed 's/^/# /' add "#" to the start of each line
| sed 's/\.ext//g' remove ".ext"
| sed -n -e 2p print the second line
| head -n 2 - print the first 2 lines
| tail -n 2 - print the last 2 lines

One-line shell script can loop over many files using find(1) and xargs(1) to perform quite complicated tasks. See Section 10.1.5, “Idioms for the selection of files” and Section 9.5.9, “Repeating a command looping over files”.

When using the shell interactive mode becomes too complicated, please consider to write a shell script (see Section 12.1, “The shell script”).