The following are the basic commands used in Linux

1.1 Access to the system and logout
Let's start with something wrong.

login: redio[Enter]

Even if the account doesn't exist you'll be asked for a password

Password: radio2001[Enter]

Login incorrect

Let's start again

login: radio[Enter]
Password: 2001[Enter]

Last login: Sun Nov 11 10:45:11 on tty1

You get information about your last access

The prompt shell means the system is ready to receive your commands.
$

1.1.1 Switching account

logging out and then logging in with a different account
using su.

$ su root[Enter]

Password: smr1301[Enter]

If it is the right password you get identity and the rights of the typed user.

1.1.2 Who am I?

When you have more then one account may be necessary to check which account you are using.

$ whoami[Enter]

root

whoami let you know under which identity you are working on.

$ logname[Enter]

radio

logname allows you to know what account you used when you first entered in the system (login).

1.1.3 Ending a session

Just need to end the shell, that is the program that shows the prompt.

$ whoami[Enter]

root

$ exit[Enter]

In case we had used su

$ whoami[Enter]

radio

This exit closes last shell, and logs the user out of the system.

$ exit[Enter]
1.1.4 Shutting down

login: root[Enter]

Password: smr1301[Enter]

# shutdown -h now[Enter]

System is going down NOW!!

...
System halted

When this happens it is possible to reboot the system.

---------

You can use su to become root and shutdown the system.
1.2 Password management

1.2.1 User root changes password to regular user

# passwd radio[Enter]

New UNIX password: 123[Enter]

The password is too short, the system advises you but it doesn't stop you from using it.

BAD PASSWORD: it's a WAY too short

Retype new UNIX password: 123[Enter]

passwd: all authentication tokens updated successfully

1.2.2 Regular user changes its password

$ passwd[Enter]

You'll be asked for the old password.

Changing password for radio

(current) UNIX password: 123[Enter]

New UNIX password: elpoep[Enter]

BAD PASSWORD: it is based on a (reversed) dictionary word

passwd: Authentication token manipulation error

$ passwd[Enter]

Changing password for radio

(current) UNIX password: 123[Enter]

New UNIX password: I cannot understand PCs[Enter]

Retype new UNIX password: I cannot understand PCs[Enter]

passwd: all authentication tokens updated successfully

1.3 Moving around directories

The filesystem is organizated in directory and subdirectory.

1.3.1 Current Directory

$ cd /usr/bin[Enter]

$ pwd[Enter]

/usr/bin

1.3.2 Absolute and relative path

The absolute path starts from the root directory while the relative one starts from the current directory.

$ cd /usr/local[Enter]

This is an absolute path since it starts with /.

$ pwd[Enter]

/usr/local

$ cd bin[Enter]

This is a relative one: it starts from the local directory moving downward to bin.

$ pwd[Enter]

/usr/local/bin

1.3.3 Moving backward

Every directory holds two references to special subdirectory:

single (.) that is a reference to the current directory.
double (..) that is a reference to the previous directory.

These symbols are effective directory names.

$ cd ..[Enter]

$ pwd[Enter]

/usr/local

$ cd ../bin[Enter]

$ pwd[Enter]

/usr/bin

You may go backward for more then one level.

$ cd ../../var/tmp[Enter]

$ pwd[Enter]

/var/tmp

$ cd /usr/bin/../local/bin/..[Enter]

$ pwd[Enter]

/usr/local

1.3.4 The precise reference to the current directory

The current directory can be seen as a single dot. In practice all relative paths can start with the prefix ./.

$ cd ./bin[Enter]

$ pwd[Enter]

/usr/local/bin

1.3.5 Home Directory

Every user has his own personal directory, known as home, it keeps all the data owned by the user. Typing just cd the user can reach his home directory.

$ cd[Enter]

$ pwd[Enter]

/home/radio

Some shells replace tilde (~) at the begging of a path with the path of the home directory of the working user.

$ cd ~[Enter]

$ pwd[Enter]

/home/radio

In the same way if tilde is set before a user account it will be replaced with the path of user's home directory.

$ cd ~ftp[Enter]

$ pwd[Enter]

/home/ftp

Going back to home directory.

$ cd[Enter]

1.4 File content

1.4.1 Directory content

To list the directory content command ls is used

$ ls /bin[Enter]

…..
……

The command ls /bin lists the content of /bin/.

A more expressive listing can be obtened using option -l.

$ ls -l /bin[Enter]
-rwxr-xr-x 1 root root 2612 Mar 7 11:29 arch
-rwxr-xr-x 1 root root 60592 Feb 3 20:12 ash
-rwxr-xr-x 1 root root 263064 Feb 3 20:12 ash.static
-rwxr-xr-x 1 root root 9968 Feb 3 19:04 aumix-minimal
lrwxrwxrwx 1 root root 4 Apr 13 23:28 awk -> gawk
-rwxr-xr-x 1 root root 5756 Mar 7 12:15 basename
-rwxr-xr-x 1 root root 316848 Feb 27 18:44 bash
... many lines ...
-rwxr-xr-x 1 root root 4320 Mar 7 12:15 true
-rwsr-xr-x 1 root root 26608 Feb 3 15:14 umount
-rwxr-xr-x 1 root root 6196 Mar 7 12:15 uname
lrwxrwxrwx 1 root root 14 Apr 13 23:49 userconf -> /bin/linuxco
nf
-rwxr-xr-x 1 root root 16252 Mar 8 17:26 usleep
-rwxr-xr-x 1 root root 346352 Mar 7 18:18 vi
lrwxrwxrwx 1 root root 2 Apr 14 00:00 view -> vi
-rwxr-xr-x 1 root root 362 Mar 7 18:18 vimtutor
lrwxrwxrwx 1 root root 8 Apr 13 23:51 ypdomainname -> hostname
-rwxr-xr-x 3 root root 46384 Feb 15 17:04 zcat

$ cd[Enter]

$ ls[Enter]

To list hidden files

$ ls -a[Enter]
. .bash_history .enlightenment .mc
.. .bash_logout .gnome .tcshrc
.ICEauthority .bash_profile .gnome-desktop .xsession-errors
.Xauthority .bashrc .gnome-help-browser
.Xdefaults .cshrc .gnome_private

1.4.2 File Content

In order to analyse the file content cat,less and more ca be used (it doesn't make sense if it is a binary file).

$ cat /etc/issue[Enter]

Red Hat Linux release 6.2 (Zoot)
Kernel 2.2.14-5.0 on an i586

1.4.3 Find out the file type

file is command the uses the so called magic number (Unix tradition) to figure out the file type..

$ file /etc/*[Enter]

/etc/CORBA: directory
/etc/DIR_COLORS: English text
/etc/HOSTNAME: ASCII text
/etc/X11: directory
/etc/adjtime: ASCII text
/etc/aliases: English text
...many lines...
/etc/shells: ASCII text
/etc/skel: directory
/etc/smb.conf: English text
/etc/smrsh: directory
/etc/snmp: directory
/etc/sound: directory
/etc/sysconfig: directory
/etc/sysctl.conf: English text
/etc/syslog.conf: English text
/etc/termcap: English text
/etc/up2date.conf: can't read `/etc/up2date.conf' (Permission denied).
/etc/vga: directory
/etc/yp.conf: English text
/etc/ypserv.conf: English text

This method is not realible but can be useful.

1.4.4 Used and free space

In order to check the free space on disk you can use df.

$ df[Enter]

The result maybe something similar to this one.
Filesystem 1k-blocks Used Available Use% Mounted on
/dev/hda5 1511968 805300 629860 56% /
/dev/hda1 99521 2482 91900 3% /boot
To check the directory used space: du.

$ du /bin[Enter]

5240 /bin

In this case the /bin/ directory holds files for a total amount of 5240KB.

1.5 File creation, copy and deletion

1.5.1 File creation

There are different ways to create a file. The easiest way to create an empty file si to use touch. First move to the home directory, the best place where to play.

$ cd[Enter]

$ touch myfile[Enter]

$ ls -l myfile[Enter]

-rw-rw-r-- 1 radio radio 0 Dec 23 10:49 myfile

The file was created..

You can use cat too:

$ cat > myfile2[Enter]

there are better ways to write[Enter]

text.[Enter]

This is a oneway writing.[Enter]

[Ctrl+d]

$ cat myfile2[Enter]

1.5.2 File copy

$ cp myfile2 myfile3[Enter]

Group copy is possible only if the last file is an existing directory.

$ cp myfile myfile2 myfile3 /tmp[Enter]

$ cp myfile* /tmp[Enter]

1.5.3 File deletion

Becareful when you delete something as root!!

$ rm myfile myfile2[Enter]

There is noway to recover deleted files.

You can use the wild chars:* and ?.

$ ls myfile*[Enter]

myfile3

$ rm myfile*[Enter]

1.6 Working with directory

1.6.1 Directory creation

$ cd[Enter]

$ mkdir mydir[Enter]

Let's check with ls.

$ ls -l[Enter]

...
drwxr-xr-x 8 radio radio 1024 Dec 23 12:11 mydir
...

The d character at the beginning of the string tells has that the file is a directory.

1.6.2 Directory copy

cp with option -r or-R.

$ cp -r mydir mydir2[Enter]

1.6.3 Directory deletion

You may delete an empty directory using rmdir.

$ rmdir mydir2[Enter]

Let's try with something more complex.

$ mkdir carbon[Enter]

$ mkdir carbon/hydrogen[Enter]

$ mkdir carbon/oxygen[Enter]

$ mkdir carbon/hydrogen/helium[Enter]

$ rmdir carbon[Enter]

rmdir: carbon: Directory not empty

$ rm -r carbon[Enter]

1.7 Moving around directories

In Unix environment rename and moving a file are the same thing.

1.7.1 Moving and renaming

The command used is mv.

$ touch white[Enter]

$ touch green[Enter]

$ mkdir purple[Enter]

Let's check.

$ ls -l[Enter]

...

-rw-rw-r-- 1 radio radio 0 Dec 25 12:46 white

-rw-rw-r-- 1 radio radio 0 Dec 25 12:46 green

drwxrwxr-x 2 radio radio 1024 Dec 25 12:46 purple

...

Let's rename white file to make it brown.

$ mv white brown[Enter]

$ ls -l[Enter]

...

-rw-rw-r-- 1 radio radio 0 Dec 25 12:46 brown

...

To move more file all in once the destination must be a directory.

$ mv brown green purple[Enter]

$ ls -l purple[Enter]

-rw-rw-r-- 1 radio radio 0 Dec 25 12:46 green

-rw-rw-r-- 1 radio radio 0 Dec 25 12:46 brown

$ mv purple /tmp[Enter]
1.8 The shell

The shell is the way to interact with the operative systme. The shell bash is the one used in our exercises.

1.8.1 Automatic fulfilling

The shell can fulfill a command using [Tab], this feature is particularly useful when you have file with long names.

$ touch microprocessor[Enter]

$ touch microscopic[Enter]

$ touch supersonic[Enter]

$ ls sup[Tab]

$ ls sup[Tab]ersonic[Enter]

$ ls mic[Tab]ro

$ ls mic[Tab]rop[Tab]rocessor[Enter]

1.9.2 Substitution: wild char

This is an alternative way to fulfill a command, it is the the shell that changes symbols with the right information.

1.8.2.1 Asterisk *

That symbol can be replaced with a sequence, from 0 to infinity, of symbols.

$ ls[Enter]

$ ls *[Enter]

This command is different, the shell changes the * with the list of files and directory held in the current directory. This means that if there is any subdirectory its content
would be displayed.

$ ls micro*[Enter]

microprocessor microscopic

* can be changed with null string:

$ touch millimicro[Enter]

$ ls *micro*[Enter]

microprocessor microscopic millimicro

8.2.2 Question mark ?

The question mark ? can be change with just one symbol

Let's create some files.

$ touch xy123j4[Enter]

$ touch xy456j5[Enter]

$ touch xy789j111[Enter]

$ touch xy78j67[Enter]

$ ls [Enter]

xy123j4
xy456j5
xy789j111
xy78j67

$ ls ?????j?[Enter]

xy123j4
xy456j5

We would have a different result using *

$ ls *j*[Enter]

xy123j4 xy456j5 xy789j111 xy78j67

1.8.2.3 Square brackets [ ]

The square brackets are used to have a range of symbols from which to choose the substitution symbols. Just one symbol from the one listed is used.

$ ls xy????[4567]*[Enter]

xy123j4 xy456j5

$ ls xy????[4-7]*[Enter]

1.8.2.4 Escape

For some special symbols you need to use escape

$ touch six\*height[Enter]

$ ls[Enter]

...

six*height

If you create a filename with a space inside

$ touch my\ letter[Enter]

$ ls[Enter]

...

my letter

six*height

1.8.3 Input/Output redirection and pipeline

The shell allows you to redirect command output from the standard output (usually the screen). The same for the input.

1.8.4 Redirection

$ ls -l > mylist[Enter]

$ cat mylist[Enter]

For the input .. instead of using the standard input (keyboard).

$ cat < mylist[Enter]

Appending to a file.

$ ls -l /tmp >> mylist[Enter]

$ cat mylist[Enter]

1.8.5 Pipeline

The pipeline is a way of redirecting input and output commands.

$ cat mylist sort[Enter]

$ cat < mylist sort[Enter]

Make it easier but without using pipeline.

$ sort < mylist[Enter]

1.8.6 Alias

Aliases allows you to create an alternative name to an existing command.

$ alias ll='ls -l'[Enter]

$ ll[Enter]

This alias take options as an ordinary command.

$ ll micro*[Enter]

It's the same as ls -l micro*.

-rw-rw-r-- 1 radio radio 0 Dec 26 10:19 microprocessor

-rw-rw-r-- 1 radio radio 0 Dec 26 10:19 microscopic


This are the aliases that are usually created to avoid mistake

$ alias rm='rm -i'[Enter]

$ alias cp='cp -i'[Enter]

$ alias mv='mv -i'[Enter]

Now try to remove a file.

$ rm microprocessor[Enter]

rm: remove `microprocessor'?:

n[Enter]

In this way the file was not removed.

1.9 Searching

Searching files or directories it's an important task in a filesystem as complex as the Linux (or Unix) one.

1.9.1 Find

Searching a file or directory using its name or other external feature find command is used.

$ find / -name bash -print[Enter]

This command search for files and directory named bash inside all the directory starting from the root directory (/).

/bin/bash

...

find: /var/run/sudo: Permission denied

find: /var/spool/at: Permission denied

find: /var/spool/cron: Permission denied

...

You can use wild chars, in this case it will be the find command that will have to manage them, without using shell.

$ find / -name \*sh -print[Enter]

The escape char \ tells the shell not to translate the * as wild char.

/bin/bash

/bin/ash

/bin/sh

...
1.9.2 Grep

To search inside the content of a file grep is used.

$ grep radio /etc/*[Enter]

/etc/group:radio::500:radio

/etc/passwd:radio:Ide2ncPYY1234:500:500:radiolab:/home/radio:/bin/bash

grep: /etc/skel: Is a directory

grep: /etc/sudoers: Permission denied

Thanks!!!