When working with Linux using CLI (command line interface), you are interacting with a program called a shell. All you type is passed to the shell, which interprets what you have typed, does parameters expansion (which is somewhat similar to brace expansion in algebra) and executes programs for you. Shell which we will be using is called Bash, which stands for Bourne Again Shell, which is in turn a pun. Now I will show you general overview of how the bash works in plain English:

YOU
    Log into a Linux box
    Your identity is verified by username (user1) and password (123qwe)
    Bash is executed
Bash
    reads and executes initial commands from your profile, which define:
        What your command prompt will look like
        What colors you will see when working in Linux
        What you editor is
        What you viewer is
        ...
    After initial commands are read, Bash enters in a loop
        While not ordered to exit by typing 'exit', 'logout' or hitting <CTRL+D>:
            Read a line
            Parse this line, expand braces
            Execute command with expanded parameters

I repeat, any command which you type is not executed directly, but expanded firstly and only then executed. For example, when you type ls * the star * is expanded into list of all files in current directory.

Now you will learn how to modify your profile and how to write and view your history.

 1: ls -al
 2: cat .profile
 3: echo Hello, $LOGNAME!
 4: cp -v .profile .profile.bak
 5: echo 'echo Hello, $LOGNAME!' >> .profile
 6: tail -n 5 .profile
 7: history -w
 8: ls -altr
 9: cat .bash_history
10: exit

user1@vm1's password:
Linux vm1 2.6.32-5-amd64 #1 SMP Sun May 6 04:00:17 UTC 2012 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.
Last login: Thu Jun  7 12:03:29 2012 from sis.site
Hello, user1!
user1@vm1:~$ ls -al
total 20
drwxr-xr-x 2 user1 user1 4096 Jun  7 12:18 .
drwxr-xr-x 3 root  root  4096 Jun  6 21:49 ..
-rw-r--r-- 1 user1 user1  220 Jun  6 21:48 .bash_logout
-rw-r--r-- 1 user1 user1 3184 Jun  6 21:48 .bashrc
-rw-r--r-- 1 user1 user1  697 Jun  7 12:04 .profile
user1@vm1:~$ cat .profile
# ~/.profile: executed by the command interpreter for login shells.
# This file is not read by bash(1), if ~/.bash_profile or ~/.bash_login
# exists.
# see /usr/share/doc/bash/examples/startup-files for examples.
# the files are located in the bash-doc package.
 
# the default umask is set in /etc/profile; for setting the umask
# for ssh logins, install and configure the libpam-umask package.
#umask 022
 
# if running bash
if [ -n "$BASH_VERSION" ]; then
    # include .bashrc if it exists
    if [ -f "$HOME/.bashrc" ]; then
        . "$HOME/.bashrc"
    fi
fi
 
# set PATH so it includes user's private bin if it exists
if [ -d "$HOME/bin" ] ; then
    PATH="$HOME/bin:$PATH"
fi
echo Hello, $LOGNAME!
user1@vm1:~$ echo Hello, $LOGNAME!
Hello, user1!
user1@vm1:~$ cp -v .profile .profile.bak
`.profile' -> `.profile.bak'
user1@vm1:~$ echo 'echo Hello, $LOGNAME!' >> .profile
user1@vm1:~$ tail -n 5 .profile
# set PATH so it includes user's private bin if it exists
if [ -d "$HOME/bin" ] ; then
    PATH="$HOME/bin:$PATH"
fi
echo Hello, $LOGNAME!
user1@vm1:~$ history -w
user1@vm1:~$ ls -altr
total 28
-rw-r--r-- 1 user1 user1 3184 Jun  6 21:48 .bashrc
-rw-r--r-- 1 user1 user1  220 Jun  6 21:48 .bash_logout
drwxr-xr-x 3 root  root  4096 Jun  6 21:49 ..
-rw-r--r-- 1 user1 user1  741 Jun  7 12:19 .profile.bak
-rw------- 1 user1 user1  308 Jun  7 12:21 .bash_history
-rw-r--r-- 1 user1 user1  697 Jun  7 12:25 .profile
drwxr-xr-x 2 user1 user1 4096 Jun  7 12:25 .
user1@vm1:~$ cat .bash_history
ls -al
cat .profile
echo Hello, $LOGNAME!
cp -v .profile .profile.bak
echo 'echo Hello, $LOGNAME!' >> .profile
tail -n 5 .profile
history -w
ls -altr
user1@vm1:~$ exit
logout

Don not be afraid, all this commands will be explained. Line numbers correspond to those “Now type this” section.

1. Prints out all files in current directory, including hidden ones. Options -al tell ls to print file list in long format and include all files including hidden ones. .profile and .bash_rc are hidden files, because they start with dot .. Every file starting with dot is hidden, it is that simple. This two particular files are shell scripts, They contain instructions which are executed when you log in.
2. Prints out your .profile file. Just that.
3. Tells your shell, which is in your case bash, to output a string Hello, $LOGNAME!, substituting $LOGNAME with environment variable $LOGNAME which happens to contain your login.
4. Copies your .profile file to .profile.bak. Key -v tells cp to be verbose, which means it will print out all operations. Remember this key, it is often used to tell commands to feed you more information than they do by default.
5. Adds a line to your .bash_rc profile. From now on, every time you will log into vm1 this command will be executed. Attention, >> means to add something to file, but > means replacing file with something. If you accidentally replaced your .profile instead of appending to it, command

   cp -v .profile.bak .profile

   will return your old .profile file.

6. Prints out exactly five last lines from .profile file.
7. Writes all your command history to .bash_history file. Normally this is done at the end of your session, when you close it by typing exit or by pressing <CTRL>+D.
8. Prints out files in current directory. Options -tr mean that file list is sorted by time in reverse direction. This means that most recently created and modified files are printed last. Notice how you have 2 new files now.
9. Prints out file which keeps history of your commands. Notice how all that you have type is here.
10. Closes the session.

1. Search online why ls -al tells you “total 20” then there are only 5 files present. What does it mean? Notice that “.” and “..” are special file entries, which correspond to the current directory and parent directory respectively.
2. Log in vm1 and type man -K /etc/profile, now using cursor keys scroll to “INVOCATION” section and read it. To exit man, type “q”. Type man man to find out what -K man option means.
3. Type in uname with space before the command. Now, type history. You see? If you prepend a command with space it won't be saved in history! Hint: usable when you need to specify the password ring on the command line.
4. Find a wiki page about bash and try to read it. No worries if it scares you, just omit frightening sections for now.