IT 244: Introduction to Linux/Unix

IT 244: Introduction to Linux/Unix
Class 19 Exercise

Log in to users3.cs.umb.edu

Run the ssh client

Connect to users3.cs.umb.edu.

Directory for This Exercise

Go to your ex directory
```
    cd  ~/it244/ex
```
Create an ex19 directory
```
    mkdir  ex19
```
Go to your ex19 directory
```
    cd  ex19
```

The `history` mechanism

Run history
```
    history
```
The list you see here is every one of your last 500 command lines.
Display your last 10 command lines
```
    history 10
```
When you give history a numeric argument, it lists only that number of commands.
Execute again some previous command using the event number
```
    !EVENT_NUMBER
```
where EVENT_NUMBER is a number you obtained from the history command.

Notice that the old command line appears first, and then the output that the command line produced.
Execute again some previous command using the first letter of that command line
```
    !h
```

Look at the Values of the `history` Variables

Display HISTFILE
```
    echo $HISTFILE
```
This is the file that contains your history record.
Examine the most recent contents of your history file
```
    tail $HISTFILE
```
Display HISTFILESIZE
```
    echo $HISTFILESIZE
```
This is the maximum number of command lines the history mechanism will record in ~/.bash_history when you quit
Count the number of lines in your history file
```
    cat $HISTFILE | wc -l 
```
The wc (word count) command with the -l option counts the number of lines in the file given to it as an argument.

The number this command returns should be less than our equal to the value of HISTFILE.
Display HISTFILESIZE
```
    echo $HISTSIZE
```
The value of this variable sets the maximum number of command lines saved by the history mechanism between individual terminal sessions.

Using `fc` to Edit a Command

Go to my home directory and execute pwd on the same line
```
    cd ~tsoro ; pwd
```
Run fc
```
    fc
```
The command line you just entered appears in an editing window.
Edit the command line

Make it read like this:
```
    cd ~; pwd; ls
```
Save changes and quit

Save these changes (Control O) and quit (control X), using the key combinations you see at the bottom of the editing window.

The command line you just edited should appear first, followed by the output of the edited command.

The edited command line has taken you to your home directory, run pwd and list the contents of that directory.
Look at your last 16 command lines
```
    fc -l
```
Edit some command line using its event number
```
    fc EVENT_NUMBER
```
Make some changes save and quit.
Edit the last call to history
```
    fc h
```
Change the command to read
```
    history 5
```
Save these changes and quit
The edited command will now run.

Command Line Editing

Enter a long command line
```
    echo foo; echo bar; echo bletch
```
Recover this last command line but do not hit Enter
```
    ↑
```
Go to the beginning of the command line
```
    Control A
```
Go to the end of the command line
```
    Control E
```
Go back to just before the last echo command

Use the left arrow key (←) to do this.
Remove the last echo command
```
    Control K
```
Remove the command line completely
```
    Control U
```

Command Completion

Go back to your ex19 directory
```
    cd ~/it244/ex/ex19
```
Enter the first two letters of the mkdir command then hit Tab
```
    mk[Tab]
```
The terminal should beep.
Hit Tab a second time
```
    [Tab]
```
You should now see a list of all commands that begin with "mk".
Type "di" after "mk" and hit Tab

The mkdir command is complete.
Create the directory dir1

Add the argument "dir1" after mkdir.

Pathname Completion

Go to your it244 directory using pathname completion
```
    cd  ~/it2[Tab]  [Enter]
```
You should now be in your it244 directory.
Show your current directory and it's contents
```
    pwd ; ls
```
Enter the first few letters of a pathname, then hit Tab
```
    cd ~/it244/e[Tab]
```
The Readline library adds an "x"
Hit Tab again

The Readline library adds "ex" to the pathname.
Hit the Tab twice more

The Readline library lists the example directories.
Add "20" and hit Return

This will take you back to your ex19 directory.

Variable Completion

Create the variable team
```
    team='Red Sox'
```
Use variable completion to print the value of this variable
```
    echo $t[Tab][Enter]
```
Create two new variables
```
    blar=green ; bletch=red
```
Use variable name completion
```
    echo $b[Tab]
```
Readline completion adds an l.
Use variable completion to see your choices
```
[Tab][Tab]
```
Completion shows you the variables that match.
Complete the command
```
r[Enter]
```

Aliases

Go to your home directory
```
    cd
```
Make sure you are in your home directory
```
    pwd
```
Edit .bash_profile using an editor
```
    nano .bash_profile
```
Create the global variable ex in .bash_profile
```
    export ex=it244/ex
```
Create an alias using your ex variable
```
    alias ex='cd $ex'
```
Save these changes and quit
Make this change take effect
```
    source ~/.bash_profile
```
Use this alias
```
    ex
```
Show your current directory
```
    pwd
```
You should be in your ex directory inside your it244 directory.
Go back to your home directory
```
    cd
```
Open .bash_profile in an editor
```
    nano .bash_profile
```

Create two new aliases

    alias h='history 10'
    alias hg='history | grep '

Save these changes and quit
Make this change take effect
```
    source .bash_profile
```
Run the first alias
```
    h
```
You should see your last 10 command lines.
Run the second alias
```
    hg cd
```
You should see every time you have run cd in the last 500 command lines.

Single and Double Quotes

Create a variable
```
    city=Boston
```
Display the value of this variable
```
    echo $city
```

Define another variable using single quotes

    capitol='$city is the capitol of Massachusetts'

Display the value of this variable
```
    echo $capitol
```
The variable city is not evaluated because it was enclosed in single quotes when defined.

Redefine city using double quotes

    capitol="$city is the capitol of Massachusetts"

Display this redefined variable
```
    echo $capitol
```
The double quotes permitted the shell to evaluate city when assigning a value to capitol.
Make sure you are in your home directory
```
    cd
```
Display the value of PWD
```
    echo $PWD
```
PWD has the address of your home directory.
Go to my home directory
```
    cd ~tsoro
```
Display the value of PWD
```
    echo $PWD
```
PWD is now set to the address of my home directory.
Create an alias using this variable in double quotes
```
    alias new_pwd="echo $PWD"
```
Use this alias in your current location
```
    new_pwd
```
It displays the address of my home directory.
Return to your home directory
```
    cd
```
Make sure you are in your home directory
```
    pwd
```
Use the alias in this new location
```
    new_pwd
```
The alias again returns the address of my home directory.

Since you created the alias in my home directory, and used double quotes, the value of PWD at the time of definition was used in creating the alias.

But you want alias to use the value of PWD at the time the alias is used.
Redefine the alias using single quotes
```
    alias new_pwd='echo $PWD'
```
Use the newly redefined alias
```
    new_pwd
```
It displays the address of your home directory.
Go to my home directory
```
    cd ~tsoro
```
Use the redefined alias again
```
    new_pwd
```
It displays the address of my home directory.

Since you defined the alias using single quotes, PWD was not evaluated when it was defined, but instead, when it is used.

Functions

Go to your home directory
```
    cd
```
Open .bash_profile using nano
```
    nano  .bash_profile
```

Define a function

    cheer ()
    {
        echo Go $1'!'
    }

Define another function

    print_args ()
    {
        echo "arg1: $1"
        echo "arg2: $2"
        echo "arg3: $3"
    }

Save and quit
Make these functions available in your login shell
```
    source  ~/.bash_profile
```
Use the first function
```
    cheer 'Red Sox'
```
Use the second function
```
    print_args  foo  bar  bletch
```

Checking Your Work

Tell the instructor you have finished making changes to .bash_profile

There is no Bash script for this Class Exercise.

Log in to users3.cs.umb.edu

Directory for This Exercise

The history mechanism

Look at the Values of the history Variables

Using fc to Edit a Command

Command Line Editing

Command Completion

Pathname Completion

Variable Completion

Aliases

Single and Double Quotes

Functions

Checking Your Work

The `history` mechanism

Look at the Values of the `history` Variables

Using `fc` to Edit a Command