A Rabbit Trail: Menu-Driven Bash Script -Part II

Introduction

In Part 1, we took a detour from our ongoing software journey, where we have been exploring and developing, and growing some software (Shape Calculator) into  various front-end Web Applications and Web Services, in order to start a command-line, menu-driven tool that could serve us in our normal project development.

It is written as a Bash shell script, and so far, it can start and stop our local MySQL database.

I want us to see more of what's going on in our MySQL database. For instance, what databases are available. Or what tables do they contain?

This will give us the motivation to see how we can incorporate the use of the mysql command-line client into our Bash scripting. This will introduce the idea of a HERE document.  (Wierd name, I know).

Latest Scripts

Here is the latest script.

First We Change Main Menu Selection

Choose your favorite editor (I use either gvim, or vim), open that file.

Since we are going to add some new MySQL-related features, we need to change the Main Menu selection to:

    echo "|=============================================|"
    echo "|              Main Menu                      |"
    echo "|=============================================|"
    echo "|1) MySQL Utiliies                            |"
    echo "|=============================================|"
    echo "|e) Exit                                      |"
    echo "|=============================================|"

Add New Selection To MySQL Menu

Then move up the file to the MySQL Menu and add a new selection:

3) Show Databases

    echo "|=============================================|"
    echo "|             MySQL Menu                      |"
    echo "|=============================================|"
    echo "|1) Start MySQL                               |"
    echo "|2) Stop  MySQL                               |"
    echo "|                                             |"
    echo "|3) Show Databases                            |"
    echo "|=============================================|"
    echo "|b) Back                                      |"
    echo "|e) Exit This Tool                            |"
    echo "|=============================================|"

Need New Menu Function

Now... since we'll probably want to (later) do something with these databases, let's make them menu selections.   That means we need a new function.  Let's call it mysql_show_databases_menu.

#######################################################################
function mysql_show_databases_menu {
#######################################################################
while [ 1 ];
do

    echo "|=============================================|"
    echo "|             MySQL Databases Menu            |"
    echo "|=============================================|"
                   ## DATABASE NAMES GO HERE ##
    echo "|=============================================|"
    echo "|b) Back                                      |"
    echo "|e) Exit This Tool                            |"
    echo "|=============================================|"


    menu_choice="";
    read -p "Please make a selection:" menu_choice

    case $menu_choice in
        b)
            break;
            ;;
        e)
            exit 0;
            ;;
        *)
            echo;echo;
            echo "Invalid selection: $menu_choice"
            echo;echo;
            ;;
    esac
done

return 0;
#######################################################################
} # end function mysql_show_databases_menu
#######################################################################

Let's See How To 'Show Databases'

First, let's do what we want, the simple way, by using the mysql command-line client:

Above, I've shown the commands we need, and the output we would like to capture.

Do Same Thing With HERE Document

A HERE document follows this basic template:

some-command  << SOME_LABEL
  inner commands understood by the 'some-command'
  etc...
SOME_LABEL

By convention, use capital letters for the identifying label.

So, we will use the above template, wth the MySQL command-line client.

In the above image, the 'some-command' is mysql, 

and the 'SOME_LABEL'  is MYSQL,

and the 'inner commands understood by ...'  is the show databases;

Ok?  Let's run it.

Remove Unwanted Text

This looks good.  In fact, the only output we DON'T want is the heading (Database). We will eliminate that.

But first, since we want to run the above inside our script, we need to capture that output into a variable.  So let's demo that part.

You already know what a HERE doc looks like.  All we did was place it inside   $( .... ); .

That runs any series of commands in a separate shell.   Then we captured the output by assigning it to a variable   databases=$(....);  .

The end of the screenshot shows us displaying the contents of the  $databases  variable.

Now... in order to remove the Database out of it, we could do it many different ways.

One way is using the sed  command.  The most common template for this command is as follows:

 sed -e 's/<replace-this>/<with-this>/' .

Let's try it.

Cygwin(Unix/Linux) commands can be chained together, by "piping" the output of one command, into the input of another.   And the symbol to do that is the "|" (pipe).

FOR Loops

We need one more thing.  A loop.   In Bash, a for loop usually looks like so:

for avar in <some white space delimited list>;
do
     something;
done;

Let's try a simple example:

Above, we entered the for-loop script, and hit <ENTER> and it executes immediately.

We can do the same with the  $databases  variable.

Remember that the $databases  variable contained the list of databases.

Ok, we are ready to put all that we have learned into our menu-driven tool.

Updated mysql_show_databases_menu  Function

#######################################################################
function mysql_show_databases_menu {
#######################################################################
while [ 1 ];
do

# a HERE DOCUMENT, output into a local Bash variable 'databases'.
databases=$(mysql -h 127.0.0.1 -u root << 'MYSQL'
show databases;
MYSQL
)

    databases=$(echo $databases|sed 's/Database//');

    echo "|=============================================|"
    echo "|             MySQL Databases Menu            |"
    echo "|=============================================|"

    ### for-loop to display our database list, numbered.
    i=1;
    for db in $databases;
    do
    echo "| $i) $db"
    i=$((i+1));
    done

    echo "|=============================================|"
    echo "|b) Back                                      |"
    echo "|e) Exit This Tool                            |"
    echo "|=============================================|"


    menu_choice="";
    read -p "Please make a selection:" menu_choice

    case $menu_choice in
        b)
            break;
            ;;
        e)
            exit 0;
            ;;
        *)
            echo;echo;
            echo "Invalid selection: $menu_choice"
            echo;echo;
            ;;
    esac
done

return 0;
#######################################################################
} # end function mysql_show_databases_menu
#######################################################################

Test The Script

It works.  We have a list of databases.   And, later, we can make them selectable as menu items.

Latest Scripts

You can get it here.

Next

In the next "A Rabbit Trail:" article, we'll tackle how to gracefully manage the database server when hitting CTRL-C in the main script tool.

Stay tuned!