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Programming with BASH

Discussion in 'Linux Advanced Tutorials' started by Rob, Jul 10, 2013.

  1. Rob

    Rob Administrator
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    Programming with BASH

    Thought it might sound like it, BASH isn't one of those captions that pop up (along with Ooff! and Biff!) when Batman and Robin are fighting the bad guys on the old 60's TV show. BASH actually stands for Bourne Again Shell. The reason for the name goes back to Steve Bourne who wrote the original Bourne Shell for Unix. When the GNU created a Free Software equivalent, they named it after Steve's shell and added a little pun on his last name.

    If you're a system administrator, making BASH scripts is going to be one of those mandatory things. But far from being a chore, you'll learn that it's going to make your work and your life a whole lot easier.

    Our First BASH Script

    The first thing that a BASH script needs is the proverbial 'shebang'. These are two characters:

    Code:
    #!
    Following this, you should include the path to the BASH interpreter. So the first line of your script should look like this:

    Code:
    #!/bin/bash
    If your default shell is BASH, the line:

    Code:
    #!/bin/sh
    does the same thing. It's just a symbolic link to /bin/bash. But if your default shell isn't BASH, you won't be invoking /bin/bash if your write a shell script with that first line. Since on Linux systems, BASH is normally the default shell, you'll see most BASH scripts start with

    Code:
    #!/bin/sh
    From there on, you're free to do what the shell allows. Shell scripts created for administration purposes (which are the majority of scripts) are made up of lines that invoke other commands. Let's look at a simple example. Let's say you have email users on your system but you don't have a quota in place. Still, you want to monitor the sizes of the mailboxes to make sure that people aren't taking up more space than they should. This script, run from crontab, would do the trick nicely:

    Except for the shebang, comment lines start with #


    Code:
    #!/bin/sh
     
    # show us the size of email spools email spools
    # date in YYYY-MM-DD format
    today=`date +%Y-%m-%0e`;
    # subject and recipient variables
    subject="Mailcheck";
    sendto="[email protected]";
    cd /var/spool/mail
    ls -lSh | awk '{print $5, $9}' | grep "(G|M)" | mail -s $subject-$today $sendto
    # end script

    First off, you'll see that we've declared some variables. These variables are not prefixed by any characters when declared but they are prefixed by the dollar sign ($) when used. You've also noticed that variables can be other commands, as in this example with the date command. When you use a command as a variable, it must be put inside backticks (` `).

    First, the script changes to the directory where the mail spools are located. The the scripts performs an 'ls' with options and presents a list where the biggest spools are displayed first with their size in human readable format. This is piped to awk, which sorts out the size and the user name. The awk output is grepped for those spools which are in the Megabytes and Gigabytes. This is then piped to the 'mail' command and sent to the admin account with the subject plus the date we declared in those variables. The admin will then have a nice sorted list of who is using the most space in /var/spool/mail.

    Built in Variables

    Though we created our own variables in the previous example, BASH also comes with what are known as built invariables. Here is an example of a script with frequently used built in variables.


    Code:
    #!/bin/sh
     
    echo "You are user $UID on $HOSTNAME"
    echo "Your home directory is: $HOME"
    echo "$HOSTNAME is running $OSTYPE"

    The output of this script should yield something similar to this:





    As you can see, we didn't have to previously declare any of these. That's why they're known as built-in variables. Their use will save you a lot of time in writing your scripts. You can find a complete list of built-in variables in theGNU BASH Reference Manual

    Interactive Scripts

    Though we mentioned that the main use of BASH scripts is for automating administrative tasks, there may be times when you need users to interact with scripts. If you want a user to input information, you need to use the variableread. Let's take a look at the following example:


    Code:
    #!/bin/sh
     
    echo -n "Enter the name of a city: "
    read CITY
    echo -n "$CITY is "
    case $CITY in
    London | Paris | Berlin | Rome) echo -n "in Europe";;
    'New York' | Chicago | Washington) echo -n "in The USA";;
    Tokyo | Bejing | Bangalore) echo -n "in Asia";;
    *) echo -n "some place - but I don't know where";;
    esac

    As you can see, we've declared a variable that's going to depend on what the user types in when he/she is prompted for the name of a city. After, we have several options for each case. If the user types in the name of a city we've contemplated here, he/she will be given a message as to where the city is. If not, the script will display a message that it doesn't know where the city is. Any answer is represented by the asterisk (*)

    Making Sure You Have What You Need

    If you have to manipulate the contents of a file, it's a good idea to check if this file exists first. Here is a simple BASH routine to do this using the if command:


    Code:
    #!/bin/sh
     
    if test -f /var/log/mail.log; then
    printf "The file existsn";
    fi

    This is a good idea, as it would render your script useless if you had it set to manipulate a file that didn't exist.

    If Loops: A Practical Example

    I was a full-time English as a foreign language teacher for 12 years, so I can't resist giving you this example of a multiple-choice test using a BASH script.

    Code:
    #!/bin/sh
     
    PS3="Choose the number of the correct word to fill in the blank: "
    echo "The emergency brake let go and car rolled ______ the hill"
    select SENT1 in up down along beside
    do
    if [ "$SENT1" == "" ]; then
    echo -e "You need to enter somethingn"
    continue
    elif [ "$SENT1" != down ]; then
    echo -e "Sorry. Incorrectn"
    echo "1. Incorrect" >> eoiexam.dat
    elif [ "$SENT1" == down ]; then
    echo -e "Great!n"
    echo "No. 1 - Correct" >> eoiexam.dat
    break
    fi
    done

    The script makes use of the 'elif' routine to sort out answers that aren't correct. You will also notice that it writes the results, whether correct or not, to a file.

    If you're in the teaching profession, you could expand on this to give your students a quick quiz.
     
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