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Programming with bash

   System Administration
   Linux and Subversion
--
   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:
   #!
   Following this, you should include the path to the BASH interpreter. So
   the first line of your script should look like this:
   #!/bin/bash
   If your default shell is BASH, the line:
   #!/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
--
   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 #
   #!/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
--
   $sendto
   # end script
   First off, you'll see that we've declared some variables. These
   variables are not prefixed by any characters when declared but they are
   prefixed by the dollar sign ($) 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
--
   comes with what are known as built invariables. Here is an example of a
   script with frequently used built in variables.
   #!/bin/sh
   echo "You are user $UID on $HOSTNAME"
   echo "Your home directory is: $HOME"
   echo "$HOSTNAME is running $OSTYPE"
   The output of this script should yield something similar to this:
   You are user 500 on penguin.linux.ork
   Your home directory is: /home/mike
   penguin.linux.ork is running linux-gnu
   As you can see, we didn't have to previously declare any of these.
   That's why they're known as built-in variables. Their use will save you
   a lot of time in writing your scripts. You can find a complete list of
   built-in variables in theGNU BASH Reference Manual
   Interactive Scripts
   Though we mentioned that the main use of BASH scripts is for automating
   administrative tasks, there may be times when you need users to
   interact with scripts. If you want a user to input information, you
   need to use the variableread. Let's take a look at the following
   example:
   #!/bin/sh
   echo -n "Enter the name of a city: "
   read CITY
   echo -n "$CITY is "
   case $CITY in
--
   After, we have several options for each case. If the user types in the
   name of a city we've contemplated here, he/she will be given a message
   as to where the city is. If not, the script will display a message that
   it doesn't know where the city is. Any answer is represented by the
   asterisk (*)
   Making Sure You Have What You Need
   If you have to manipulate the contents of a file, it's a good idea to
   check if this file exists first. Here is a simple BASH routine to do
   this using the if command:
   #!/bin/sh
   if test -f /var/log/mail.log; then
   printf "The file existsn";
   fi
--
   #!/bin/sh
   PS3="Choose the number of the correct word to fill in the blank: "
   echo "The emergency brake let go and car rolled ______ the hill"
   select SENT1 in up down along beside
   do
   if [ "$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"