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Adding commands to the shell script
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Create Shell Scripts
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FIGURE 9-48
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Adding non-declared variables together
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and so on) it will contain The bash shell is a little more forgiving The bash shell will create the variable in memory dynamically for you from the read command and assign the user s input as its value Of course, bash does let you declare and type the variable if you want to This is done using the declare command in the script This can be useful if you want to have the user enter numbers in a read command The issue here is that the bash shell interprets anything entered at the read command as text, even if the user enters a number Consider the script shown in Figure 9-48 When run, this script asks the user for two numbers, adds them together and assigns the result to a variable named TOT, and echoes the value of TOT on the screen Notice what happens when the script is run in Figure 9-49 Because we didn t declare the NUM1, NUM2, or TOT variables, bash treated them all as simple text No arithmetic took place However, if we declare these
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FIGURE 9-49
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9: Using the Linux Shell
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FIGURE 9-50
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Declaring and typing variables in a script
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variables and type them as integers, something very different will happen Consider a revised version of this script, shown in Figure 9-50 Notice that NUM1, NUM2, and TOT are declared at the beginning of the script using the i (integer) option Now the bash shell will interpret the user s input as a whole number, not as text When we use the + operator on the variables in the script, they are actually added, not just concatenated, as shown in Figure 9-51 You re doing great! We ve gone from relatively simple scripts to ones that are a little more complex Let s ratchet things up a notch and add some control structures to our scripts
FIGURE 9-51
Adding declared variables in a script
Create Shell Scripts
SCENARIO & SOLUTION
You need to write a script that will mount a CD in your CD-ROM drive and then display the contents of the CD on screen How would you do this You could use the following script:
#!/bin/bash #A script for mounting a CD mount t iso9660 /dev/cdrom /media/cdrom ls l /media/cdrom exit 0
You ve saved the script above as mntmycd What command would you enter to make it executable by the file owner
You would enter chmod u+x mntmycd
Using Control Structures in Scripts
In addition to interactivity, it can also be very beneficial to add control structures to your shell scripts Our scripts, to this point, have executed straight through from beginning to end This works fine, but what if we need the script to make some decisions Based on user input or output from a command, you may want the script to determine a course of action This is done by implementing control structures in the script We re going to discuss the following about this type of script element:
Using if/then/else structures Using case structures
Let s begin by reviewing if/then statements
Using if/then Structures
Using an if/then/else structure within your shell script gives your script the ability to execute different commands based on whether or not a particular condition is true or false The structure appears as follows:
if condition then commands else commands fi
The if part of the structure tells the shell to determine if the specified condition is true or false If it is true, then the commands under the then part of the structure are run
9: Using the Linux Shell
If the condition evaluates to false, then the commands under the else part of the structure are run For example, in the script we ve been working with so far in this part of the chapter, we ve asked the user to enter the name of a directory they want to add to the PATH environment variable When we add the directory to PATH, the shell doesn t check to see if the directory the user entered actually exists It would be beneficial to run a quick test and verify that the specified directory exists If it does, we should go ahead and add it to the PATH variable If not, we should post an error message on the screen telling the user what happened The script shown in Figure 9-52 does this very thing In this example, the [ e $MYNEWPATH ] condition calls a utility called test and directs it to check and see if the directory contained in the MYNEWPATH variable exists (as specified by the e option) If test returns a value of TRUE, then the steps immediately under the if statement are executed However, if the test program returns a value of FALSE, then the statements under the else portion of the structure are executed In this case, an error message will be displayed indicating that the directory doesn t exist When we run this script and supply a valid directory, the output in Figure 9-53 is displayed You can also use a related structure in your scripts called case Let s discuss how to use it next
FIGURE 9-52
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