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#!/bin/bash
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The first line of any shell script must specify which shell the script is written to run under In this case, the /bin/bash shell is specified When a script is run, a subshell will be created using the shell specified here and the script contents will be processed in it This part of the script is a comment that describes what the script does Notice that this part of the script begins with a # character to indicate that the text that comes after it is a comment Because it is a comment, this part of the script is not displayed on the screen when it is run Comments are optional The script will run just fine without them
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#A sample script that
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FIGURE 9-42
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A sample shell script
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9: Using the Linux Shell
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However, it s considered good form to include a comment at the beginning of your scripts right after the shell declaration that describes what the script does and, optionally, who wrote it
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echo, date
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These elements in the script are simple commands that are typically used at the shell prompt The echo command is used to display text on the screen The date command is used to display the current date and time on the screen This part of the script is its end It tells the shell what to do after it is done running the commands in the script In this case, it tells the shell to exit the script
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exit 0
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When we run this script, the output shown in Figure 9-43 is displayed on the screen Notice that this script first displayed the text specified by the echo command Then the next command was processed, which directed the shell to run the date command When the script was done, the shell exited the script and returned you to the command prompt
How Shell Scripts Are Executed
Notice in Figure 9-43 that, to run the script I created, I had to call the shell (/bin/bash) and then tell it what script to execute (/runme) This is one option for running a script However, there s a second option (and a better one in my opinion) for running shell scripts Remember in an earlier chapter when we talked about permissions Recall that one of the file permissions you could assign to a file is named execute
Running the sample shell script
FIGURE 9-43
Create Shell Scripts
FIGURE 9-44
Changing the execute permission on a script file
With the execute permission assigned, any file, including a text file, can be allowed to execute from the shell prompt This is a great option for making scripts easy for end users to run To do this, simply enable the execute attribute for Owner, Group, and/or Others As you learned earlier, this is done using the chmod command In the example we re working with here, I can configure the runme file to be executed by the file Owner by entering chmod u+x /runme at the shell prompt Then I can run the runme script by simply entering its name at the shell prompt, just as I would with any other system command This is shown in Figure 9-44 If desired, I could have also given the execute permission to Group and/or Others as well There is one other issue you need to be aware of when working with scripts and that is the issue of paths Notice in Figure 9-44 that I had to enter /runme at the shell prompt Even though the file resided in the current directory, the current directory (in this case, /home/ksanders) is not in my PATH environment variable If you re creating scripts for yourself, then this probably doesn t pose a problem By now, you should be familiar enough with Linux to understand why you have to specify the path to the file at the shell prompt and know how to do it However, if you re creating a script for your end users to run, this may present a problem To save yourself frustration, you should consider putting the script in a directory that is part of the PATH or adding the directory to your existing PATH One option you can use is the ~/bin directory in each user s home directory Most Linux distributions automatically create a directory named /bin for each user Then, one of the bash configuration files, discussed at the beginning of this chapter, is used to automatically add ~/bin to the PATH environment variable when a shell is started This is shown in Figure 9-45
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