zebra barcode printer vb net 9: Using the Linux Shell in Software

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9: Using the Linux Shell
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FIGURE 9-21
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Using the set command to view the value of a user-defined variable
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Creating Aliases
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Aliases are really cool Aliases are similar to variables in many ways, but have a very different function An alias is basically a shortcut to a different file or command on your Linux system When your system boots, a series of aliases are automatically created for you You can view these by entering alias at the shell prompt This is shown in Figure 9-22 Notice in Figure 9-22 that a series of commands are listed that aren t really commands at all Instead, they are aliases that point to real commands For example, if you type dir at the shell prompt, the output from alias tells us that the ls l command is actually run Likewise, typing will actually execute the cd command You can create your own aliases too To do this, just enter alias name= command at the shell prompt For example, suppose you want to be able to enter longlist at the
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Manage Linux Environment Variables
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FIGURE 9-22
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Viewing system-generated aliases
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shell prompt to view a long listing of a directory You could enter alias longlist= ls l at the shell prompt This is shown in Figure 9-23 Now, if I enter longlist at the shell prompt, the ls l command is executed and its output is displayed on screen This is shown in Figure 9-24 You can even include multiple commands within a single alias To do this, separate the commands in the alias command line with a semi-colon (;) For example, if you wanted to create an alias that would mount a CD inserted in your CD drive and then generate a long listing of the files it contains, you could enter the following at the shell prompt:
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alias mntcd="mount t iso9660 /dev/cdrom /media/cdrom;ls l /media/cdrom"
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After doing so, you can enter mntcd at the shell prompt to mount the CD and generate a listing of its files As with variables, any aliases you define with the alias command are not persistent If you reboot the system, they will be gone when the system comes back up As with variables, you can make your aliases persistent by adding them to one of the bash
9: Using the Linux Shell
FIGURE 9-23
Creating a new alias using the alias command
configuration files You can add them to /etc/profile if you want to make the aliases available to all users on the system You can also add them to one of the hidden bash configuration files in a particular user s home directory if you want them to be available only to that particular user
SCENARIO & SOLUTION
You need to view all of the aliases on your system What command could you enter at the shell prompt to do this You want to add an alias named logs to your system that will display the last few lines of your /var/log/messages file What command would you enter to do this To view all of the aliases, you enter alias at the shell prompt You would enter alias logs= tail /var/log/messages at the shell prompt
Manage Linux Environment Variables
FIGURE 9-24
Running the longlist alias at the shell prompt
Let s practice working with aliases in the following exercise
EXERCISE 9-4
ON THE CD
Working with Aliases
In this exercise, you will practice creating aliases in the bash shell To complete this exercise, you will need a DOS-formatted floppy diskette that is compatible with your computer s hardware Once you have it ready, do the following: 1 If necessary, boot your Linux system and log in as a standard user If you used the lab exercise in 3 to install your system, you can log in as tux with a password of M3linux273 2 Change to your root user account by entering su followed by your root password
9: Using the Linux Shell
3 Create an alias named mntflp that will automatically mount a floppy diskette in your floppy drive and display a list of mounted file systems by entering alias mntflp= mount t vfat /dev/fd0 /media/floppy;mount at the shell prompt If you re using a Fedora or Red Hat distribution, use a mount point of /mnt/ floppy instead of /media/floppy 4 At the shell prompt, enter alias and verify that the alias has been created 5 Insert a DOS-formatted floppy diskette in your floppy drive 6 Test your alias by entering mntflp at the shell prompt 7 Verify that your floppy is mounted in the mount table 8 Dismount your floppy by entering umount /dev/fd0 at the shell prompt 9 At your shell prompt, enter ls a Which file would you edit on your particular distribution if you wanted to make the alias persistent Now that you know how to manage variables, it s time to discuss how to redirect output from shell commands Let s do that next
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