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Manage Linux Environment Variables
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Using the set command to view variables and their values
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Setting the Value of a Variable
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As we discussed earlier, most of the environment variables used on a Linux system work great using the values assigned to them by the system In fact, there are many environment variables that you should not change! For example, changing the value of the HOSTNAME variable could cause problems with many services running on your system However, there are times when you will need to change the value assigned to an environment variable For example, you may need to add an additional directory to the end of the PATH variable This can be a handy way to make life easier for your users (and for you as well) In addition, you may need to edit the DISPLAY variable to configure your X Windows System to send its display to a remote computer Likewise, you may want to alter the shell prompt to display different information To do these tasks, you need to change the value of an environment variable This is relatively easy to do Simply enter variable=value at the shell prompt For example, suppose you installed an application in /var/opt/mydb named myapp
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9: Using the Linux Shell
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Using the env command to view variables and their values
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This path doesn t currently exist in your PATH variable and you want to add it so you won t have to use the full path when you want to run the program To do this, you can enter PATH=$PATH:/var/opt/mydb at the shell prompt, as shown in Figure 9-16 An important point to notice here is that I specified $PATH in the variable assignment command This includes the current value of the PATH variable in
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Setting the value of the PATH environment variable
Manage Linux Environment Variables
the new value assignment I then concatenated :/var/opt/mydb to the existing list of paths If you don t include $PATH in your reassignment, then the current directories in your PATH variable will be erased and replaced by the new path you specify in the command! When this happens, your system will start to experience a host of problems! Remember that setting the value of an environment variable will erase its current contents If you want to preserve the current contents, use the technique just shown If you do actually want to erase the contents of the variable and replace it with a new value, then you can simply enter variable=value at the shell prompt However, we ve still one more task to complete We ve assigned the value of PATH to include an additional directory However, the new value of the PATH variable applies only to the current shell If I were to open up another terminal session, the change that I made to PATH will not be applied This is shown in Figure 9-17 As you can see in Figure 9-17, the value of PATH doesn t include /var/opt/mydb even though I m still logged in as the same user To make the assignment apply to all shells, I need to export the new value of the variable To do this, I enter export variable at the shell prompt In this example, I need to enter export PATH at the shell prompt, as shown in Figure 9-18 After doing so, the new value assigned to PATH is made available to all other shells, including any subshells created by the current shell
Making Variables Persistent
One problem you will encounter in this process, however, is the fact that any new value you add to an environment variable will be lost after the system reboots If the
FIGURE 9-17
New variable value not exported
9: Using the Linux Shell
FIGURE 9-18
Exporting the new value of a variable
change you made needs to be persistent across system restarts, then you need to edit one of your bash configuration files, discussed at the beginning of the chapter, and add the variable assignment to the file In the preceding example, I probably want my new PATH variable value assignment to be automatically made each time the system restarts I don t want to have to manually set the value of PATH and export it each time the system reboots To do this, I can use the command to set the value of PATH to include /var/opt/ mydb as well as the command to export the PATH variable to one of the following bash configuration files:
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