Changes to the Linux environment variables (permanent, temporary) and the time format for modifying the LS display

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Linux modifies environment variables, very simple but important

I. Variant types of Linux

According to the lifetime of the variables, the Linux variables can be divided into two categories:

1. Permanent: Need to modify the configuration file, the variable is permanent.

2. Temporary: Using the Export command-line declaration, the variable is invalidated when the shell is closed.

Ii. three ways to set up variables

1. Add the variable "to all users (permanent)" in the/etc/profile file

Using VI to add a variable to the file/etc/profile file, the variable will be valid for all users under Linux and is "permanent".

Example: Edit the/etc/profile file, add the CLASSPATH variable

# Vi/etc/profile

Export classpath=./java_home/lib; $JAVA _home/jre/lib

Note: If you want to run the file immediately after you modify it, you can only take effect the next time you re-enter the user. Source/etc/profile.

2. Add variable "to single user (permanent)" in. bash_profile file in User directory

Use VI to add variables to the. bash_profile file in the user directory, and change the amount to be valid only for the current user and be "permanent".

For example: Edit the. Bash_profile under the Guok User directory (/home/guok)

$ vi/home/guok/.bash.profile If you do not change the. bashrc file.

Add the following content:

Export classpath=./java_home/lib; $JAVA _home/jre/lib

Note: If you want to take effect immediately after modifying a file, run $ source/home/guok/.bash_profile or you can only take effect the next time you re-enter the user.

3. Run the export command directly to define the variable "valid only for the current shell (BASH)"

The variable is defined directly under the Shell's command line using the [export variable name = variable value], which is valid only under the current shell (bash) or its child shell (bash), the shell is closed, the variable is invalidated, and the new shell is opened without this variable. You need to redefine the words you need to use.
Third, another

1. Use the readonly command to set the read-only variable, and if the readonly command is used, the variable cannot be modified or erased.

2. Use the unset command to clear the environment variable $ unset temp_kevin #删除环境变量TEMP_KEVIN

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Changes to the Linux environment variables (permanent, temporary) and the time format for modifying the LS display