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This article will explain in detail how to use the export command in linux. Xiaobian thinks it is quite practical, so share it with you as a reference. I hope you can gain something after reading this article.

I have not been in contact with linux for a long time. Recently, when I was learning the export command of BASH, I encountered a problem (the book says export is to change a custom variable into a system environment variable): I defined a variable in a script file, and then exported the variable. According to my own ideas, after executing the script, I must be able to use echo to display its value at the prompt, but the result is not like this. After the script is executed, I use set to not see the existence of this variable. Why is that? I was puzzled and finally posted the question. A senior told me that it was OK to use source+ script files. I tried it and it worked, but a new question came out. After I delete the export command from the script, I can use source as well. Well, this export doesn't seem to be useful.

After many attempts, I found something that I guessed myself. If there is anything wrong, please correct me. Thank you.

When executing a script, it opens a child shell environment (I don't know if this is the case for executing other programs), then copies all system environment variables from the parent shell, and the statements in this script are executed in the child shell. (That is to say, the environment variables of the parent shell can be called in the child shell, but not vice versa. If the environment variables are defined in the child shell, they are only valid for the shell or its child shell. When the child shell ends, it can also be understood that the variables disappear when the script is executed.) To prove this, look at the script:

test='value'

export test

After such a script is executed, the test does not actually exist. Then look at the following:

test='value'

export test

bash

Here in the last line of the script and then open a subshell, the shell should be the script file in the shell of the subshell, after the script is executed, you can see the test variable, because it is now in its subshell, when exit subshell, the test variable disappears.

If you use source to execute the script, if you do not add export, you will not see this variable in the subshell, because it is not a system environment variable, such as the script content is:

test='value'

After executing with source, you can see this variable under the shell, but when you execute bash to open a subshell, test will not be copied to the subshell, because the execution script file is actually running in a subshell, so when I build another script file, I will not enter anything, such as echo $test. So pay special attention to this point, obviously in the prompt can use echo $test output variable value, why put it into the script file will not work?

So the conclusion is: 1. The script is executed in a child shell environment, and the child shell automatically exits after the script is executed;2. The system environment variables in a shell are copied to the child shell (variables defined with export);3. The system environment variables in a shell are only valid for the shell or its child shell, and the variables disappear at the end of the shell (and cannot be returned to the parent shell). Variables defined without export are valid only for the shell and are invalid for subshells.

Later, according to the moderator's prompt, I sorted out the post: Why is a script executed directly and executed with source not one line? This is also a problem I have encountered myself. The original manual is like this: Read and execute commands from filename in the current shell environment and return the exit status of the last command executed from filename. Directly executing a script file runs in a subshell, whereas source runs in the current shell environment. From the above, you already understand the truth.

The problem that has been bothering me for a few days can finally be solved satisfactorily.

About "how to use export command in linux" this article is shared here, I hope the above content can be of some help to everyone, so that you can learn more knowledge, if you think the article is good, please share it for more people to see.

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