Question

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root@VM-0-11-debian:~/linux/2023/01# ls
root@VM-0-11-debian:~/linux/2023/01# ls > list
root@VM-0-11-debian:~/linux/2023/01# ls
list
root@VM-0-11-debian:~/linux/2023/01# cat list
list

I know the > symbol redirects standard output to a file. If the file doesn’t exist, it creates the file; otherwise, it replaces the file.

I want to ask whether the execution process of the shell command ls > list is as I describe below:

1. Because a file named “list” doesn’t exist, it first creates a file named “list”.

2. The ls command lists the contents of the directory (list), and the listed content is sent to standard output.

3. In a replacement mode, the content from standard output (list) is added to the file named “list”.

My personal understanding of the execution process is as described above. I hope you can give me some guidance, thank you.

Answer

The file redirection operator > is handled by the shell and creates/truncates whatever file you write to before starting the binary (ls). That’s why you can see the filename “list” in the file contents: the file was created before the ls process started.

Yes, your understanding is correct.

This is why it’s not possible to do things like sort txt > txt — before sort reads the file, the file named txt will be truncated. You’ll end up with an empty file. (Note: the sort command is used to sort all lines in a text file)

Summary

The execution process of ls > list:

1. > list creates a file named “list”

2. ls command lists the directory contents (result is “list”)

3. The result “list” is written to the file named “list” in replacement mode.

There’s still a lot worth exploring in shell/Linux commands. Today I saw an article What happens when you open a terminal and enter ’ls’. The article explains what happens when you open a terminal and enter the ls command. After browsing through it, I found many concepts and terms I didn’t understand. Reading this article was somewhat painful for me, but it precisely points out the direction for my future efforts.

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