From manpage of sudo:
-S, --stdin
Write the prompt to the standard error and read the password from the standard input instead of using the terminal device. The password must be followed by a newline character.
What is the purpose of using sudo -S instead of just sudo?
Is it correct thatsudo by default read password from standard input?
What is the purpose of "Write the prompt to the standard error"? Does sudo by default write it to the standard output?
Do they both require that the password must be followed by a newline character?
For example, in https://stackoverflow.com/a/39553081/156458, sudo -S true still requires typing in password, so how does it solve the original question in that post? I found that link when I searched for solution to Shall I run a sudo-required script in some shell configuration file?
Thanks.
Update:
The reply by J.Taylor said
sudodoes not read the password from stdin by default - it reads it from the terminal interface.
I was wondering how to understand it in terms of implementation.
Is it correct that when a program reads from standard input, it reads from file descriptor 0 to which the standard input is always binded?
Why can't I tell whether sudo uses standard input or terminal when usingsudo without -S?
How can a program (such as sudo -S) achieve to read from terminal instead of standard input?
sudo does not read the password from stdin by default - it reads it from the terminal interface. Using sudo -S allows you to pipe the password in from another command/file like this: printf "yourpassword\n" | sudo -S nano /etc/apt/sources.list
This could be used in a shell script to log in to sudo without being prompted for a password, but you need to be careful not to execute this kind of thing from the shell directly, because then your sudo password would be in the shell history.
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