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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

sudo does 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?

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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.

  • Thanks. " it reads it from the terminal interface." But I always provide password to sudo from standard input. Isn't standard input connected to the terminal interface/emulator by default? – Tim Apr 1 '18 at 0:55
  • By default, yes it is. The point of -S is when you're overriding the default somehow, such as by piping to sudo. – Joseph Sible Apr 1 '18 at 3:39
  • @Joseph Usually a program accepts input from standard input (i.e. file descriptor 0). sudo doesn't do so. Does sudo open a fixed device file which represents the terminal? – Tim Apr 1 '18 at 4:05
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    For the password prompt, yes. For example, echo abcd | sudo cat will let you type the password, rather than using abcd as the password. – Joseph Sible Apr 1 '18 at 4:06
  • @JosephSible Thanks. (1) I was wondering what you meant by "For the password prompt, yes" ? (2) Why can't I tell if sudo uses standard input or terminal when using just sudo? How does a program use terminal instead of standard input? – Tim Apr 1 '18 at 12:59

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