background process's outputs show staircase effect when other command is running in the foreground with sudo

[jeffy1009]

Minimized test case:
(for i in {1..5}; do echo "FOO"; sleep 1; done) & sudo sleep 5
Running this show FOO outputs in staircases.

As soon as I enter the passwords in sudo, it changes stty -a outputs in the background process.
icrnl is one of them.

Tested on Ubuntu 25.04 with Sudo version 1.9.17p2.

Also applies to Ubuntu 22.04 and 24.04 but not on 20.04.

[millert]

This is because sudo now runs commands in a pseudo-terminal by default. Sudo sets the terminal to raw mode so that it can copy the output from the pseudo-terminal to the real terminal properly. All processes in the same session share the same terminal settings.

The simplest workaround is to add the following to your sudoers file:

Defaults !use_pty

Another option is to use the exec_background option to run sudo commands in the background by default. Sudo will grant the controlling terminal to the command if it tries to write output to the terminal. Unfortunately, some programs don't behave correctly when run in the background.

Defaults exec_background

It may be possible to defer setting the terminal to raw mode until sudo needs to actually copy output from the pseudo-terminal the command is running. I'll add that to my todo file as something to try in the future.