How to check from commandline if sudo authentication failed? - linux

Question

I was making a graphical front end to sudo for myself in bash scripting. Using something like gksudo is not an option for me, because the OS that I use doesn't provide one.
I designed it this way-
#!/bin/bash
sudo -S $# <<< $(/usr/lib/gtkdialog/box_passwd "$USER password" "Enter password" 2>/dev/null)
STATUS=$?
if [ $STATUS -eq 127 ]; then
/usr/lib/gtkdialog/box_splash -bg red -fg white -timeout 5 -text "sudo not found!"
exit 1
fi
while [ $STATUS -ne 0 ]; do
sudo -S $# <<< $(/usr/lib/gtkdialog/box_passwd "$USER password" "Authentication failed! Please retry" 2>/dev/null)
export STATUS=$?
done
(Note that /usr/lib/gtkdialog/box_passwd and /usr/lib/gtkdialog/box_splash are specific to this OS).
It works if I remove the while loop. But then the program would ask for the password only once, even if it was wrong. But currently, the while loop many a times doesn't end, because even if authentication is correct, sudo may also return 1 if the program it runs is not found or the program returns 1.
So how can I make this code run the while loop only if authentication for sudo is failed, and not other times when sudo returns 1?
EDIT: Just to make it clear, /usr/lib/gtkdialog/box_passwd is used here as graphical program to prompt the user for his/her password, which /usr/lib/gtkdialog/box_passwd passes to its stdout (prints to its stdout). /usr/lib/gtkdialog/box_splash is a program which just creates a new window and show some specific message

Answer 1

In some OSes, the man page for sudo specifies which error messages would come from sudo. If your OS does that, you can grep stderr for these messages.
Otherwise, sudo isn't very helpful in that department, but if your usage of sudo is simple (no fancy configuration in /etc/sudoers, and no specific commands are configured to be allowed by sudo), then you can just do something like:
#!/bin/bash
sudo -S true <<< $(/usr/lib/gtkdialog/box_passwd "$USER password" "Enter password" 2>/dev/null)
export STATUS=$?
if [ $STATUS -eq 127 ]; then
/usr/lib/gtkdialog/box_splash -bg red -fg white -timeout 5 -text "sudo not found!"
exit 1
fi
while [ $STATUS -ne 0 ]; do
sudo -S true <<< $(/usr/lib/gtkdialog/box_passwd "$USER password" "Authentication failed! Please retry" 2>/dev/null)
STATUS=$?
done
sudo "$#"
Namely, you don't run the command, you just run a dummy command (true in this case, which never fails).
In the default configuration, sudo remembers you for a few minutes.
So if you run sudo "$#" immediately after sudo true succeeded, sudo wouldn't ask you for a password in order to run the actual command.