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When I'm doing something that requires root be typed in dozens of times in a row, I prefer to switch my session to a root session. In the various tutorials and instructions I have used on the Internet, I see sudo su, sudo su -, sudo -i and sudo /bin/bash being used to open a root session, but I'm not clear on the difference between these and when or if that difference matters.

Can someone clear this up for me?

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You forgot to ask about sudo -s and sudo su. –  Radu Rădeanu Nov 13 '13 at 6:49
    
    
    
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@RaduRădeanu When I typed the question in I reviewed the suggested questions and none of those really addressed my complete question. The same is true for the questions you linked to. Though they do contain a lot of new information for me, and thank you for linking to them, I found them not as complete as the answer user chaos below provides, which is much more in line with what I was looking for. –  Paul Nov 23 '13 at 23:57

1 Answer 1

To explain this you need to know what the programs do:

su - The command su is used to switch to another user (s witch u ser), but you can also switch to the root user by invoking the command with no parameter. su asks you for the password of the user to switch, after typing the password you switched to the user's environment.

sudo - sudo is meant to run a single command with root privileges. But unlike su it prompts you for the password of the current user. This user must be in the sudoers file (or a group that is in the sudoers file). By default, Ubuntu "remembers" your password for 15 minutes, so that you don't have to type you password every time.

bash - A text-interface to interact with the computer. It's important to understand the difference between login, non-login, interactive and non-interactive shells:

  • login shell: A login shell logs you into the system as a spiecified user, necessary for this is a username and password. When you hit ctrl+alt+F1 to login into a virtual terminal you get after successful login a login shell.
  • non-login shell: A shell that is executed without logging in, necessary for this is a current logged in user. When you open a graphic terminal in gnome it is a non-login shell.
  • interactive shell: A shell (login or non-login) where you can interactively type or interrupt commands. For example a gnome terminal.
  • non-interactive shell: A (sub)shell that is probably run from an automated process you will see neither input nor output.

sudo su Calls sudo with the command su. Bash is called as interactive non-login shell. So bash only executes .bashrc. You can see that after switching to root you are still in the same directory:

user@host:~$ sudo su
root@host:/home/user#

sudo su - This time it is a login shell, so /etc/profile, .profile and .bashrc are executed and you will find yourself in root's home directory with root's environment.

sudo -i It is nearly the same as sudo su - The -i (simulate initial login) option runs the shell specified by the password database entry of the target user as a login shell. This means that login-specific resource files such as .profile, .bashrc or .login will be read and executed by the shell.

sudo /bin/bash This means that you call sudo with the command /bin/bash. /bin/bash is started as non-login shell so all the dot-files are not executed, but bash itself reads .bashrc of the calling user. Your environment stays the same. Your home will not be root's home. So you are root, but in the environment of the calling user.

sudo -s reads the $SHELL variable and executes the content. If $SHELL contains /bin/bash it invokes sudo /bin/bash (see above).

Check: To check if you are in a login shell or not (works only in bash because shopt is a builtin command):

shopt -q login_shell && echo 'Login shell' || echo 'No login shell'
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Just a clarification: sudo allows a permitted user to execute a command as the superuser or another user. Anyway, +1 for your effort. –  Radu Rădeanu Nov 13 '13 at 10:38
    
@chaos Thank you for this excellent answer! It mostly answers my questions, so I went ahead and marked the question answered, but I'm not understanding when it is desirable to run a particular shell. I really only use Ubuntu through command line and I find my most common use for opening a root user session (vs. using sudo) is when I'm doing something that requires heavy usage of root privileges, such as when installing something new or performing a major reconfiguration. I've been using sudo /bin/bash, but apparently that method has a bad rap for some reason I don't understand. –  Paul Nov 23 '13 at 22:22
    
Also, it is more "correct" to say that su stands for switch user, rather than super user. I.e. run a php script: su www-data /usr/share/script.php or just su www-data for interactive shell. But su without any user name will asume the root (super user) account. –  oblivian Feb 19 at 22:29
    
Chaos - "shopt -q login_shell && echo 'Login shell' || echo 'No login shell'" These kinds of suggestions I see often, but why make it so long? The && operator means if exit code 0 then "do next command" and the double pipes (||) means else (if not 0) then do this command. So what it is basically saying if exit code 0 then echo "Login Shell", if exit code 1 (fail) then echo "No login". Why not just "shopt -q login_shell ; echo $?" $? means the result/exit code of the preceding command. In most if not all programs 0 means success, 1 or more means fail. So if the echo is 0 = success... –  oblivian Feb 19 at 22:45
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@Paul: sudo -i is suggested. Read here: ubuntuforums.org/showthread.php?t=1817402 and here: unix.stackexchange.com/questions/98531/… –  Lucas Malor Sep 10 at 8:21

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