linux "Id" Command

https://www.geeksforgeeks.org/id-command-in-linux-with-examples/

Linux: List of all Users and All Groups

https://linuxize.com/post/how-to-list-users-in-linux/

• less /etc/passwd

• getent passwd
• getent passwd | cut -d: -f1

https://linuxize.com/post/how-to-list-groups-in-linux/

• less /etc/group

• getent group
• getent group | cut -d: -f1

Linux : /etc/passwd vs /usr/bin/passwd (passwd Utility) vs /etc/shadow vs /etc/group

https://stackoverflow.com/questions/50904342/etc-passwd-vs-usr-bin-passwd

The two files are different, and serve different purpose.

• /etc/passwd is user database (fun fact: contrary to its name, it doesn't store passwords - those are stored (possibly in hashed form) in /etc/shadow) - see man 5 passwd (i.e. passwd(5)), man 5 shadow (i.e. shadow(5)).

• /usr/bin/passwd is utility that is supposed to modify user records stored in /etc/passwd and /etc/shadow. See man 1 passwd (i.e. passwd(1))

Linux : File Hierarchy Standard : /bin vs /usr/bin vs usr/local/bin

https://en.wikipedia.org/wiki/Filesystem_Hierarchy_Standard

https://unix.stackexchange.com/questions/5915/difference-between-bin-and-usr-bin

https://askubuntu.com/questions/308045/differences-between-bin-sbin-usr-bin-usr-sbin-usr-local-bin-usr-local

/sbin - Binaries needed for booting, low-level system repair, or maintenance (run level 1 or S)

/bin - Binaries needed for normal/standard system functioning at any run level.

/usr/bin - Application/distribution binaries meant to be accessed by locally logged in users

/usr/sbin - Application/distribution binaries that support or configure stuff in /sbin.

/usr/share/bin - Application/distribution binaries or scripts meant to be accesed via the web, i.e. Apache web applications

*local* - Binaries not part of a distribution; locally compiled or manually installed. There's usually never a /local/bin but always a /usr/local/bin and /usr/local/share/bin.

• /bin : For binaries usable before the /usr partition is mounted. This is used for trivial binaries used in the very early boot stage or ones that you need to have available in booting single-user mode. Think of binaries like cat, ls, etc.

• /sbin : Same, but for binaries with superuser (root) privileges required.

• /usr/bin : Same as first, but for general system-wide binaries.

• /usr/sbin : Same as above, but for binaries with superuser (root) privileges required.

Linux : PAM : Pluggable Authentication Module

https://www.redhat.com/sysadmin/pluggable-authentication-modules-pam
https://www.tecmint.com/use-pam_tally2-to-lock-and-unlock-ssh-failed-login-attempts/

sudo pam_tally2 -u 703247699 --reset

Config Files for PAM
/etc/pam.d
/etc/security

sudo   pam_tally2   -u    userID123
---------------------------------------------------------
Login           Failures Latest failure     From
userID123    0

Linux - Special Dollar Sign Variables - $0, S1

https://stackoverflow.com/questions/5163144/what-are-the-special-dollar-sign-shell-variables

• $1, $2, $3, ... are the positional parameters.
• "$@" is an array-like construct of all positional parameters, {$1, $2, $3 ...}.
• "$*" is the IFS expansion of all positional parameters, $1 $2 $3 ....
• $# is the number of positional parameters.
• $- current options set for the shell.
• $$ pid of the current shell (not subshell).
• $_ most recent parameter (or the abs path of the command to start the current shell immediately after startup).
• $IFS is the (input) field separator.
• $? is the most recent foreground pipeline exit status.
• $! is the PID of the most recent background command.
• $0 is the name of the shell or shell script.

Most of the above can be found under Special Parameters in the Bash Reference Manual. There are all the environment variables set by the shell.

For a comprehensive index, please see the Reference Manual Variable Index.

Linux : "sudo su userid" vs sudo "su userid -" ? What does dash mean - Login Shell vs Non Login Shell

https://askubuntu.com/questions/376199/sudo-su-vs-sudo-i-vs-sudo-bin-bash-when-does-it-matter-which-is-used

• sudo su              interactive non-login shell. 

• sudo su -           It is a login shell

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 your 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:

Types of shells:

• login shell: A login shell logs you into the system as a specified 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 currently 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.

So the cases are:

• 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).