07-24-2014
The reason that the su command is SUID is that without this attribute, a non-superuser would not have the privilege to switch user at all, it would simply be running as the original account. The SUID means that for the execution of the code, you will be as the owner of the code, which must be root or another superuser account (i.e. User ID zero)
The code will then be powerful enough to do what it needs and within the code, it will verify that you are allowed to do what you are asking it to, i.e. it may prompt for the target user password.
Robin
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CHSH(1) User Commands CHSH(1)
NAME
chsh - change login shell
SYNOPSIS
chsh [options] [LOGIN]
DESCRIPTION
The chsh command changes the user login shell. This determines the name of the user's initial login command. A normal user may only change
the login shell for her own account; the superuser may change the login shell for any account.
OPTIONS
The options which apply to the chsh command are:
-h, --help
Display help message and exit.
-R, --root CHROOT_DIR
Apply changes in the CHROOT_DIR directory and use the configuration files from the CHROOT_DIR directory.
-s, --shell SHELL
The name of the user's new login shell. Setting this field to blank causes the system to select the default login shell.
If the -s option is not selected, chsh operates in an interactive fashion, prompting the user with the current login shell. Enter the new
value to change the shell, or leave the line blank to use the current one. The current shell is displayed between a pair of [ ] marks.
NOTE
The only restriction placed on the login shell is that the command name must be listed in /etc/shells, unless the invoker is the superuser,
and then any value may be added. An account with a restricted login shell may not change her login shell. For this reason, placing /bin/rsh
in /etc/shells is discouraged since accidentally changing to a restricted shell would prevent the user from ever changing her login shell
back to its original value.
FILES
/etc/passwd
User account information.
/etc/shells
List of valid login shells.
/etc/login.defs
Shadow password suite configuration.
SEE ALSO
chfn(1), login.defs(5), passwd(5).
shadow-utils 4.5 01/25/2018 CHSH(1)