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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shells(4) File Formats shells(4)
NAME
shells - shell database
SYNOPSIS
/etc/shells
DESCRIPTION
The shells file contains a list of the shells on the system. Applications use this file to determine whether a shell is valid. See getuser-
shell(3C). For each shell a single line should be present, consisting of the shell's path, relative to root.
A hash mark (#) indicates the beginning of a comment; subsequent characters up to the end of the line are not interpreted by the routines
which search the file. Blank lines are also ignored.
The following default shells are used by utilities: /bin/bash, /bin/csh, /bin/jsh, /bin/ksh, /bin/pfcsh, /bin/pfksh, /bin/pfsh, /bin/sh,
/bin/tcsh, /bin/zsh, /sbin/jsh, /sbin/sh, /usr/bin/bash, /usr/bin/csh, /usr/bin/jsh, /usr/bin/ksh, /usr/bin/pfcsh, /usr/bin/pfksh,
/usr/bin/pfsh, and /usr/bin/sh, /usr/bin/tcsh, /usr/bin/zsh. Note that /etc/shells overrides the default list.
Invalid shells in /etc/shells may cause unexpected behavior (such as being unable to log in by way of ftp(1)).
FILES
/etc/shells lists shells on system
SEE ALSO
vipw(1B), ftpd(1M), sendmail(1M), getusershell(3C), aliases(4)
SunOS 5.10 4 Jun 2001 shells(4)