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23.6 `su': Run a command with substitute user and group ID

`su' allows one user to temporarily become another user.  It runs a
command (often an interactive shell) with the real and effective user
ID, group ID, and supplemental groups of a given USER. When the -l
option is given, the su-l PAM file is used instead of the default su
PAM file.  Synopsis:

     su [OPTION]... [USER [ARG]...]

   If no USER is given, the default is `root', the super-user.  The
shell to use is taken from USER's `passwd' entry, or `/bin/sh' if none
is specified there.  If USER has a password, `su' prompts for the
password unless run by a user with effective user ID of zero (the

   By default, `su' does not change the current directory.  It sets the
environment variables `HOME' and `SHELL' from the password entry for
USER, and if USER is not the super-user, sets `USER' and `LOGNAME' to
USER.  By default, the shell is not a login shell.

   Any additional ARGs are passed as additional arguments to the shell.

   GNU `su' does not treat `/bin/sh' or any other shells specially
(e.g., by setting `argv[0]' to `-su', passing `-c' only to certain
shells, etc.).

   `su' can optionally be compiled to use `syslog' to report failed,
and optionally successful, `su' attempts.  (If the system supports

   This version of `su' has support for using PAM for authentication.
You can edit `/etc/pam.d/su' to customize its behaviour.

   The program accepts the following options.  Also see *note Common

     Pass COMMAND, a single command line to run, to the shell with a
     `-c' option instead of starting an interactive shell.

     Pass the `-f' option to the shell.  This probably only makes sense
     if the shell run is `csh' or `tcsh', for which the `-f' option
     prevents reading the startup file (`.cshrc').  With Bourne-like
     shells, the `-f' option disables file name pattern expansion
     (globbing), which is not likely to be useful.

     Make the shell a login shell.  This means the following.  Unset all
     environment variables except `TERM', `HOME', and `SHELL' (which
     are set as described above), and `USER' and `LOGNAME' (which are
     set, even for the super-user, as described above), and set `PATH'
     to a compiled-in default value.  Change to USER's home directory.
     Prepend `-' to the shell's name, intended to make it read its
     login startup file(s).  Additionaly `DISPLAY' and `XAUTHORITY'
     environment variables are preserved as well for PAM functionality.

     Do not change the environment variables `HOME', `USER', `LOGNAME',
     or `SHELL'.  Run the shell given in the environment variable
     `SHELL' instead of the shell from USER's passwd entry, unless the
     user running `su' is not the super-user and USER's shell is
     restricted.  A "restricted shell" is one that is not listed in the
     file `/etc/shells', or in a compiled-in list if that file does not
     exist.  Parts of what this option does can be overridden by
     `--login' and `--shell'.

`-s SHELL'
     Run SHELL instead of the shell from USER's passwd entry, unless
     the user running `su' is not the super-user and USER's shell is
     restricted (see `-m' just above).

   Exit status:

     125 if `su' itself fails
     126 if subshell is found but cannot be invoked
     127 if subshell cannot be found
     the exit status of the subshell otherwise