`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. 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' to `-su', passing `-c' only to certain
`su' can optionally be compiled to use `syslog' to report failed,
and optionally successful, `su' attempts. (If the system supports
`syslog'.) However, GNU `su' does not check if the user is a member of
the `wheel' group; see below.
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).
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 superuser 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'.
Run SHELL instead of the shell from USER's passwd entry, unless
the user running `su' is not the superuser and USER's shell is
restricted (see `-m' just above).
Why GNU `su' does not support the `wheel' group
(This section is by Richard Stallman.)
Sometimes a few of the users try to hold total power over all the
rest. For example, in 1984, a few users at the MIT AI lab decided to
seize power by changing the operator password on the Twenex system and
keeping it secret from everyone else. (I was able to thwart this coup
and give power back to the users by patching the kernel, but I wouldn't
know how to do that in Unix.)
However, occasionally the rulers do tell someone. Under the usual
`su' mechanism, once someone learns the root password who sympathizes
with the ordinary users, he or she can tell the rest. The "wheel
group" feature would make this impossible, and thus cement the power of
I'm on the side of the masses, not that of the rulers. If you are
used to supporting the bosses and sysadmins in whatever they do, you
might find this idea strange at first.
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