Substitute user identity
Run a command with substitute user and group id, allow 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
SYNTAX su [OPTION]... [USER [ARG]...] OPTIONS `-c COMMAND' `--command=COMMAND' Pass COMMAND, a single command line to run, to the shell with a `-c' option instead of starting an interactive shell. `-f' `--fast' 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. `-' `-l' `--login' 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). `-m' `-p' `--preserve-environment' 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'. `-s SHELL' `--shell=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).
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 super-user).
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 shells, etc.). `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 options above.
"It was just like Romeo and Juliet, only it ended in tragedy"
chroot - Run a command with a different root directory
id - Print user and group id's
logname - Print current login name