su

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[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 `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" - Milhouse

Related commands:

chroot - Run a command with a different root directory
id - Print user and group id's
logname - Print current login name