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 [options]... [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" ~ Milhouse
Related linux commands:
chroot - Run a command with a different root directory.
id - Print user and group id's.
logname - Print current login name.
Equivalent Windows command: RUNAS /user.