When called with no user specified, su defaults to running an interactive shell as root. When user is specified, additional arguments can be supplied, in which case they are passed to the shell.
For backward compatibility, su defaults to not change the current directory and to only set the environment variables HOME and SHELL (plus USER and LOGNAME if the target user is not root). It is recommended to always use the --login option (instead of its shortcut -) to avoid side effects caused by mixing environments.
This version of su uses PAM for authentication, account and session management. Some configuration options found in other su implementations, such as support for a wheel group, have to be configured via PAM.
su is mostly designed for unprivileged users, the recommended solution for privileged users (e.g., scripts executed by root) is to use non-set-user-ID command runuser(1) that does not require authentication and provide separate PAM configuration. If the PAM session is not required at all then the recommend solution is to use command setpriv(1).
Note that su in all cases use PAM (pam_getenvlist(3)) to do the final environment modification. Command-line options such as --login and --preserve-environment affect the environment before it is modified by PAM.
- -c, --command=command
- Pass command to the shell with the -c option.
- -f, --fast
- Pass -f to the shell, which may or may not be useful, depending on the shell.
- -g, --group=group
- Specify the primary group. This option is available to the root user only.
- -G, --supp-group=group
- Specify a supplementary group. This option is available to the root user only. The first specified supplementary group is also used as a primary group if the option --group is not specified.
- -, -l, --login
- Start the shell as a login shell with an environment similar to a real login:
- clears all the environment variables except TERM and variables specified by --whitelist-environment
- initializes the environment variables HOME, SHELL, USER, LOGNAME, and PATH
- changes to the target user's home directory
- sets argv of the shell to '-' in order to make the shell a login shell
- -m, -p, --preserve-environment
- Preserve the entire environment, i.e., do not set HOME, SHELL, USER or LOGNAME. This option is ignored if the option --login is specified.
- -P, --pty
- Create a pseudo-terminal for the session. The independent terminal provides better security as the user does not share a terminal with the original session. This can be used to avoid TIOCSTI ioctl terminal injection and other security attacks against terminal file descriptors. The entire session can also be moved to the background (e.g., "su --pty - username -c application &"). If the pseudo-terminal is enabled, then su works as a proxy between the sessions (copy stdin and stdout).
- This feature is mostly designed for interactive sessions. If the standard input is not a terminal, but for example a pipe (e.g., echo "date" | su --pty), then the ECHO flag for the pseudo-terminal is disabled to avoid messy output.
- -s, --shell=shell
- Run the specified shell instead of the default. The shell to run is selected according to the following rules, in order:
- the shell specified with --shell
- the shell specified in the environment variable SHELL, if the --preserve-environment option is used
- the shell listed in the passwd entry of the target user
- If the target user has a restricted shell (i.e., not listed in /etc/shells), the --shell option and the SHELL environment variables are ignored unless the calling user is root.
- Same as -c, but do not create a new session. (Discouraged.)
- -w, --whitelist-environment=list
- Don't reset the environment variables specified in the comma-separated list when clearing the environment for --login. The whitelist is ignored for the environment variables HOME, SHELL, USER, LOGNAME, and PATH.
- -V, --version
- Display version information and exit.
- -h, --help
- Display help text and exit.
The environment variable PATH may be different on systems where /bin and /sbin are merged into /usr; this variable is also affected by the --login command-line option and the PAM system setting (e.g., pam_env(8)).
Exit status generated by su itself:
- Generic error before executing the requested command
- The requested command could not be executed
- The requested command was not found
- default PAM configuration file
- PAM configuration file if --login is specified
- command specific logindef config file
- global logindef config file
session required pam_lastlog.so nowtmp