table of contents
su - run a command with substitute user and group ID
su [options] [-] [user [argument...]]
su allows commands to be run with a substitute user and group ID.
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 provides separate PAM configuration. If the PAM session is not required at all then the recommended solution is to use command setpriv(1).
Note that su in all cases uses 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.
Since version 2.38 su resets process resource limits RLIMIT_NICE, RLIMIT_RTPRIO, RLIMIT_FSIZE, RLIMIT_AS and RLIMIT_NOFILE.
-, -l, --login
-m, -p, --preserve-environment
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.
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.
Upon receiving either SIGINT, SIGQUIT or SIGTERM, su terminates its child and afterwards terminates itself with the received signal. The child is terminated by SIGTERM, after unsuccessful attempt and 2 seconds of delay the child is killed by SIGKILL.
su reads the /etc/default/su and /etc/login.defs configuration files. The following configuration items are relevant for su:
ENV_ROOTPATH (string), ENV_SUPATH (string)
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)).
su normally returns the exit status of the command it executed. If the command was killed by a signal, su returns the number of the signal plus 128.
Exit status generated by su itself:
For security reasons, su always logs failed log-in attempts to the btmp file, but it does not write to the lastlog file at all. This solution can be used to control su behavior by PAM configuration. If you want to use the pam_lastlog(8) module to print warning message about failed log-in attempts then pam_lastlog(8) has to be configured to update the lastlog file as well. For example by:
session required pam_lastlog.so nowtmp
This su command was derived from coreutils' su, which was based on an implementation by David MacKenzie. The util-linux version has been refactored by Karel Zak.
setpriv(1), login.defs(5), shells(5), pam(8), runuser(1)
For bug reports, use the issue tracker at <https://github.com/util-linux/util-linux/issues>.
The su command is part of the util-linux package which can be downloaded from Linux Kernel Archive <https://www.kernel.org/pub/linux/utils/util-linux/>.