1SU(1) User Commands SU(1)
2
6 su - run a command with substitute user and group ID
7
9 su [options] [-] [user [argument...]]
10
12 su allows to run commands with a substitute user and group ID.
13 When called without arguments, su defaults to running an interactive
15 shell as root.
16 For backward compatibility, su defaults to not change the current
18 directory and to only set the environment variables HOME and SHELL
19 (plus USER and LOGNAME if the target user is not root). It is recom‐
20 mended to always use the --login option (instead of its shortcut -) to
21 avoid side effects caused by mixing environments.
22 This version of su uses PAM for authentication, account and session
24 management. Some configuration options found in other su implementa‐
25 tions, such as support for a wheel group, have to be configured via
26 PAM.
27 su is mostly designed for unprivileged users, the recommended solution
29 for privileged users (e.g. scripts executed by root) is to use non-set-
30 user-ID command runuser(1) that does not require authentication and
31 provide separate PAM configuration. If the PAM session is not required
32 at all then the recommend solution is to use command setpriv(1).
33
36 -c, --command=command
37 Pass command to the shell with the -c option.
38 -f, --fast
40 Pass -f to the shell, which may or may not be useful, depending
41 on the shell.
42 -g, --group=group
44 Specify the primary group. This option is available to the root
45 user only.
46 -G, --supp-group=group
48 Specify a supplemental group. This option is available to the
49 root user only. The first specified supplementary group is also
50 used as a primary group if the option --group is unspecified.
51 -, -l, --login
53 Start the shell as a login shell with an environment similar to
54 a real login:
55 o clears all the environment variables except TERM and
57 variables specified by --whitelist-environment
58 o initializes the environment variables HOME, SHELL,
60 USER, LOGNAME, and PATH
61 o changes to the target user's home directory
63 o sets argv[0] of the shell to '-' in order to make the
65 shell a login shell
66 -m, -p, --preserve-environment
68 Preserve the entire environment, i.e. it does not set HOME,
69 SHELL, USER nor LOGNAME. This option is ignored if the option
70 --login is specified.
71 -P, --pty
73 Create pseudo-terminal for the session. The independent terminal
74 provides better security as user does not share terminal with
75 the original session. This allow to avoid TIOCSTI ioctl termi‐
76 nal injection and another security attacks against terminal file
77 descriptors. The all session is also possible to move to back‐
78 ground (e.g. "su --pty - username -c application &"). If the
79 pseudo-terminal is enabled then su command works as a proxy
80 between the sessions (copy stdin and stdout).
81 This feature is EXPERIMENTAL for now and may be removed in the
83 next releases.
84 -s, --shell=shell
87 Run the specified shell instead of the default. The shell to
88 run is selected according to the following rules, in order:
89 o the shell specified with --shell
91 o the shell specified in the environment variable SHELL,
93 if the --preserve-environment option is used
94 o the shell listed in the passwd entry of the target
96 user
97 o /bin/sh
99 If the target user has a restricted shell (i.e. not listed in
101 /etc/shells), the --shell option and the SHELL environment vari‐
102 ables are ignored unless the calling user is root.
103 --session-command=command
105 Same as -c but do not create a new session. (Discouraged.)
106 -w, --whitelist-environment=list
108 Don't reset environment variables specified in comma separated
109 list when clears environment for --login. The whitelist is
110 ignored for the environment variables HOME, SHELL, USER, LOG‐
111 NAME, and PATH.
112 -V, --version
114 Display version information and exit.
115 -h, --help
117 Display help text and exit.
118
120 Upon receiving either SIGINT, SIGQUIT or SIGTERM, su terminates its
121 child and afterwards terminates itself with the received signal. The
122 child is terminated by SIGTERM, after unsuccessful attempt and 2 sec‐
123 onds of delay the child is killed by SIGKILL.
124
126 su reads the /etc/default/su and /etc/login.defs configuration files.
127 The following configuration items are relevant for su(1):
128 FAIL_DELAY (number)
130 Delay in seconds in case of an authentication failure. The number
131 must be a non-negative integer.
132 ENV_PATH (string)
134 Defines the PATH environment variable for a regular user. The
135 default value is /usr/local/bin:/bin:/usr/bin.
136 ENV_ROOTPATH (string)
138 ENV_SUPATH (string)
139 Defines the PATH environment variable for root. The default value
140 is /usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin.
141 ALWAYS_SET_PATH (boolean)
143 If set to yes and --login and --preserve-environment were not spec‐
144 ified su initializes PATH.
145 The environment variable PATH may be different on systems where /bin
147 and /sbin are merged into /usr.
148
150 su normally returns the exit status of the command it executed. If the
151 command was killed by a signal, su returns the number of the signal
152 plus 128.
153 Exit status generated by su itself:
155 1 Generic error before executing the requested command
157 126 The requested command could not be executed
159 127 The requested command was not found
161
163 /etc/pam.d/su default PAM configuration file
164 /etc/pam.d/su-l PAM configuration file if --login is specified
165 /etc/default/su command specific logindef config file
166 /etc/login.defs global logindef config file
167
169 For security reasons su always logs failed log-in attempts to the btmp
170 file, but it does not write to the lastlog file at all. This solution
171 allows to control su behavior by PAM configuration. If you want to use
172 the pam_lastlog module to print warning message about failed log-in
173 attempts then the pam_lastlog has to be configured to update the last‐
174 log file as well. For example by:
175 session required pam_lastlog.so nowtmp
177
179 setpriv(1), login.defs(5), shells(5), pam(8), runuser(8)
180
182 This su command was derived from coreutils' su, which was based on an
183 implementation by David MacKenzie. The util-linux has been refactored
184 by Karel Zak.
185
187 The su command is part of the util-linux package and is available from
188 Linux Kernel Archive ⟨https://www.kernel.org/pub/linux/utils/util-
189 linux/⟩.
190util-linux July 2014 SU(1)