1SU(1) User Commands SU(1)
2
6 su - run a command with substitute user and group ID
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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
57 o initializes the environment variables HOME, SHELL,
59 USER, LOGNAME, and PATH
60 o changes to the target user's home directory
62 o sets argv[0] of the shell to '-' in order to make the
64 shell a login shell
65 -m, -p, --preserve-environment
67 Preserve the entire environment, i.e. it does not set HOME,
68 SHELL, USER nor LOGNAME. This option is ignored if the option
69 --login is specified.
70 -P, --pty
72 Create pseudo-terminal for the session. The independent terminal
73 provides better security as user does not share terminal with
74 the original session. This allow to avoid TIOCSTI ioctl termi‐
75 nal injection and another security attacks against terminal file
76 descriptors. The all session is also possible to move to back‐
77 ground (e.g. "su --pty - usename -c application &"). If the
78 pseudo-terminal is enabled then su command works as a proxy
79 between the sessions (copy stdin and stdout).
80 This feature is EXPERIMENTAL for now and may be removed in the
82 next releases.
83 -s, --shell=shell
86 Run the specified shell instead of the default. The shell to
87 run is selected according to the following rules, in order:
88 o the shell specified with --shell
90 o the shell specified in the environment variable SHELL,
92 if the --preserve-environment option is used
93 o the shell listed in the passwd entry of the target
95 user
96 o /bin/sh
98 If the target user has a restricted shell (i.e. not listed in
100 /etc/shells), the --shell option and the SHELL environment vari‐
101 ables are ignored unless the calling user is root.
102 --session-command=command
104 Same as -c but do not create a new session. (Discouraged.)
105 -V, --version
107 Display version information and exit.
108 -h, --help
110 Display help text and exit.
111
113 Upon receiving either SIGINT, SIGQUIT or SIGTERM, su terminates its
114 child and afterwards terminates itself with the received signal.
115
117 su reads the /etc/default/su and /etc/login.defs configuration files.
118 The following configuration items are relevant for su(1):
119 FAIL_DELAY (number)
121 Delay in seconds in case of an authentication failure. The number
122 must be a non-negative integer.
123 ENV_PATH (string)
125 Defines the PATH environment variable for a regular user. The
126 default value is /usr/local/bin:/bin:/usr/bin.
127 ENV_ROOTPATH (string)
129 ENV_SUPATH (string)
130 Defines the PATH environment variable for root. The default value
131 is /usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin.
132 ALWAYS_SET_PATH (boolean)
134 If set to yes and --login and --preserve-environment were not spec‐
135 ified su initializes PATH.
136
138 su normally returns the exit status of the command it executed. If the
139 command was killed by a signal, su returns the number of the signal
140 plus 128.
141 Exit status generated by su itself:
143 1 Generic error before executing the requested command
145 126 The requested command could not be executed
147 127 The requested command was not found
149
151 /etc/pam.d/su default PAM configuration file
152 /etc/pam.d/su-l PAM configuration file if --login is specified
153 /etc/default/su command specific logindef config file
154 /etc/login.defs global logindef config file
155
157 For security reasons su always logs failed log-in attempts to the btmp
158 file, but it does not write to the lastlog file at all. This solution
159 allows to control su behavior by PAM configuration. If you want to use
160 the pam_lastlog module to print warning message about failed log-in
161 attempts then the pam_lastlog has to be configured to update the last‐
162 log file as well. For example by:
163 session required pam_lastlog.so nowtmp
165
167 setpriv(1), login.defs(5), shells(5), pam(8), runuser(8)
168
170 This su command was derived from coreutils' su, which was based on an
171 implementation by David MacKenzie. The util-linux has been refactored
172 by Karel Zak.
173
175 The su command is part of the util-linux package and is available from
176 Linux Kernel Archive ⟨https://www.kernel.org/pub/linux/utils/util-
177 linux/⟩.
178util-linux July 2014 SU(1)