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 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 mostly designed for interactive sessions. If the
83 standard input is not a terminal, but for example pipe (e.g.
84 echo "date" | su --pty) than ECHO flag for the pseudo-terminal
85 is disabled to avoid messy output.
86 -s, --shell=shell
88 Run the specified shell instead of the default. The shell to
89 run is selected according to the following rules, in order:
90 o the shell specified with --shell
92 o the shell specified in the environment variable SHELL,
94 if the --preserve-environment option is used
95 o the shell listed in the passwd entry of the target
97 user
98 o /bin/sh
100 If the target user has a restricted shell (i.e. not listed in
102 /etc/shells), the --shell option and the SHELL environment vari‐
103 ables are ignored unless the calling user is root.
104 --session-command=command
106 Same as -c but do not create a new session. (Discouraged.)
107 -w, --whitelist-environment=list
109 Don't reset environment variables specified in comma separated
110 list when clears environment for --login. The whitelist is
111 ignored for the environment variables HOME, SHELL, USER, LOG‐
112 NAME, and PATH.
113 -V, --version
115 Display version information and exit.
116 -h, --help
118 Display help text and exit.
119
121 Upon receiving either SIGINT, SIGQUIT or SIGTERM, su terminates its
122 child and afterwards terminates itself with the received signal. The
123 child is terminated by SIGTERM, after unsuccessful attempt and 2 sec‐
124 onds of delay the child is killed by SIGKILL.
125
127 su reads the /etc/default/su and /etc/login.defs configuration files.
128 The following configuration items are relevant for su(1):
129 FAIL_DELAY (number)
131 Delay in seconds in case of an authentication failure. The number
132 must be a non-negative integer.
133 ENV_PATH (string)
135 Defines the PATH environment variable for a regular user. The
136 default value is /usr/local/bin:/bin:/usr/bin.
137 ENV_ROOTPATH (string)
139 ENV_SUPATH (string)
140 Defines the PATH environment variable for root. ENV_SUPATH takes
141 precedence. The default value is /usr/local/sbin:/usr/local/bin:
142 /sbin:/bin:/usr/sbin:/usr/bin.
143 ALWAYS_SET_PATH (boolean)
145 If set to yes and --login and --preserve-environment were not spec‐
146 ified su initializes PATH.
147 The environment variable PATH may be different on systems where /bin
149 and /sbin are merged into /usr.
150
152 su normally returns the exit status of the command it executed. If the
153 command was killed by a signal, su returns the number of the signal
154 plus 128.
155 Exit status generated by su itself:
157 1 Generic error before executing the requested command
159 126 The requested command could not be executed
161 127 The requested command was not found
163
165 /etc/pam.d/su default PAM configuration file
166 /etc/pam.d/su-l PAM configuration file if --login is specified
167 /etc/default/su command specific logindef config file
168 /etc/login.defs global logindef config file
169
171 For security reasons su always logs failed log-in attempts to the btmp
172 file, but it does not write to the lastlog file at all. This solution
173 allows to control su behavior by PAM configuration. If you want to use
174 the pam_lastlog module to print warning message about failed log-in
175 attempts then the pam_lastlog has to be configured to update the last‐
176 log file as well. For example by:
177 session required pam_lastlog.so nowtmp
179
181 setpriv(1), login.defs(5), shells(5), pam(8), runuser(8)
182
184 This su command was derived from coreutils' su, which was based on an
185 implementation by David MacKenzie. The util-linux has been refactored
186 by Karel Zak.
187
189 The su command is part of the util-linux package and is available from
190 Linux Kernel Archive ⟨https://www.kernel.org/pub/linux/utils/util-
191 linux/⟩.
192util-linux July 2014 SU(1)