SU(1)

✨ SU(1) ✨

okay bestie, sit down, hydrate, because we're talking about su — the glow-up command that lets you run stuff as a whole different user. it's giving borrowed identity but make it legit 💅

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🌸 NAME 🌸

su — run a command with substitute user and group ID

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🎀 SYNOPSIS 🎀

su [options] [-] [user|UID [argument...]]

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💖 DESCRIPTION 💖

su lets you run commands with a substitute user and group ID. like, you're you, but the command thinks it's somebody else. iconic behavior honestly.

When you call it with no user specified, su defaults to running an interactive shell as root. When a user is specified, you can also hand it extra arguments, and those get passed straight to the shell. simple as that 🫶

For backward compatibility, su defaults to not changing the current directory, and to only setting 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. no cap, mixing environments is messy and you do not want that energy in your life.

This version of su uses PAM for authentication, account and session management. Some configuration options found in other su implementations — like support for a wheel group — have to be configured via PAM. iykyk.

su is mostly designed for unprivileged users. The recommended solution for privileged users (e.g., scripts executed by root) is to use the non-set-user-ID command runuser(1), which 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 the command setpriv(1). main character energy, but the responsible kind 👑

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. okayyy limits-resetting queen 🔥

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✨ OPTIONS ✨

• -c, --command command — pass command to the shell with the -c option. Creates a new session via setsid(2). Refer to --session-command if you want to keep the same session.
• -f, --fast — pass -f to the shell, which may or may not be useful, depending on the shell. it's giving "we tried our best" 🙈
• -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.

Note that on systemd(1)-based systems, a new session may be defined as a real entry point to the system. However, su does not create a real session (by PAM) from this point of view. You need to use tools like systemd-run(1) or machinectl(1) to initiate a complete, real session. so like... it's a session, but it's not that kind of session, bestie.

su does:

• clear all the environment variables except TERM, COLORTERM, NO_COLOR and variables specified by --whitelist-environment
• initialize the environment variables HOME, SHELL, USER, LOGNAME, and PATH
• change to the target user's home directory
• set argv[0] 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. sorry babe, --login wins.
• -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. Please note the security advice about the TIOCSTI vulnerability below. 👀

The entire session can also be moved to the background (e.g., su --pty - user -c application &). If the pseudo-terminal is enabled, then su works as a proxy between the sessions (sync 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. we love a tidy terminal ✨

• -s, --shell shell — run the specified shell instead of the default. 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.

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
• /bin/sh

• --session-command=command — same as -c, but do not create a new session. (Discouraged.) Please note the security advice about the TIOCSTI vulnerability below. like... bestie... why 😭
• -T, --no-pty — do not create a pseudo-terminal, the opposite of --pty and -P. Please note the security advice about the TIOCSTI vulnerability below.
• -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. those five are on their own, no exceptions 💅
• -h, --help — display help text and exit.
• -V, --version — display version and exit.

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🦋 SIGNALS 🦋

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, and after an unsuccessful attempt and 2 seconds of delay, the child is killed by SIGKILL. yeah, it goes down swinging ⚔️

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💜 CONFIG FILES 💜

su reads the /etc/default/su and /etc/login.defs configuration files. The following configuration items are relevant for su:

FAIL_DELAY (number)

Delay in seconds in case of an authentication failure. The number must be a non-negative integer.

ENV_PATH (string)

Defines the PATH environment variable for a regular user. The default value is /usr/local/bin:/bin:/usr/bin.

ENV_ROOTPATH (string), ENV_SUPATH (string)

Defines the PATH environment variable for root. ENV_SUPATH takes precedence. The default value is /usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin.

ALWAYS_SET_PATH (boolean)

If set to yes and --login and --preserve-environment were not specified, su initializes PATH.

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)).

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🌈 EXIT STATUS 🌈

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:

1. 1 — generic error before executing the requested command
2. 126 — the requested command could not be executed
3. 127 — the requested command was not found

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🍓 FILES 🍓

• /etc/pam.d/su — default PAM configuration file
• /etc/pam.d/su-l — PAM configuration file if --login is specified
• /etc/default/su — command specific logindef config file
• /etc/login.defs — global logindef config file

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🔥 SECURITY NOTES 🔥

If su shares a terminal with the original session, it is potentially vulnerable to privilege escalation through TIOCSTI/TIOCLINUX ioctl command injection. There are two built-in ways to prevent this: either you can use su with the -c option, which starts a new session via setsid(2) without a controlling terminal. Or, if your use case requires a controlling terminal — for example an interactive session — you can instruct su to use a pseudo terminal with the --pty or -P option. pick your fighter, bestie, but pick one 🥊

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 a 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

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💀 HISTORY 💀

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. a whole lineage, we love a documented origin story 🌱

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🧚 SEE ALSO 🧚

setpriv(1), login.defs(5), shells(5), pam(8), runuser(1)

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🎀 REPORTING BUGS 🎀

For bug reports, use the issue tracker https://github.com/util-linux/util-linux/issues. don't be shy, the maintainers want to hear it 🫶

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⭐ AVAILABILITY ⭐

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/.

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and that's the whole thing, bestie 💖 this page is from util-linux 2.42.4, dated 2026-09-21, manual page SU(1). go forth and substitute your user responsibly, mwah 💅✨