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One free scan finds every outdated or missing driver and matches the right update for your exact hardware.Free scan · exact hardware matchsu lets you switch to another user identity to start a shell or run a command. This guide covers the util-linux implementation: without a user argument, it opens an interactive root shell; use --login for a login-style shell or --command to run a specific command. Options and defaults can differ in other implementations.
What does su do?
The name su means “substitute user.” The util-linux command runs a shell or command using a selected user and group ID. It is commonly used to become another account, including root, subject to the system’s authentication and account policies.
Basic syntax:
su [options] [-] [user|UID [argument...]]
If you omit the user, util-linux su starts an interactive shell as root. What authentication is required depends on local PAM configuration and policy.
How do you run a command as another user with su?
Use --command (or -c) followed by a command string and the target user:
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su --command 'id' USER
su passes the string to the target user’s shell using that shell’s -c option; it does not parse the string as its own command language. The command mode starts a new session. If the command needs a controlling terminal, consider whether a pseudoterminal is needed instead.
To open a shell as a user with a login-style environment, run:
su --login USER
The short form is su - USER. With no user supplied, su --login requests a login shell as root.
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Login shell or bare su?
Bare su USER retains backward-compatible environment behavior and does not change to the target user’s home directory. That can leave a shell with a mixture of the caller’s environment and the target identity. The util-linux manual recommends using --login to avoid side effects from mixing environments.
In util-linux, login mode clears most environment variables, initializes login variables, changes to the target user’s home directory, and marks the shell as a login shell. It sets HOME, SHELL, USER, LOGNAME, and PATH; keeps TERM, COLORTERM, and NO_COLOR, as well as variables explicitly whitelisted where permitted. PAM may modify the final environment, so exact results depend on local configuration.
Preserving selected environment variables
-m, -p, or --preserve-environment asks su to keep the environment; this option is ignored with --login. For selective preservation in login mode, --whitelist-environment LIST retains listed variables, except HOME, SHELL, USER, LOGNAME, and PATH, which cannot be whitelisted.
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Useful util-linux su options
| Option | What it does |
|---|---|
-c, --command COMMAND |
Passes a command string to the target shell with its -c option and starts a new session. |
-, -l, --login |
Starts a login shell, changes to the target home directory, and initializes the login environment. |
-m, -p, --preserve-environment |
Preserves the environment; ignored with --login. |
-g GROUP, --group GROUP |
Selects a primary group; root-only. |
-G GROUP, --supp-group GROUP |
Selects supplementary groups; root-only. If --group is omitted, the first supplementary group is also used as the primary group. |
-P, --pty |
Allocates a pseudoterminal, mainly for interactive sessions. |
-s SHELL, --shell SHELL |
Selects a shell, subject to restricted-shell behavior. |
-w LIST, --whitelist-environment LIST |
Retains permitted variables while clearing the login environment. |
--session-command COMMAND |
Runs a command without creating a new session; the manual discourages this option. |
When choosing a shell, util-linux checks an explicit --shell first, then a preserved $SHELL when preserving the environment, then the target account’s configured shell, and finally /bin/sh.
Security and local policy
Util-linux su uses PAM for authentication, account checks, and session management. PAM configuration can affect authentication, environment changes, and group-based restrictions; do not assume behavior is identical across distributions.
The util-linux manual warns of a terminal-injection risk involving the TIOCSTI and TIOCLINUX ioctls when su shares a terminal with the original session. For a command that does not need a controlling terminal, -c starts a new session without one. For an interactive session that needs a terminal, --pty allocates a pseudoterminal to isolate it from the original session. Choose based on the command’s terminal needs; a PTY is not a universal security guarantee.
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Since util-linux version 2.38, su resets RLIMIT_NICE, RLIMIT_RTPRIO, RLIMIT_FSIZE, RLIMIT_AS, and RLIMIT_NOFILE. This version-specific behavior should not be assumed for other implementations or earlier util-linux releases.
When should you use runuser, setpriv, or sudo instead?
| Tool | When it fits | Key distinction |
|---|---|---|
su |
A user needs to switch identity, typically through an interactive shell or command. | Uses PAM for authentication, account, and session management; local policy governs access. |
runuser |
A privileged caller, such as a root-run script, needs to run a command as another user. | The util-linux manual recommends it for privileged users; it does not require authentication. |
setpriv |
A privileged caller needs to change process privileges without a PAM session. | The util-linux manual recommends it when no PAM session is needed. |
sudo |
A command should be authorized according to sudo policy. | Policy can constrain user and group execution, but permitting an interactive shell may expose more commands than a single authorized invocation. |
These tools are not interchangeable. Compare the intended caller, PAM session needs, environment and working-directory behavior, terminal handling, and the system’s local policy. On systemd-based systems, su does not create a complete real session as systemd defines one; the util-linux manual points to systemd-run or machinectl for that use case.
Exit status and logging
Normally, su returns the status of the command it ran. If that command is killed by a signal, util-linux su returns the signal number plus 128. Errors from su include status 1 for a generic pre-execution error, 126 when the requested command cannot be executed, and 127 when it cannot be found.
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The util-linux manual says failed login attempts are logged to btmp and that su itself does not write to lastlog. PAM configuration may affect related logging behavior.
Implementation matters
This article describes util-linux su, as documented in its su(1) manual. A separate shadow-utils manual is also used by some systems; option details and defaults should not be transferred between implementations without checking the local manual.
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