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Su (Unix)

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Title: Su (Unix)  
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Subject: Passwd, Linux security software, SU, Runas, Comparison of privilege authorization features
Collection: System Administration, Unix User Management and Support-Related Utilities
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Su (Unix)

The Unix command su, derived from set user , is used by a computer user to execute a command with the privileges of another user account. When executed it invokes a shell without changing the current working directory or the user environment.

When the command is used without specifying the new user id as a command line argument, it defaults to using the superuser account (user id 0) of the system.


  • History 1
  • Usage 2
  • See also 3
  • References 4
  • External links 5


The command su, including the Unix permissions system and the setuid system call, was part of Version 1 Unix. Encrypted passwords appeared in Version 3.[1]


When run from the command line, su asks for the target user's password, and if authenticated, grants the operator access to that account and the files and directories that account is permitted to access.

john@localhost:~$ su jane
jane@localhost:/home/john$ exit

When used with a hyphen (su -) it can be used to start a login shell. In this mode users can assume the user environment of the target user:

john@localhost:~$ su - jane

The command sudo is related, and executes a command as another user but observes a set of constraints about which users can execute which commands as which other users (generally in a configuration file named /etc/sudoers, best editable by the command visudo). Unlike su, sudo authenticates users against their own password rather than that of the target user (to allow the delegation of specific commands to specific users on specific hosts without sharing passwords among them and while mitigating the risk of any unattended terminals).

Some Unix-like systems have a wheel group of users, and only allow these users to su to root.[2] This may or may not mitigate these security concerns, since an intruder might first simply break into one of those accounts. GNU su, however, does not support a wheel group for philosophical reasons. Richard Stallman argues that because a wheel group would prevent users from utilizing root passwords leaked to them, the group would allow existing admins to ride roughshod over ordinary users.[3]

See also


  1. ^  
  2. ^ Levi, Bozidar (2002). UNIX Administration: A Comprehensive Sourcebook for Effective Systems and Network Management. CRC Press. p. 207.  
  3. ^ "Why GNU su does not support the wheel group". 

External links

  • su — manual pages from GNU coreutils.
  •  – Linux User Commands Manual
  •  – FreeBSD General Commands Manual
  •  – Solaris 10 System Administration Commands Reference Manual
  • The su command — by The Linux Information Project (LINFO)
  • Definition of su —
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