The su (substitute or switch user) command lets you change from the current account to another user or group ID [1]. It can be used for security purposes, such as running the web in a more locked-down account without access to other files, but its most common purpose is to switch from a user account to root in distributions that are not set up to use sudo. Although the target account must be logged into, using su requires fewer steps than logging out of one account and into another. When the session is no longer needed, the command exit will return the terminal to the original account.

su Options

Frequently used to switch from a user account to root, the bare command switches to the root account given, but it does not change the current directory or environment. This option is convenient for small tasks, such as entering a single command, but it can cause problems if the root account differs greatly from the original account. By contrast, adding --login (-, -l) opens a login shell that changes to the target account's home directory (in this case, /root) and activates the environment variables HOME, SHELL, USER, LOGNAME, and PATH (Figure 1). This option can be inconvenient if you plan to work on files in the original directory, but it removes the chance of a problem caused by a change in the environments, making it suited for a longer session in the target account.

However, those who use su only for either of these purposes might be surprised to discover that the command is more versatile. To start with, su can specify any account to log in to. The root user can also use --group=GROUP (-g) to access all accounts in a group, when, for example, an option such as --command is used (see the following features list). In addition, a specific shell can be logged into with --shell=shell (-s).

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