runat - execute command in extended attribute name space
/usr/bin/runat file [command]
The runat utility is used to execute shell commands in a file's hidden attribute directory. Effectively, this utility changes the current working directory to be the hidden attribute directory associated with the file argument and then executes the specified command in the bourne shell (/bin/sh). If no command argument is provided, an interactive shell is spawned. The environment variable $SHELL defines the shell to be spawned. If this variable is undefined, the default shell, /bin/sh, is used.
The file argument can be any file, including a directory, that can support extended attributes. It is not necessary that this file have any attributes, or be prepared in any way, before invoking the runat command.
The following operands are supported:
A non-zero exit status will be returned if runat cannot access the file argument, or the file argument does not support extended attributes.
See fsattr(5) for a detailed description of extended file attributes.
The process context created by the runat command has its current working directory set to the hidden directory containing the file's extended attributes. The parent of this directory (the ".." entry) always refers to the file provided on the command line. As such, it may not be a directory. Therefore, commands (such as pwd) that depend upon the parent entry being well-formed (that is, referring to a directory) may fail.
In the absence of the command argument, runat will spawn a new interactive shell with its current working directory set to be the provided file's hidden attribute directory. Notice that some shells (such as zsh and tcsh) are not well behaved when the directory parent is not a directory, as described above. These shells should not be used with runat.
Example 1 Using runat to list extended attributes on a file
example% runat file.1 ls -l example% runat file.1 ls
Example 2 Creating extended attributes
example% runat file.2 cp /tmp/attrdata attr.1 example% runat file.2 cat /tmp/attrdata > attr.1
Example 3 Copying an attribute from one file to another
example% runat file.2 cat attr.1 | runat file.1 "cat > attr.1"
Example 4 Using runat to spawn an interactive shell
example% runat file.3 /bin/sh
This spawns a new shell in the attribute directory for file.3. Notice that the shell will not be able to determine what your current directory is. To leave the attribute directory, either exit the spawned shell or change directory (cd) using an absolute path.
Recommended methods for performing basic attribute operations:
runat file chmod mode attribute runat file chgrp group attribute runat file chown owner attribute
runat file /bin/sh or set your $SHELL to /bin/sh and runat file
The above list includes commands that are known to work with runat. While many other commands may work, there is no guarantee that any beyond this list will work. Any command that relies on being able to determine its current working directory is likely to fail. Examples of such commands follow:
Example 5 Using man in an attribute directory
example% runat file.1 man runat >getcwd: Not a directory
Example 6 Spawning a tcsh shell in an attribute directory
example% runat file.3 /usr/bin/tcsh tcsh: Not a directory tcsh: Trying to start from "/home/user"
A new tcsh shell has been spawned with the current working directory set to the user's home directory.
Example 7 Spawning a zsh shell in an attribute directory
example% runat file.3 /usr/bin/zsh example%
While the command appears to have worked, zsh has actually just changed the current working directory to '/'. This can be seen by using /bin/pwd:
example% /bin/pwd /
The following exit values are returned:
Otherwise, the exit status returned is the exit status of the shell invoked to execute the provided command.
See attributes(5) for descriptions of the following attributes:
open(2), attributes(5), fsattr(5)
It is not always obvious why a command fails in runat when it is unable to determine the current working directory. The errors resulting can be confusing and ambiguous (see the tcsh and zsh examples above).
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Created 1996-2021 by Maxim Chirkov
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