MATLAB: Are changes lost in MATLAB 7.0 (R14) when using the “!” (bang) operator or the SYSTEM function to change environmental variables

bangdosenvenvironmentexclamationMATLABsystem

I am using the ! (bang) operator to change system environmental variables. The command executes without problem, but the variables are not changed afterwards.

Best Answer

This information has been added to the documentation in Release 14 Service Pack 2 (R14SP2). For previous releases, please read below for any possible workarounds:

When you use the ! (bang) operator or SYSTEM, a new thread is created for the commands. So, in the new process, the environmental variables are changed. However, once the commands are completed, the process is ended. When you check afterwards, the environmental variables are unchanged because you are looking at the same thread as before.

In order to execute commands with specific environmental variables set, place all of the commands inside the call to SYSTEM. One helpful note, you can separate commands in DOS using an ampersand (&) and a semi-colon (;) on UNIX.

Alternatively, you can set the environmental variables before starting MATLAB.

Related Solutions

MATLAB: How to set a system-wide environment variable from within MATLAB

This is a limitation of the Windows operating system. Environment variables that are set from within a process, instead of being set through the control panel, are visible only to that process.

A possible workaround would be to use ActiveX functionality, as implemented in the attached file. Please note that setting a system-wide variable this way requires a restart in order for the changes to take effect.

As another possible workaround, Microsoft offers a tool called "setx" which allows you to set system-wide environment variables.This command is available in Windows versions later than Windows XP Service Pack 2 and Windows 2000. More information on the "setx" command can be found at:

http://technet.microsoft.com/en-us/library/cc755104.aspx

Here is an example how to use it from within MATLAB:

>> !setx my_var some_value
This way of setting system-wide environmental variable does not require a restart of Windows or user re-logging on. However, note that if you use the Windows command window and
the "set" command to view the environmental variables, you will need to restart the command window to see the change.

MATLAB: How to modify the SYSTEM command or ! (bang) operator in MATLAB

MATLAB makes operating system calls through the functions SYSTEM, UNIX and DOS and also by using the ! (bang) operator. These are all built-in functions.

You can overload a built-in function by creating a MATLAB function with the same name which is higher in the MATLAB search path. Then from within that file you can call the built-in function using the BUILTIN function. In this way you can overload a function like SYSTEM.

For example, if you want to prepend the command pbs_attach to all calls made with the system command, you can create a file called "system.m" which contains the following code:

function [s,w] = system( cmd )
prefix = 'echo ';
if nargout == 2
    [s,w] = builtin( 'system', [prefix cmd] );
else
    s = builtin( 'system', [prefix cmd] );
end

You then need to make sure that this file is in a directory which is higher on the MATLAB path than $MATLABROOT\toolbox\matlab\general\ (where $MATLABROOT is the MATLAB root directory on your machine, as returned by typing:

matlabroot

at the MATLAB command prompt.)

To do this, you can put this file in any directory and use the ADDPATH and SAVEPATH commands in MATLAB or the Set Path dialog box to modify the path. For more information on the MATLAB path see the following web page:

Search Path :: Workspace, Search Path, and File Operations (Desktop Tools and Development Environment)

https://www.mathworks.com/help/matlab/search-path.html

To ensure that all system commands go through your functions, you will need to create files with the names "system.m", "unix.m", and "dos.m".

It is also possible to overload the ! (bang) operator by creating a file called "!.m" and having this file be before the directory $MATLABROOT\toolbox\matlab\general in the MATLAB search path. However, the ! (bang) operator is considered punctuation by the MATLAB language and this behavior may change without notice in future releases of MATLAB.

Please note that when you create a file which overloads a MATLAB built-in function, a warning message is displayed. These warning messages will be displayed whenever these files are in your current directory.\n

Warning: Function T:\1-3S35CL\unix.m has the same name as a MATLAB builtin.
We suggest you rename the function to avoid a potential name conflict.
Warning: Function T:\1-3S35CL\dos.m has the same name as a MATLAB builtin.
We suggest you rename the function to avoid a potential name conflict.
Warning: Function T:\1-3S35CL\system.m has the same name as a MATLAB builtin.
We suggest you rename the function to avoid a potential name conflict.

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MATLAB: How to set a system-wide environment variable from within MATLAB

MATLAB: How to modify the SYSTEM command or ! (bang) operator in MATLAB

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