Re: Can system modify an environment variable in the current environment?

[Taylan Kammer <taylan.kammer@gmail.com>]

On 02.09.2021 21:49, Mortimer Cladwell wrote:
> Hi,
> 
> Consider my file test.scm:
> 
> (define (main args)
>   (let* ((myvar (string-append "export
> GUILE_LOAD_PATH=/some/random/text:$GUILE_LOAD_PATH"))
>            (statement1 (system (string-append "echo " myvar " >>
> $HOME/.bashrc")))
>             (statement2 (system myvar)))
>     (write myvar)))
> 
> At the terminal:
> 
> mbc@HP8300:~/temp$ echo $GUILE_LOAD_PATH
> /home/mbc/.guix-profile/share/guile/site/3.0
> 
> mbc@HP8300:~/temp$ guile -e main -s test.scm
> "export GUILE_LOAD_PATH=/some/random/text:$GUILE_LOAD_PATH"
> 
> mbc@HP8300:~/temp$ echo $GUILE_LOAD_PATH
> /home/mbc/.guix-profile/share/guile/site/3.0
> 
> At the end of .bashrc I see the last line is:
> 
> export
> GUILE_LOAD_PATH=/some/random/text:/home/mbc/.guix-profile/share/guile/site/3.0
> 
> If I close and reopen the terminal and:
> 
> mbc@HP8300:~$ echo $GUILE_LOAD_PATH
> /some/random/text:/home/mbc/.guix-profile/share/guile/site/3.0
> 
> So statement1 works as expected, modifying .bashrc which is then effective
> in modifying GUILE_LOAD_PATH on future invocations of terminal.
> 
> statement2 is an attempt to modify the current running environment, but
> fails.
> Note that if I paste export
> GUILE_LOAD_PATH=/some/random/text:$GUILE_LOAD_PATH directly into the
> terminal, that successfully modifies the variable.
> 
> Why does  (system myvar) i.e. (system "export
> GUILE_LOAD_PATH=/some/random/text:$GUILE_LOAD_PATH") fail?
> Thanks
> Mortimer
> 

Hi Mortimer,

The "system" procedure starts a child-process in the operating system.
In the Unix process model, the changes to the environment made by a child
process do not affect the parent.

Note that anything for which you use "system" above can be done directly
within Guile by using the appropriate procedures, instead of starting a
child process using the system shell.

You might want to look for I/O related procedures to figure out how you
can append text to an existing file.  This might help:

https://www.gnu.org/software/guile/manual/html_node/File-Ports.html

Setting an environment variable is simple, just use 'setenv':

https://www.gnu.org/software/guile/manual/html_node/Runtime-Environment.html

Hope that helps. :-)

-- 
Taylan