On 28/06/2019 06:24, Mayo Adams wrote: > What are these environment variables, exactly, and why is it necessary to > set them?
When you run a program under an operating system the OS sets up an "environment" (or context) for the program to run in. (This includes the OS shell that the user interacts with - each user gets their own environment.) Environment variables are variables that are accessible within that environment but not from other environments. Thus two users may have different values for the same variable, such as the HOME variable which dictates the users home directory. Or PATH which tells the OS where the users programs can be found. When you start a new program the OS creates a copy of the current environment(your user environment) and runs the program within that copy. Thus if the program modifies any of the environment variables it does not affect the parent process environment since it is modifying its own copy. (So if it changes HOME to give itself its own default directory that doesn't change the user's HOME - or any other program environment for that matter) Some applications define their own environment variables as a way of setting user specific values. So a web server might define the root directory for the server as an environment variable then each user can set a different root without having to pass that in each time they run the program. Environment variable have fallen out of favour for user settings and config files are now preferred. But some things are a bit easier via en environment variable - especially where you spawn new sub-processes and don't want the sub-process to have to re-read the config file each time. -- Alan G Author of the Learn to Program web site http://www.alan-g.me.uk/ http://www.amazon.com/author/alan_gauld Follow my photo-blog on Flickr at: http://www.flickr.com/photos/alangauldphotos _______________________________________________ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: https://mail.python.org/mailman/listinfo/tutor
