I want to know how the standard environment variables given by the linux OS like PATH, HOME are set automatically. In which file are these read from. There should be some file from which these variables are set when a particular user login.
The answer to your question can be found in
INVOCATION section of
Here's relevant excerpt:
When bash is invoked as an interactive login shell, or as a non-inter- active shell with the --login option, it first reads and executes com- mands from the file /etc/profile, if that file exists. After reading that file, it looks for ~/.bash_profile, ~/.bash_login, and ~/.profile, in that order, and reads and executes commands from the first one that exists and is readable. The --noprofile option may be used when the shell is started to inhibit this behavior. When a login shell exits, bash reads and executes commands from the file ~/.bash_logout, if it exists. When an interactive shell that is not a login shell is started, bash reads and executes commands from ~/.bashrc, if that file exists. This may be inhibited by using the --norc option. The --rcfile file option will force bash to read and execute commands from file instead of ~/.bashrc. When bash is started non-interactively, to run a shell script, for example, it looks for the variable BASH_ENV in the environment, expands its value if it appears there, and uses the expanded value as the name of a file to read and execute. Bash behaves as if the following com- mand were executed: if [ -n "$BASH_ENV" ]; then . "$BASH_ENV"; fi but the value of the PATH variable is not used to search for the file name.
There's even more in the man page, I recommend you read it.
Things are a little more complex than just what your shell provides.
There are three main ways to login:
- login from a true terminal (nowadays, mainly the console)
- login from a pseudo-terminal (mainly network connections)
- login from a graphic environment
They all are able to set up the environment before executing your shell, and they all do (
TERM are probably not set by your shell, and even if you have no start-up files, your shell usually inherits the
PATH environment variable).
When logging in from a true terminal, the process handling the connection will probably set up the
TERM environment variable and delegate the rest of the work to the
login program. That program does some verifications (such as preventing login for root on insecure terminals), ensures that the whole environment excepted
TERM is clean, initializes
LOGNAME and then launch your login shell. Your shell will then do its own initialization.
When logging in from a pseudo-terminal, the same things happen with a little twist. Often the environment gets more initialization from the process handling the connection than just the
TERM environment variable (network protocols often have a way to transfer the environment from the other side) and thus
login is used in a mode in which it doesn't clean the environment, the clean up is done by the program handling the connection.
Graphic environments usually don't delegate to
login but behave similarly. After verifying your credentials, they create a clean environment (with usual environment variable and at least
DISPLAY set correctly; they often allow a sysadmin provided script to add things) then launch the start up script for your desktop environment; those may try to get the environment from your login shell and also often provide the possibility to provide a script to complete the set up. So when you launch any program from your desktop, its environment is the combination of what the graphical login program, your desktop environment and your login scripts set.
One last thing: when you launch a terminal emulator, you may get either a login shell (in which case the login script from the shell is executed and thus you get to see the effect of your latest changes in it, but won't see the same environment as the other programs) or not (in which case the login script from the shell isn't executed, you don't see your latest changes, but you get the same environment as the other programs — modified by the interactive initialization script of your shell).
You should take a look at the
.profile file in the root of user directories - variables like those can be set (or overridden) there.
However, if you mean where do all environmental variables get set - I'd assume that they get inherited from some parent process. Perhaps a look at something like this might help.