Environment variables of linux system

Operating system environment variables generally refer to some parameters used to specify the operating environment of the system

Common environment variables are:

PATH: Specify the search path of the command
HISTSIZE: Number of records to save history commands
SHELL: the current shell, the value is usually bin\bash
HOME: Specify the user's main working directory

View environment variables:

Use the env command to view the environment of all systems

Use echo $NAME //NAME is the environment variable you specify

Use set to display locally defined environment variables and shell variables

Set the environment variables:

Set permanent environment variables

1. Edit under the /etc/profile file, the changed environment variables are for all users
 export CLASSPATH = /../... The path is an absolute path

 2. Modify in the ./barsh_profile file in the current user directory. If you modify it, it will only take effect for the current user.
vim /home/wens/.barshc
 export CLASSPATH = /../... The path is an absolute path

 Finally, use the source command to directly make the environment variables effective
 source /home/wens/.barshc //Directly follow the environment variable file

Set temporary environment variables

Use the export command to directly set the environment variables, but it is temporary and only works under the current shell

 export environment variable = path

Clear environment variables

Use the unset command to clear the environment variables directly
// unset PATH

How environmental variables are organized

Each program has an environment variable table. The environment variables are organized by an array of character pointers. Each pointer points to an environment variable string ending in ‘’/0’.

environ -> [-]--> PATH=/home/dsd\0
           [-]--> home=/DSD/SD\0
           [ ]--> ....
           [ ]
           [NULL]

Get environment variables through code

The third parameter through the command line

int main(int argc, cahr* argv[], char* env[]){
         //Environment variables are stored in the env array
}

Obtained through third-party variable environ

int main(int argc, cahr* argv[]){
    extern cahr ** environ;
             //environ points to the global environment variable table, does not exist in any file, can only be accessed through extern
           //Environ variables stored in the array of character pointers are environment variables
}

Get environment variables through system calls

putenv
int putenv(const char *name)
getenv
char *getenv(const char *name)


 Parameters must be given in the form of path=/../..

Environment variables are global and can be inherited by child processes