修改环境变量

Linux下有两个函数用来查询和设置环境变量

char *getenv(const char *name);

int putenv(const char *string);

getenv用于查询指定的环境变量的值，成功则返回该值，否则返回null。

putenv以一个键值对的格式设置给定的环境变量的值

#include <stdlib.h>
#include <stdio.h>
#include <string.h>

int main(int argc, char **argv)
{
    char *var, *value;
    
    if(argc == 1 || argc > 3)
    {
        fprintf(stderr, "usage:environ var [value]\n");
        exit(-1);
    }

    var = argv[1];
    value = getenv(var);
    if(value)
    {
        printf("Variable %s has value %s\n", var, value);
    }
    else
    {
        printf("Variable %s has no value\n");
    }
    
    if(argc == 3)
    {
        char *string;
        value = argv[2];
        string = malloc(strlen(var) + strlen(value) + 2);
        if(!string)
        {
            fprintf(stderr, "out of memory\n");
            exit(1);
        }
        strcpy(string, var);
        strcat(string, "=");
        strcat(string, value);
        printf("calling putenv with: %s\n", string);
        if(putenv(string) != 0)
        {
            fprintf(stderr, "putenv failed\n");
            free(string);
            exit(1);
        }
        
        value = getenv(var);
        if(value)
        {
            printf("New value of %s is %s\n", var, value);
        }
        else
        {
            printf("New value of %s is null??\n", var);
        }
        
        return 0;
    }
}