Functions related to environment variables in Linux

1, in the terminal can be through the env, set command to view the current environment variables

2. The list of environment variables for the current process can be obtained by the third parameter in the main function.

int main (int argc, char *argv[], char *env[]);

Where argv and Env are an array of pointers, the last element of the array is null

3. Print the environment variables of the current process

int Main (int.Char char *env[]) {  char **p = env;　　　　 while (*p) {printf ("%s\n", *env);　　Env+ +; }}

4, getenv get the specified environment variable

char * GETENV ("Environment variable name")

int　　Main () {  char * p = getenv ("PATH");　　 if(p) {printf ("%s", p);　　}  Else{  return; }}

5. PUTENV Setting Environment variables

intMainintARGC,Char* * argv,Char**env) {printf ("%s\n", Getenv ("PATH")); Putenv ("Path=/home/hello"); printf ("%s\n", Getenv ("PATH"));}

Output Result:

/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
/home/hello

6. SETENV Setting Environment variables

The first thing to note is that this function does not add or modify the environment variables of the shell process, or the environment variables set by the SETENV function are valid only in this process and in this execution. If the SETENV function is executed at one time when the program is run, the process terminates and runs the program again, the last setting is invalid, the last environment variable was not read.

SETENV ("Variable name", "New variable Value", "whether overriding")

intMainintARGC,Char* * argv,Char**env) {printf ("%s\n", Getenv ("PATH")); intres = setenv ("PATH","/home/hello",1); The third parameter, 1, represents an overrideif(res = =-1)return; printf ("%s\n", Getenv ("PATH"));}

Output Result:

/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin

/home/hello

If the third argument is 0, that is, the environment variable already exists, it does not change its value

Output Result:

/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin

/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin

7. unsetenv Delete an environment variable

UNSETENV ("Environment variable name")

intMainintARGC,Char* * argv,Char**env) {printf ("%s\n", Getenv ("PATH")); Unsetenv ("PATH"); printf ("%s\n", Getenv ("PATH"));}

Output Result:

/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin

Path is deleted, so only one line is output

Functions related to environment variables in Linux