How can I get the local shell variables in C?

Question

I'm currently working on my own replica of the history builtin. I realize I need the HISTSIZE, HISTFILE and HISTFILESIZE variables to achieve the command options but I don't know how to get them. Is there a function similar to getenv or a variable like environ but with the local shell variables ?

Thanks in advance !

In your main declaration you can use `char **envp`, so it looks like : `int main (int argc, char **argv, char **envp)` . Hope it helps — Thomas Blanquet, Dec 01 '16 at 09:21
[`std::getenv`](http://en.cppreference.com/w/cpp/utility/program/getenv) — Danh, Dec 01 '16 at 09:27
If these are no environment variables, there is no way to get hold of them from a child process... — Stefan Hegny, Dec 01 '16 at 09:49

score 2 · Accepted Answer · answered Dec 01 '16 at 10:25

Well a builtin is executed directly by the shell for 2 possible reasons:

you avoid spawning a new processus (performance reason)
it is the only way to have access to internal shell variables (your use case)

A shell could only provide access to its internal variables by two ways:

either copy them to the environment because by definition the environment is the list of variables that are passed to child processes
either provided an inter process communication mechanism (named pipe, private sockect, etc.) to allow its childs to ask them

AFAIK, no common shell implement either, but you can export the required shell variables to the environment:

export HIST
export HISTSIZE
export HISTFILESIZE

should be enough to later get them through getenv

So how does the echo command has an executable at /bin/echo ? It needs to access intern variables — Fayeure, Mar 04 '21 at 00:56
@Fayeure It doesn't need to access internal variables. If you have `a=78` and you run the command `/bin/echo "it is $a"`, then _the shell_ replaces that with `/bin/echo "it is 78"` before the echo executable is told to do anything. The executable has no access to `a`. — Jim Danner, Jan 29 '22 at 13:15

score 1 · Answer 2 · edited May 23 '17 at 12:30

1

The main declaration can have an extra argument, char **envp, which is an 2D array of char terminated by a NULL pointer. Here is an example :

#include <stdio.h>

int main(int argc, char **argv, char **envp)
{
   int index = 0;
   while (envp[index])
       printf("%s\n", envp[index++];
   return 0;
}

EDIT

Using envp will provide you to get the environnement. To find shell variables, I found this. I think that could help you.

edited May 23 '17 at 12:30

Community

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answered Dec 01 '16 at 09:26

Thomas Blanquet

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envp only gives me the environment variables, I need HISTSIZE, HISTFILE and HISTFILESIZE which are shell variables. You can see those in the shell with the command "set" – Bob Dec 01 '16 at 09:36
`envp` as third parameter [isn't portable](http://stackoverflow.com/questions/10321435/is-char-envp-as-a-third-argument-to-main-portable). – nwellnhof Dec 01 '16 at 12:09

score 0 · Answer 3 · answered Dec 01 '16 at 09:54

0

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

int main() {

printf("SET = %s\n",getenv("SET"));

return 0;

}

To test Program, set the variable first and then execute :-

export SET=1

answered Dec 01 '16 at 09:54

Sumit Gemini

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score 0 · Answer 4 · answered Dec 01 '16 at 10:23

As Steffan Hegny pointed out, there is no way to acces these variables if they are no environment variables. So popen(), what I thought could help, isn't a solution to your problem.

However if you need acces to those variables you can export their values to a real environment variable which can be accessed via getenv(char*).

E.g.

export _HISTFILE=$HISTFILE && ./myProgram

or

_HISTFILE=$HISTFILE ./myProgram

with myProgram accessing the value like this:

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

int main() 
{
  printf("History file: %s", getenv("_HISTFILE"));
}

How can I get the local shell variables in C?

4 Answers4