Reference Linux username in string to open file

Question

I have a config file which contains the path to another file which needs to be opened. This file path references the Linux username:

/root/${USER}/workspace/myfile.txt

where $USER should be translated to the Linux username.

This doesn't work and because the string is stored in my config file, I cannot use getenv().

Is there another way to achieve this?

There are ways to [get an environment variable](http://man7.org/linux/man-pages/man3/getenv.3.html), and from that construct the filename. — Some programmer dude, Sep 10 '18 at 10:38
There are many ways to search for, extract and replace sub-strings in C++. Regular expressions comes to mind. Use in combination with `getenv`. — Some programmer dude, Sep 10 '18 at 10:42
I have just updated the question to reflect I cannot really use getenv() — user997112, Sep 10 '18 at 10:42
ok, look for variable interpolation in c/c++, e.g. http://man7.org/linux/man-pages/man3/wordexp.3.html or https://codereview.stackexchange.com/questions/172644/c-environment-variable-expansion — jsantander, Sep 10 '18 at 10:45
'I cannot use getenv' Why not, what is stopping you? Just because $USER is only a part of your string does not stop you using getenv. — john, Sep 10 '18 at 10:47
Possible duplicate of [How to get the username in C/C++ in Linux?](https://stackoverflow.com/questions/8953424/how-to-get-the-username-in-c-c-in-linux) — Mureinik, Sep 10 '18 at 10:48
@john Thank you for your valuable contribution. I will take it under advisement — user997112, Sep 10 '18 at 11:25

score 2 · Accepted Answer · answered Sep 10 '18 at 10:56

You can use wordexp to translate "~" which is a UNIX path element meaning the HOME directory. Something like this:

#include <wordexp.h>

std::string homedir()
{
    std::string s;
    wordexp_t p;
    if(!wordexp("~", &p, 0))
    {
        if(p.we_wordc && p.we_wordv[0])
            s = p.we_wordv[0];
        wordfree(&p);
    }
    return s;
}

And then extract the username from the returned path.

But I normally use std::getenv() like this:

auto HOME = std::getenv("HOME"); // may return nullptr
auto USER = std::getenv("USER"); // may return nullptr

score 2 · Answer 2 · answered Sep 10 '18 at 11:08

Get the username with getenv, replace $USER in the path with it.

Very straightforward example:

#include <iostream>
#include <string>
#include <cstdlib>

int main()
{
    std::string path = "/root/$USER/workspace/myfile.txt";
    const char* user = std::getenv("USER");
    int pos = path.find("$USER");
    if (user != nullptr && pos >= 0)
    {
        path.replace(pos, 5, user);
        std::cout << path << std::endl;
    }
}

Reference Linux username in string to open file

2 Answers2

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