Using a shell commands in c++

Question

Here's a terminal command:

awk '/^Mem/ {print $4}' <(free -m)

Here's my code:

class CSystemInfo{
public:
    std::string free_ram(){
        std::string s;
        FILE *in;
        char buff[512];
        //here is a prepared string (but with an error)
        if(!(in = popen("awk '/^Mem/ {print $4 \"\\t\"}' <(free -m)","r"))){
        return "";  
        }
        while(fgets(buff, sizeof(buff), in)!=NULL){
            cout << s;
            s=s+buff;
        }
        pclose(in);
        return s;
       }
};
    CSystemInfo info;
    std::string ram = info.free_ram();
    cout << ram;

The above code, when run, returns this message:

sh: 1: Syntax error: "(" unexpected

How can I place the '/' symbol, for this command to work correctly?

Looks like default shell - sh - is not supporting <(free -m) syntax. Consider invoking bash. — Lapshin Dmitry
– Lapshin Dmitry, Commented Oct 14, 2015 at 8:14
Your teminal shell could be bash that supports this syntax, while popen runs in sh, more primitive, low-level shell. Try echo $0. I have just tried running your command in sh, it failed with the same message. — Lapshin Dmitry
– Lapshin Dmitry, Commented Oct 14, 2015 at 8:20

Lapshin Dmitry · Accepted Answer · 2015-10-14 08:54:28Z

3

Your problem is not in C++. You are invoking your command with popen, and popen runs your command in sh shell, that does not support <() syntax, while in your terminal you are having bash, zsh or any other shell, that does support <() syntax.

Edit: Better choise! Use pipes!

popen("free -m | awk ...")

Original answer, not working!: Try invoking bash in popen:

bash -c "awk '/^Mem/ {print $4}' <(free -m)"

in code:

popen("bash -c \"awk '/^Mem/ {print $4}' <(free -m)\"")

edited Oct 14, 2015 at 8:54

answered Oct 14, 2015 at 8:23

Lapshin Dmitry

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4 Comments

Максим Максимов Over a year ago

Memory: 3398 2848 550 380 209 959 {print $4} doesn't work

Lapshin Dmitry Over a year ago

Works for me (I just changed Mem to Пам because I'm running russian localization). I added one new option to answer, check it out.

Максим Максимов Over a year ago

I use it too. But the best, what I see is Память: 3398 2785 613 339 212 918

Lapshin Dmitry Over a year ago

OK, I see, my original answer is wrong, use pipe option

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