how to copy result of the linux command into a string using c code?

Question

I have executed a command "watch grep \"cpu MHz \" /proc/cpuinfo".After executing this command i got following result. Result of The Command

But when I am trying this command using c code.

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

int main(){
    FILE *fp;
    char path[1035];
    char command[]="watch grep \"cpu MHz \" /proc/cpuinfo";
    fp = popen(command, "r");
    if (fp == NULL) {
            printf("Failed to run command\n" );
        exit(1);
    }

    /* Read the output a line at a time - output it. */
    while (fgets(path, sizeof(path)-1, fp) != NULL) {
        printf("%s",path);
    }
    pclose(fp);
    return 0;
}
    I am getting following result.

Result of The Code tell me where am I going wrong?

Try enclosing the whole command you pass to watch in single quotes. E.g. "watch 'grep \"cpu MHz\" /proc/cpuinfo'"; — Some programmer dude
– Some programmer dude, Commented Dec 8, 2015 at 9:39
Yes it is showing correct result but it is taking almost 1 min for showing the result. — Yunis Khan
– Yunis Khan, Commented Dec 8, 2015 at 9:52
That's because the output of the watch command contains no newline characters, so fgets is not returning until the path buffer is full. By the way, you don't need to use sizeof(path)-1 in your call to fgets. You can just use sizeof(path). — Ian Abbott
– Ian Abbott, Commented Dec 8, 2015 at 10:25

Ian Abbott · Accepted Answer · 2015-12-08 10:30:24Z

1

Try something like this:

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

int main(void){
    FILE *fp;
    char path[1035];
    char command[]="while grep \"cpu MHz\" /proc/cpuinfo; do sleep 2; done";
    fp = popen(command, "r");
    if (fp == NULL) {
            printf("Failed to run command\n" );
        exit(1);
    }

    /* Read the output a line at a time - output it. */
    while (fgets(path, sizeof(path), fp) != NULL) {
        printf("%s",path);
    }
    pclose(fp);
    return 0;
}

answered Dec 8, 2015 at 10:30

Ian Abbott

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2 Comments

Yunis Khan Over a year ago

yes It is working.Can you please explain it a little?How is it giving correct result now without any delay?

Armali Over a year ago

@Yunis Khan - This is showing each line immediately because not the single command watch … is called which buffers all its output to the pipe until the buffer is full, but rather the command grep … is called repeatedly.

Frederic Henri · Accepted Answer · 2015-12-08 12:39:35Z

0

I think this is what you want. Don't forget to use memset.

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

int main()
{
    FILE *fp;
    char path[1035];
    char command[]="watch grep 'cpu MHz'  /proc/cpuinfo";

    fp = popen(command, "r");
    if (fp == NULL) {
        printf("Failed to run command\n" );
        exit(1);
    }

    memset(path,'\0',sizeof(path));
    /* Read the output a line at a time - output it. */
    while (fgets(path, sizeof(path)-1, fp) != NULL) {
        printf("%s",path);
    }
    pclose(fp);
    return 0;
}

edited Dec 8, 2015 at 12:39

Frederic Henri

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answered Dec 8, 2015 at 9:53

princess

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1 Comment

Yunis Khan Over a year ago

It is showing the same result which I have included at Result of The Code link but I need the result which I have included at Result of the command link.

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