Using POPEN to store a value to a string using C programming

Question

I am trying to write a C code under UNIX to read the third word from each line of a text, and store it to a string by using POPEN. However my code is giving me an error (Modifiable lvalue required for assignment operator) at the line inside my while loop. Here is my code:

    int main() {

int license = 0;
char number[100];


FILE *file = popen("grep User results_today.TXT_05012013 > filename", "r");
if ( file != NULL)
{
    char line [128];
    while (fgets(line, sizeof line, file) != NULL)
    {

        number = popen("cut -f3 -d' '", "r");

    }
    fclose (file);

    printf("Hello %s\n", number);
}

I know there are a few errors on here as i am still kinda new to C. But please help me correct them, thanks!

Chris Dodd · Accepted Answer · 2013-05-09 02:35:48Z

2

FILE *file = popen("grep User results_today.TXT_05012013 > filename", "r");

This will run a grep command looking for User and redirect the output to the file filename. It will return a FILE * that allows you to read the output of this command, but as that output has been redirected, you won't get anything.

popen("cut -f3 -d' '", "r");

This will run the cut command which, as it has no file arguments, will read from stdin and write to stdout which can be read by the FILE * that popen returns, but which you aren't doing anything with.

You probably want something more like:

char line[128];
int number;
FILE *file = popen("grep User results_today.TXT_05012013 | cut -f3 -d' '", "r");
if (file) {
    while (fgets(line, sizeof line, file)) {
        if (sscanf(line, "%d", &number) == 1) {
            printf("It's a number: %d\n", number);
        }
    }
    pclose(file);
}

answered May 9, 2013 at 2:35

Chris Dodd

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1 Comment

Dave Wang Over a year ago

Thanks for your help. So it looks like your code will run the grep User and cut command on the text file and output to FILE *file, then use the while loop to output to number? How come when i run your code it doesn't give me anything? Not even the "It's a number" part... am i missing something?

Mirco Ellmann · Accepted Answer · 2013-05-08 23:53:55Z

1

You assign the result of popen to a fixed size char array. This is not possible.

number = popen("cut -f3 -d' '", "r");

Do it like the first popen -> assign it to FILE *file2

answered May 8, 2013 at 23:53

Mirco Ellmann

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4 Comments

Mirco Ellmann Over a year ago

he should figure that out by himself ;)

R.. GitHub STOP HELPING ICE Over a year ago

By the way, getdelim is a handy way to read the entire input.

Dave Wang Over a year ago

@MircoEllmann Thanks, I kinda guessed that would be the problem. So now that i have a file called "filename" which i previously used grep User command to extract the info i wanted, is there a different way in using that while loop from my code to extract let's say the 3rd word from each line of the file "filename"? Thanks.

Mirco Ellmann Over a year ago

You might try to combine the commands with | grep User results_today.TXT_05012013 | cut -f3 -d' ' You could save the second popen that way. give it a shot...

y0tta · Accepted Answer · 2015-09-18 09:39:00Z

First of I'm not a C programmer This is my implementation (with a lot of borrowed lines of course ;) ). I was just fed up of

popen
while fgets
 printf // I want to store in char* got it?

So here's the code. It may not be perfect but does the job :)

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

#include <string.h>

char* concatenate(char * dest, char * source) {
    char * out = (char *)malloc(strlen(source) + strlen(dest) + 1);

    if (out != NULL) {
            strcat(out, dest);
            strcat(out, source);
    }

    return out;
}

char * executeCmd(char * cmd) {
    FILE *fp;

    int BUFF_SIZE = 1024;

    int size_line; 
    char line[BUFF_SIZE];

    char* results = (char*) malloc(BUFF_SIZE * sizeof(char));

    if (cmd != NULL) {
            /* Open the command for reading. */
            fp = popen(cmd, "r");
            if (fp != NULL) {

            /* Read the output a line at a time - output it. */
            while (fgets(line, size_line = sizeof(line), fp) != NULL) {
                    results = concatenate(results, line);
            }
            }
            /* close */
            pclose(fp);
    } // END if cmd ! null

    return results;
}


int main( int argc, char *argv[] ) {
    char * out = executeCmd("ls -l");
    printf("%s\n", out);

    return 0;
}

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