Store output from Unix command in C code

Question

int main()  {
    char buf[100];
    FILE *fp = popen("df -P /myLoc", "r");
    while (fgets(buf, 100, fp) != NULL) {
        printf( "%s", buf);
    }
    pclose(fp);

    return 0;
}

Output:

Filesystem             512-blocks  Used  Available Capacity Mounted on
/dev0/vol0             123456      3456   5464675     4%    /sys

I got the output of command in buf variable. But I need to get the value of Capacity (4 in this case) into an integer variable. I think cut or awk command can be used but not sure how to make it work exactly.

Any help is appreciated.

May be fstatfs or statfs is better option than running the df command. — Rohan
– Rohan, Commented May 28, 2013 at 7:27
df -P | awk '{print $5}' --this gives the column name along with the value. I just need the value. — user32262
– user32262, Commented May 28, 2013 at 7:50

pmr · Accepted Answer · 2013-05-28 07:41:32Z

6

If you want to use shell tools, you should write a shell-script.

If this really has to be in C you should use the system calls provided by POSIX instead of cobbling things together through popen. In you case that would be statvfs and the member f_favail of the statvfs struct.

#include <stdio.h>
#include <sys/types.h>
#include <sys/statvfs.h>

int main()
{
  struct statvfs buf;
  statvfs("/my/path", &buf);
  printf("Free: %lu", buf.f_favail);

  return 0;
}

edited May 28, 2013 at 7:41

answered May 28, 2013 at 7:27

pmr

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3 Comments

user32262 Over a year ago

Could you be able to please give an example as to how to use statvfs in a C code?

user32262 Over a year ago

I am not getting the capacity value (4 in the question example). I am getting some value in the output though but not sure what it reflects.

pmr Over a year ago

@user32262 If I can bother you to read the documentation: linux.die.net/man/2/statvfs You will need to calculate the percentage yourself from the total inodes and the available ones.

Nobilis · Accepted Answer · 2013-05-28 07:37:04Z

2

Replace your df command with df -P /myLoc | awk '{print $5}' | tail -n 1 | cut -d'%' -f 1.

This will return 4 in your case. Then you can use atoi() to convert it to an int.

But as others have suggested using system calls should at least be more portable.

edited May 28, 2013 at 7:37

answered May 28, 2013 at 7:28

Nobilis

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3 Comments

user32262 Over a year ago

I am getting this error - sed: Function s/\%//gi cannot be parsed.

Nobilis Over a year ago

Try using cut, I'll update my answer to reflect that. Just out of curiosity what OS is that?

Nobilis Over a year ago

The sed example I gave you was tested on Linux, didn't realise that it was not universal. The GNU utils really do spoil :) EDIT: This works on HPUX, just tested it - sed 's/\%//' , adding gi is just a convention which was unnecessary here.

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