How to use String in System() Command

Question

i wanted to make a program for wget which asks you from which URL you want do download and then download, but i don't know how to add the string "wget" and the url and put it in the system() command. I know there are several possibilities to add strings, but nothing worked for me. Could you please help me? (The code should look like this:)

char url[32];
char wget[32];
scanf("%s", &url);
strcpy(wget, "wget");
strcat(wget, url);
system(wget);

@SakthiKumar Thats right, bit it doesnt fix the problem. When i use `printf("%s", wget)` to see the output, it shows me `wget ��` — Lorenz, Mar 07 '14 at 12:22

score 1 · Answer 1 · answered Mar 07 '14 at 12:22

1

scanf("%s", &url); strip of & ampersand as it is not required. url itself is base address of the array needed for scanf().

Arrays basically decays to pointers, so there is no need to use the & operator on an array to get a pointer. It can be dangerous if you think you have an array but actually have a pointer.

answered Mar 07 '14 at 12:22

Sunil Bojanapally

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What do you mean? When i remove the `&` i get an error. – Lorenz Mar 07 '14 at 12:26
In `scanf()`, format '%s' expects type `char *`, but argument 2 you passed is of type `char (*)[32]`. – Sunil Bojanapally Mar 07 '14 at 12:31

score 1 · Answer 2 · answered Mar 07 '14 at 12:52

Others have pointed out the missing space faster than I did, but there is actually much more "wrong" with your code, so if you excuse me I'll switch to tutorial mode for a minute or two.

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

// The maximum allowed length of an input line, including the newline.
// Longer lines result in an error message.
const size_t INPUTMAXSIZE = 32;

int main()
{
    // You can change the command at will. Length is dynamic.
    // Do not forget the space at the end.
    char * command = "wget ";

    char parameter[INPUTMAXSIZE];
    char * ptr;

    // Using fgets, which allows us to avoid buffer overflow.
    if ( fgets( parameter, INPUTMAXSIZE, stdin ) == NULL )
    {
        puts( "Error while reading input." );
        exit( EXIT_FAILURE );
    }

    // fgets stores the newline as well
    if ( ( ptr = strchr( parameter, '\n' ) ) != NULL )
    {
        // Replace newline with terminating null
        *ptr = 0;
    }
    else
    {
        // Input longer than INPUTMAXSIZE
        puts( "URL too long." );
        exit( EXIT_FAILURE );
    }

    // Allocating the buffer memory dynamically allows us to avoid
    // a second magic number. Re-using 'ptr'.
    if ( ( ptr = malloc( strlen( command ) + strlen( parameter ) + 1 ) ) == NULL )
    {
        puts( "Memory allocation failed." );
        exit( EXIT_FAILURE );
    }

    sprintf( ptr, "%s%s", command, parameter );

    printf( "system( \"%s\" ) returned %d.\n", ptr, system( ptr ) );

    free( ptr );

    return( EXIT_SUCCESS );
}

Always provide code in complete, compilable form.
Reduce the use of "magic numbers" as much as possible.
Use constants where possible.
Make your code stable in the face of unexpected / malformed input. Failing with error is excuseable, dumping core is not.
Do check the return code of functions you are using that might fail.

I don't say my code above is perfect, but I think there's a lesson or two in there. I hope it helps.

score 0 · Accepted Answer · edited Mar 07 '14 at 12:59

You require a space between wget and url, so rather than use strcat just use sprintf as follows:

  int main ()
    {
        char url[32];
        scanf("%s", url);
        int len=strlen(url)+4+2; //4 for wget and 2 for space and safe
        char buffer[len];
        sprintf(buffer,"wget %s",url);
        system(buffer);
    }

In your case if i enter url www.google.com then final command after strcat become

wgetwww.google.com

which is not valid but it should be wget www.google.com

How to use String in System() Command

3 Answers3