Execute bash commands in C

Question

I’m using the function system("comando"); to execute bash commands in a C program.

However, I am having difficulty printing the value of a variable.

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

int main(void) {

    char* letra= "A";
    system("echo letra");
}

What I can do to work with the contents of a C variable for bash and vice versa??

Answer 1

You want to print formatted, but instead of sending directly to the console will capture this, it would be something like this:

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

int main(void) {
    char* letra = "A";
    char *buffer = malloc(strlen(letra) + 6); //6 é o tamanho de "echo ", incluindo o terminador, dá para optimizar
    if (sprintf(buffer, "echo %s", letra) > 0) system(buffer);
    else printf("algo deu errado");
}

Behold working in the ideone. And in the Coding Ground. Also put on the Github for future reference. It’s a little different because I can’t access the system() in those places.

Documentation of sprintf().

Note that although the variable is written letra is allowing a word, phrase, or a whole text.

Answer 2

You can write a Variadic Function capable of receiving a string de formatação as argument, enabling the manipulation of all primitive types in a standard way, see only:

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

#define CMD_TAM_MAX   (1024)

int systemf( const char * fmt, ... )
{
    va_list args;
    char cmd[ CMD_TAM_MAX + 1 ];

    va_start( args, fmt );
    vsnprintf( cmd, CMD_TAM_MAX + 1, fmt, args );
    va_end(args);

    return system(cmd);
}

int main( void )
{
    int i = 123;
    char ch = 'X';
    char * txt = "Ola Mundo!";
    float pi = 3.1415f;
    double dbl = 1234567890L;

    systemf( "echo int: %d", i );
    systemf( "echo char: %c", ch );
    systemf( "echo char*: %s", txt );
    systemf( "echo float: %f", pi );
    systemf( "echo double: %f", dbl );

    return 0;
}

Exit:

int: 123
char: X
char*: Ola Mundo!
float: 3.141500
double: 1234567890.000000

See working on Ideone.com

To access the variáveis de ambiente of your shell, you can use a main() with a slightly different signature than usual:

int main( int argc, char **argv, char **envp );

Where char ** envp is the list of all variáveis de ambiente available in the format VARIAVEL=VALOR, with a terminator NULL indicating the end, see only:

#include <stdlib.h>

int main( int argc, char **argv, char **envp )
{
    char **env = NULL;

    for( env = envp; *env != NULL; env++ )
    {
        char * variavel = *env;
        printf( "%s\n", variavel );
    }

    return 0;
}

Exit:

LANG=en_US.UTF-8
WORKSPACE=/tmp/a4a2fff3-ba98-4fda-9d1d-e31d83ac61ee
PWD=/home/0NxZg3
HOME=/home/0NxZg3
TMPDIR=/tmp/WpSkY2
TMPDIR_GLOBAL=/tmp/a4a2fff3-ba98-4fda-9d1d-e31d83ac61ee
SHLVL=0
PATH=/usr/local/bin:/usr/bin:/bin

See working on Ideone.com

To access a variavel de ambiente specific within your program, you can use the function getenv() of the standard library stdlib.h, look at you:

char * valor = getenv("PATH");
printf( "PATH=%s\n", valor );