Executing bash from a c program: No math operations.

andrew.comly · 02-28-2015, 12:58 AM

I am making a program that multiplies a user input integer. This will convert minutes into seconds(needed for the bash command rtcwake -m mem -s $x, rtcwake is convenient for taking naps). I am taking a C programming course, but I am always tempted to use my newfound C programming skills on my home computer, which uses bash(Lubuntu 14.04). I have been following the following tutorial:

http://www.unix.com/programming/2161...c-program.html

My code is as follows:

Code:

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

#define SHELLSCRIPT "\
#!/bin/bash \n\
echo \"How many minutes would you like to nap for?\"\n\
read y\n\
let x=60*$y\n\
echo \"That's $x seconds\"\n\
"

int main()
{
    puts("Initiating...");
    system(SHELLSCRIPT);
    return 0;
}

Unfortunately I get the following error message:

Code:

Initiating...a@a-NC210-NC110:~/bin$ gcc nap2.c
a@a-NC210-NC110:~/bin$ ./a.out
How many minutes would you like to nap for?
5
sh: 4: let: not found
That's  seconds
Initiating...a@a-NC210-NC110:~/bin$

I have altered the above code with

Code:

#/bin/bash \n\

(w/o the "!") like the tutorial suggests, but still the same response.

Actually it really is cumbersome setting the variables from within the SHELLSCRIPT. Maybe is there a way to pass the variable from the c program into the SHELLSCRIPT?

Why doesn't 'let' work from within SHELLSCRIPT? Does C have a function equivalent to rtcwake(a command that wakes the computer)?

T3RM1NVT0R · 02-28-2015, 04:44 AM

The code is perfectly fine, if I put minutes it returns the number of seconds. It appears that you are missing some of the libraries. Here is what I have in gcc:

Code:

gcc-locale-4.3-62.198
libgcc43-4.3.3_20081022-11.18
gcc-info-4.3-62.198
gcc43-locale-4.3.3_20081022-11.18
gcc-c++-4.3-62.198
gcc43-c++-4.3.3_20081022-11.18
gcc43-info-4.3.3_20081022-11.18
gcc-4.3-62.198
gcc43-4.3.3_20081022-11.18

NevemTeve · 02-28-2015, 05:56 AM

Function 'system' call /bin/sh. The comment in the first line is only a comment which is ignored by /bin/sh.

Actually, your C program has no function at all; you should forget it, and simply create a bash-script.

derive · 02-28-2015, 06:40 AM

#define SHELLSCRIPT "\
echo \"How many minutes would you like to nap for?\"\n \
read y\n \
bash -c \"echo \\\"That's $((60*$y)) seconds\\\"\" \
"
You should use bash for the let.. but let won't display the number, so I just replaced it with the $(( ))

But generally, you should:
1. use only a bash script, and leave C. (just write the original SHELLSCRIPT part to a file, chmod 755 filename, and run it with ./filename )
2. call an external shellscript directly ( you can use parameters, it will use the interpreter you define in the first line, etc )
3. write the whole thing in C.

andrew.comly · 03-03-2015, 06:51 AM

Quote:

Originally Posted by T3RM1NVT0R

It appears that you are missing some of the libraries.

aCtually, you're right I checked the 9 mentioned libraries you mentioned and I don't have any.

How do you reccommend to install a library?
Any good tutorials on installing missing libraries?

NevemTeve · 03-03-2015, 07:23 AM

Exactly what 'library' would change the fact that program /bin/sh doesn't support instruction let?

andrew.comly · 03-03-2015, 08:35 PM

Quote:

#define SHELLSCRIPT "\
echo \"How many minutes would you like to nap for?\"\n \
read y\n \
bash -c \"echo \\\"That's $((60*$y)) seconds\\\"\" \
"
You should use bash for the let.. but let won't display the number, so I just replaced it with the $(( ))

I tried this:

Code:

#include<stdio.h>
#include<stdlib.h>
#define SHELLSCRIPT "\
echo \"How many minutes would you like to nap for?\"\n\
read y\n \
bash -c \"echo \\\"That's $((60*$y)) seconds\\\" \" \
"

int main()
{
    system(SHELLSCRIPT);
    return 0;
}

but I still got the following error:

Code:

a@a-NC210-NC110:~/bin$ gcc nap.c
nap.c:5:21: warning: missing terminating " character [enabled by default]
 #define SHELLSCRIPT "\
                                            ^
nap.c: In function ‘main’:
nap.c:21:5: error: missing terminating " character
     system(SHELLSCRIPT);
     ^
nap.c:21:5: error: too few arguments to function ‘system’
In file included from nap.c:3:0:
/usr/include/stdlib.h:717:12: note: declared here
 extern int system (const char *__command) __wur;
                     ^

Quote:

But generally, you should:
1. use only a bash script, and leave C. (just write the original SHELLSCRIPT part to a file, chmod 755 filename, and run it with ./filename )
2. call an external shellscript directly ( you can use parameters, it will use the interpreter you define in the first line, etc )
3. write the whole thing in C.

I guess you are right. It always seems so annoying being able to do so many things in C but linux only can use 'bash' for scripting. My idea of mixing scripts wasn't so good.

derive · 03-03-2015, 08:51 PM

It's nasty....

Code:

sietch:~# cat > proba.c
#include<stdio.h>
#include<stdlib.h>
#define SHELLSCRIPT "\
echo \"How many minutes would you like to nap for?\"\n\
read y\n \
bash -c \"echo \\\"That's $((60*$y)) seconds\\\" \" \
"

int main()
{
    system(SHELLSCRIPT);
    return 0;
}
sietch:~# gcc proba.c
sietch:~# ./a.out 
How many minutes would you like to nap for?
2
That's 120 seconds
sietch:~#

It's working for me....

gcc version 4.4.7 (Debian 4.4.7-2)

Maybe you missed an " somewhere?

but it's simplier without C.

Code:

sietch:~# cat > nap
#!/bin/bash
echo "How many minutes would you like to nap for?"
read y
echo "That's $((60*$y)) seconds"
sietch:~# chmod 755 nap
sietch:~# ./nap
How many minutes would you like to nap for?
3
That's 180 seconds

evo2 · 03-03-2015, 08:52 PM

Hi,

Quote:

Originally Posted by andrew.comly

It always seems so annoying being able to do so many things in C but linux only can use 'bash' for scripting.

I'm puzzled by that statement. You are free to use many different scripting languages or compilers. You can code in C, C++, go, pascal, fortran, cobal, .... You can write and run bash, ksh, tcsh, csh, zsh .... shell scripts. You can write and run python, perl, ruby,... scripts.

Evo2.

derive · 03-03-2015, 09:02 PM

Or in C:

Code:

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

int main()
{
  int y, x;

printf ("How many minutes would you like to nap for?\n");

scanf("%d", &y);

x = y * 60 ;

printf ("That's %d seconds\n", x);

    return 0;
}

pan64 · 03-04-2015, 12:26 AM

Quote:

Originally Posted by andrew.comly

It always seems so annoying being able to do so many things in C but linux only can use 'bash' for scripting. My idea of mixing scripts wasn't so good.

No, you did not mix them, you simply tried to embed a bash script into C code. That script can work without C as it was already demonstrated.

But back to your C code. Probably you missed the \ chars are the very last ones at the end of the lines, there should be no other characters. Based on the error message you posted I think you need to remove all the spaces/tabs after \.

T3RM1NVT0R · 03-04-2015, 07:21 AM

@ andrew.comly,

Others are right that you shouldn't be mixing C with bash. As far as library install is concerned you can use either apt-get install <package name> or you can use synaptic. As Lubuntu is debian derivative those should work.

@ All,

I doubt that he is missing any character or quotes in the code as I copy pasted it in vi, saved the file and simply run gcc against it. Though I have tried that on my SuSE box not on Lubuntu.

If it is something specific to distro then I am not aware of.

NevemTeve · 03-04-2015, 07:32 AM

Which part is problematic on this error message:

Code:

sh: 4: let: not found

rtmistler · 03-04-2015, 07:50 AM

I have no problem invoking a bash script from within a program and have in fact done this as part of architectures where I created a bash script to do something and then needed to invoke that from within code logic.

My problem is the typical "don't use system()" issue. Here's a summary and example of what I recommend. Use fork() and one of the forms of exec().

Code:

    pid_t pid;
    char *args[3] = {
        "my-script.sh",
        "general-argument",
        NULL
    };
    char *newenv[] = { NULL };
    int w_status;

    pid = fork();
    if ( pid < 0 ) {
        printf("fork() error\n");
        return;
    }
    // Child, so run the intended program
    else if ( pid == 0 ) {
        if ( execve( "/home/myuser/my-script.sh", args, newenv ) < 0 ) {
            printf("Error invoking execve\n");
            return;
        }
    }

    // Parent, so wait for a signal from the child
    waitpid( pid, &w_status, 0 );

    // Process w_status using macros, and etc

derive · 03-05-2015, 04:38 AM

Quote:

Originally Posted by rtmistler

I have no problem invoking a bash script from within a program and have in fact done this as part of architectures where I created a bash script to do something and then needed to invoke that from within code logic.

Well his problem was, that he didn't call an external program, he defined the shell script in the C code, so the interpreter in the first line is ignored, and the builtin "sh" was used...