How do I provide input to a C program from bash?

Question

I have been reading a ton about bash scripts and program testing but I am still unable to make this code work.

Basically it is a simple program that asks the user for either of north east south or west. I start the program then it immediately asks for input. I just can not get the bash script to give it any input. I tried using echo and expect.

Any help is appreciated.

Here is the function used to get the players input:

int process_input(Map *game)
{
    printf("\n> ");

    char ch = getchar();
    char a = getchar(); //eat enter

    int damage = rand() % 4;

    switch(ch) {

        case -1:
            printf("you suck\n");
            return 0;
            break;
        case 'n':
            game->proto.move(game, NORTH);
            break;
        case 's':
            game->_(move)(game, SOUTH);
            break;
        case 'e':
            game->_(move)(game, EAST);
            break;
        case 'w':
            game->_(move)(game, WEST);
            break;
        case 'a':
            game->_(attack)(game, damage);
            break;
        case 'l':
                    printf("You can go:\n");
                    if(game->location->north) printf("NORTH\n");
                    if(game->location->south) printf("SOUTH\n");
                    if(game->location->east) printf("EAST\n");
                if(game->location->west) printf("WEST\n");
                    break;
        default:
            printf("Whats next?", ch);
        }
        return 1;
}

And here is the attempt at a bash script:

   #!/bin/bash
    /Desktop/c
    ./ex17 echo 'w'

What is it you are trying to do? Create a C program, or a bash script? — wallyk, May 12 '13 at 15:20
are you trying to send the input to the `c program` through a `bash script`? — abasu, May 12 '13 at 15:21
I really don't understand what you want/ask, and it looks like I'm not alone. — zmo, May 12 '13 at 15:22
Yes I am trying to send input into this program with a bash script. — blankwall, May 12 '13 at 15:29
@blankwall show what you've done in the bash script? it's easy enough to feed it lines in bash ALSO show your main() in C — Ahmed Masud, May 12 '13 at 15:33
aah ... okay you need to learn how to do command line arguments as well as piping. Show your main() as well please :) — Ahmed Masud, May 12 '13 at 15:41

Mark Reed · Accepted Answer · 2020-04-18T19:04:53.473

You can feed input into a program from bash using any of the following mechanisms.

For a single line of input, you can use a here-string:

./ex17 <<<'w'

For multiple lines, you can use a here-document:

./ex17 <<'EOF'
w
second line of input
more input
EOF

Or you can move those lines out of the script and into a separate file:

./ex17 <filename

More generally, you can run a command that generates as its output the desired input to your program, and connect them together with a pipe. For instance, the above could also be written:

cat filename | ./ex17

or the original example as

echo w | ./ex17

That's more general because you can replace cat and echo here with any sort of program, which can do all sorts of computation to determine what it outputs instead of just dumping the contents of a static string or file.

But what you can't easily do from bash is drive input, read output, and make decisions about what to send as the next input. For that, you should look at expect. An expect script would look something like this:

#!/usr/bin/env expect
spawn ./ex17
expect ">"
send "w\n"
expect "Whats next?"
send "next line here\n"
# turn it back over to interactive user
interact

score 6 · Answer 2 · answered May 12 '13 at 15:43

6

Try this: first:

 echo w | ./ex17

This will send w to the example and output the move. This is called piping; and it essentially connects the stdout of echo to the stdin of ex17

answered May 12 '13 at 15:43

Ahmed Masud

21,655
3
33
58

score -1 · Answer 3 · edited Jan 29 '14 at 09:04

-1

You can do either a C code or a Bash script, not a mixed one. Use scanf() for reading from keyboard (it stops reading when you hit ENTER) and complete this code in C language.

edited Jan 29 '14 at 09:04

Sourav Ghosh

133,132
16
183
261

answered May 12 '13 at 15:34

Arrigo Pierotti

203
4
10
18

2

he's trying to feed input from bash into his C program which is perfectly possible – Ahmed Masud May 12 '13 at 15:40

How do I provide input to a C program from bash?

3 Answers3

Linked

Related