Execute C language input file on linux

Question

I don't understand how different between

a. ./target <input
b. ./target <$(cat input)
c. ./target $(<input)

./target is a C program and input is a file or payload

I want to know that how are they different and are there any more techniques or method?

You indulge in [UUoC — Useless use of `cat`](https://stackoverflow.com/questions/11710552/useless-use-of-cat), then, @SamPagenkopf: — Jonathan Leffler, Nov 22 '17 at 08:00

score 2 · Answer 1 · answered Nov 22 '17 at 07:59

Two of the three notations are peculiar to Bash; all three are shell notations. The programs that are run need to process the data in quite different ways.

(./target <input — input redirection): the target program needs to read standard input to get the information.

#include <stdio.h>

int main(void)
{
    int c;
    while ((c = getchar()) != EOF)
        putchar(c);
    return 0;
}

(./target <$(cat input) — process substitution): the target program needs to open the file name specified in a command-line argument to get the information.

#include <stdio.h>

int main(int argc, char **argv)
{
    if (argc != 2)
    {
        fprintf(stderr, "Usage: %s file\n", argv[0]);
        return 1;
    }
    FILE *fp = fopen(argv[1], "r");
    if (fp == 0)
    {
        fprintf(stderr, "%s: failed to open file '%s' for reading\n",
                argv[0], argv[1]);
        return 1;
    }

    int c;
    while ((c = getc(fp)) != EOF)
        putchar(c);

    fclose(fp);
    return 0;
}

(./target $(<input) — command substitution): the target program gets the contents of the file split into words as arguments to the program, one word per argument.

#include <stdio.h>

int main(int argc, char **argv)
{
    int count = 0;
    for (int i = 0; i < argc; i++)
    {
       count += printf(" %s", argv[i]);
       if (count > 70)
           putchar('\n'), count = 0;
    }
    return 0;
}

The processing needed is quite different, therefore.

Execute C language input file on linux

1 Answers1