Replace character in stream

middleman.c

#include <stdio.h>

int main(int argc, char **argv)
{
    int c;
    FILE *fp = stdin;

    /* Input filename on command line ? */
    if (argc > 1) {
        fp = fopen(argv[1], "r");
        if (fp == NULL) {
            fputs("Unable to open file.", stderr);
            return 1;
        }
    }

    /* Replace character in stream */
    while (!feof(fp)) {
        c = getc(fp);
        putchar(c == '#' ? '+' : c);
    }

    return 0;
}

Usage:

middleman
middleman infile
middleman <infile
middleman >outfile
middleman infile >outfile
middleman <infile >outfile