Cyclic pipe (you can do "your_pg | program | your_pg" )

pipecyclic.c

#include<stdio.h>
#include<fcntl.h>
#include<unistd.h>
#include<stdlib.h>
#include<string.h>
void s2p(const char *path) {
    char buf[2];
    int fd = open(path, O_WRONLY);
    while(fread(buf, 1, 1, stdin)) {
        int cnt=1;
        if (buf[0] == '\\') {
            buf[1] = '\\';
            cnt++;
        }
        write(fd, buf, cnt);
    }
    write(fd, "\\z", 2);
    close(fd);
}

void p2s(const char *path) {
    char buf[2];
    int i = 0;
    int fd = open(path, O_RDONLY);
    while(read(fd, buf+i, 1)) {
        switch(i) {
            case 0:
                if (buf[i] == '\\') {
                    i++;
                    continue;
                }
                break;
            case 1:
                if (buf[i] == 'z') {
                    goto close_fp;
                } else if(buf[i] == '\\') {
                    i = 0;
                } else {
                    fprintf(stderr, "%.2s is no escape char", buf);
                    goto close_fp;
                }
                break;
        }
        fwrite(buf, i+1, 1, stdout);
        fflush(stdout);
        i = 0;
    }
close_fp:
    close(fd);
}

int main(int argc, const char*argv[]) {
    const char *cmdname = argv[0];
    if (argc == 1) {
        int pipefd[2];
        char cmd[2000];
        if(pipe(pipefd)){
            perror("ng");
            return -1;
        }
        int pid = getpid();
        sprintf(cmd, "PIPEIN=\"%s s2p /proc/%d/fd/%d\" "
                    "PIPEOUT=\"%s p2s /proc/%d/fd/%d\" "
                    "bash", cmdname, pid, pipefd[0]
                        , cmdname, pid, pipefd[1]);
        fflush(stdout);
        system(cmd);
        close(pipefd[0]);
        close(pipefd[1]);
    } else if (argc == 3) {
        const char *subcmd = argv[1];
        const char *fname = argv[2];
        if (strcmp(subcmd, "s2p")) {
            s2p(fname);
        } else if  (strcmp(subcmd, "p2s")) {
            p2s(fname);
        }

    } else {
        fprintf(stderr, "%s", "パラメータが不正です");
    }
    return 0;
}

インタプリタをサイクリックなパイプで利用することができる。
$PIPEIN | bash | tee /dev/stderr | python ./test.py | tee /dev/stderr | $PIPEOUT