unixexecpiper: A library written in Rust that execs multiple commands and connects them with pipes.

TL;DR;

My library basically does the functionality that happens when you type cat file.txt | grep -i | wc -l into a shell like bash. unixexecpiper does no parsing, but only the actual execution and connection between the (child) processes via unix pipes.

Important main parts of the library are pipe.rs and lib.rs :: execute_piped_cmd_chain().

The main purpose of this library is educational and to show people who are interested in this how it's done.

You might build a shell around this library (if it gets more functionality in the future).

Basics you need to know

Please make yourself familiar with the UNIX/Posix concepts: - pipe() - fork() - file descriptors and "Everything is a file" - exec()

Supported features

• Creating pipes between processes where STDOUT of one process gets connected to STDIN of the next process. \ ($ cat file.txt | grep -i | wc -l)
• I/O redirection into files \ $ cat < file.txt | grep -i | wc -l > out.txt

(not yet) supported

• I/O redirection with STDERR
• a lot of other redirection primitives listed here: https://tldp.org/LDP/abs/html/io-redirection.html

Basic idea

The parent process loops n times (for n commands) and creates n-1 Pipes. Therefore n child processes are created through fork(). Each child process has two variables in its address space:

let mut pipe_to_current: Option<Pipe>;
let mut pipe_to_next: Option<Pipe>;

Pipes communicates across process boundaries in the following way:

child process 0    child process 1    child process n
_______________    _______________    _________
| cat foo.txt |    | grep -i abc |    | wc -l |
---------------    ---------------    ---------
            ^        ^         ^        ^
      WRITE |--------|  R / W  |--------| READ
      END               E   E             END
                   (current child)
        -Pipe to Current-   -Pipe to Next-

Each process uses pipe_to_current (if present) as "read end" (as it's stdin) and pipe_to_current (if present) as "write end" (duplicate it's STDOUT file descriptor into the write end of the pipe).