unix_exec_piper: 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. unix_exec_piper 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 features

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

example

See src/bin/example.rs.

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).