Pipex is a small command-line program that emulates the functionality of the shell's pipe feature. It reads input from a file, runs two commands as separate child processes, and then sends the output of the first command as input to the second command. The output is then written to another file. Pipex is implemented using the C programming language and various system calls, such as fork, pipe, dup2, and execve.

1. Subprocesses: Let's work with different processes. One for the first command to be executed, and one to the second.For this, we will use the fork() function that creates a child process.
2. Pipes: To connect the processes, let's use pipes. Pipes are bufffers that allow communication between processes. To create a pipe, we use the pipe() function.
3. Redirection: To redirect the input and output of the processes, we will use the dup2() function.It receives two file descriptors as a parameter, and causes the file descriptor received as a second parameter to be a copy of the file descriptor received as the first parameter. Thus, we can redirect the input and exit of the processes to the pipes.
4. Execution of commands: To execute the commands, we will use the execve() function. It receives as parameters the path of the executable file, a string array with the command arguments. With this, in the command being called, the process that called it is replaced by the command process.

1. The program receives as parameters the name of the input file, the first command, the second command, and the name of the output file.
2. The program creates a pipe (read-end fd[3] | write-end fd[4]).
3. The program creates a child process.
   • It open the input file fd[5].
   • It redirects the stdout fd[1] to the pipe fd[4].
   • Then redirects the input file fd[5] to the stdin fd[0].
   • Close the read-end of the pipe fd[3].
   • Execute the first command.
   • The output of the first command is written to the pipe because the previous redirection.
   • The child process exits.
4. The program wait for the child process to finish.
5. The program call the parent process.
   • It open the output file fd[5].
   • It redirects the stdin fd[0] to the pipe fd[3], that is, the output buffer of the first command.
   • Then redirects the output file fd[5] to the stdout fd[1].
   • Close the write-end of the pipe fd[4].
   • Execute the second command.
   • The output of the second command is written to the output because the previous redirection.
   • The parent process exits.
6. If you don't have any errors, the program ends successfully.

To compile this program, run the following commands:

make

Then, to run the program, run the following command:

./pipex file1 cmd1 cmd2 file2

For test memory leaks, run:

valgrind --leak-check=full --trace-children=yes ./pipex file1 cmd1 cmd2 file2