notes.txt

< file1 cmd1 | cmd2 > file2

< : read to standard output
> : put in file instead of standard output

argc[0] -> ./pipex   (your program name)
argc[1] -> file1     (input file)
argc[2] -> cmd1      (first command)
argc[3] -> cmd2      (second command)
argc[4] -> file2     (output file)

1. read input file1
2. process input using cmd1 
3. pipe output from cmdd1 to cmd2
4. process input using cmd2 
5. put ourput in file2

functions: open, close, read, write,
malloc, free, perror,
strerror, access, dup, dup2,
execve, exit, fork, pipe,
unlink, wait, waitpid

open: open fd 
close: close fd 
read: read from fd 
write: write in fd 
malloc: reserve space 
free: free space 
perror: print own error message before error message
strerror: takes error number and points to string with error message
access: checks in code for permissions and if file exists
dup: duplicate a fd, using lowest numbered unused fd
dup2: duplicate a fd, specify fd
execve: switch to new program, give new program and parameters (piping?)
exit: exit program, close fd's and all processes
fork: every fork call lets the parent have a child. next call everyone is parent and gets a child. 2^n
pipe: creates a pipe
unlink: remove specified fd
wait: await completion (certain) processes
waitpid: wait for the child process to change state

pipefd[0] refers to the read end of the pipe. 
pipefd[1] refers to the write end of the pipe.