Processes and Redirection

pid_t fork(void); clones current process. Copies memory, global vars, stack, heap, env vars. Current directory, file descriptor table, execution state… actually everything holy. PID is process ID. Original!

When the parent runs fork(), it can return any number !=-1 and !=0 (the ID of the child process). If you’re the child process and run the same fork(), then it will return 0!

Note, when calling fork(), both your process and the child process start doing the stuff the line right after. It doesn’t start the program again. Holy heck!!!!

execlp(path, arg0, arg1, ..., (char*)NULL)

• Used to switch to running another program via one of the “exec” system calls (there’s a family of them!)
• Erases and replaces code/data on current thingy. Preserves PID, env vars, directory, file descriptor table, etc.
• Used for file redirecting / pipelining
  • I would have to fork the program, and then execlp the program to replace it! fork is the only way to launch new processes. All processes except one have parents.
• The kernel boots the first process with PID 1. Known as init. It launches system processes
  • Logic screens for instance.

List processes via

• ps
• pgrep
• top
• htop Commands to end process
• kill
• pkill (find like pgrep, then kill)

pid_t wait(int *wstatus);

• wstatus is for the child’s exit code + why it died (killed)
• “To wait for a child, or don’t ‘hang waiting‘“
• pid_t waitpid(pid_t pid, int *wstatus, int options);
  • pid > 0 = wait for child
  • pid = -1 wait for any child.
  • options = WNOHANG (don’t “hang waiting”. Use 0 otherwise) There are “useful macros” but lowkey justtodo this is bouncing off of me left off at slide 6
• We went over these in lab 11. They are pretty cool but I don’t think are relevant for us

how 2 file redirect

First, remember what that even is lol. File redirection sends the fd of one file to another one. For instance, we can redirect ‘2’ (stdout) into a file “err.txt” such that everything written into stdout instead is shoved there!

1. Open the file you wanna redirect to
2. Fork it!
   1. Just like when calling echo hi! you fork! The shell is the program that is forked, remember that! That means if we want to file redirect, the parent will have to create a fork to replace w/ a program.
   2. The parent should close the fd for err.txt considering it won’t need it (unlike the children, who do)
3. In the parent, close the file descriptor
4. In the child, dup2 the file descriptor with the one you wanna override
   1. Also in the child, run the program via exec

Think of dup2(fd_inp, 0); like the first one “replacing” the second one.

Pipes

int pipe(int pipefd[2]);

• pipefd[0] is the FD for reading; pipefd[1] is the FD for writing

Hygiene

• You should close FDs as soon as you aren’t using them.

FD = File descriptor in most cases!

Thinking time

uniq -c <(sort -r violinists.txt)

Main process:

• Calls child process to run uniq -c /dev/fd/5
• Main process