Computer Systems

First: Homework Questions

Part 2: Shell Concepts

File I/O

Progress from fgets / to pure syscall I/O.

Fork & Exec

Cover simple example.

  • A running instance of a program is a process.
  • Fork copies a process.
  • Exec loads a binary and replaces the contents of a running process, like some sort of horror movie.

Shell Operators

Demonstrate each of these in a shell:

  • Redirect input: sort < foo.txt
  • Redirect output: sort foo.txt > output.txt
  • Pipe: sort foo.txt | uniq
    • man 2 pipe
  • Background: sleep 10 &
  • And: true && echo one
    • Return value from main
    • Success (rv = 0) is true
    • Return val of last command in $?
  • Or: true || echo one
  • Semicolon: echo one; echo two

Redirect Example

  • redir.c

Pipe Examples

  • pipe0, pipe1
  • sort-pipe

Shell Evaluation Plan

  • Base case: "command arg1 arg2"
    • fork
    • in child: exec(command, arg1, arg2)
    • in parent: wait on child
  • Semicolon: "command1; command2"
    • split token list on semicolon
    • execute command1 (recursively)
    • execute command2 (recursively)
  • And / Or: "command1 OP command2"
    • split token list on operator
    • execute command1 (recursively)
    • wait for exit code
    • if correct exit code: execute command2 (recursively)
  • Background
    • fork
    • in child: execute command (recursively)
    • in parent:
      • don't wait right away
      • if you wait in the future, keep in mind it may be the background job
      • you should wait at some point to avoid zombies.
  • Redirect: "command OP file"
    • fork
    • in child: change the file descriptor table to accomplish the redirect
    • in child: execute command (recursively)
    • in parent: wait on child
  • Pipe: "command1 | command 2"
    • fork
    • in child:
      • pipe syscall
      • fork
      • in child/child: hook pipe to stdout, close other side
      • in child/child: execute command1 (r)
      • in child/parent: hook pipe to stdin, close other side
      • in child/parent: execute command2 (r)
      • in child/parent: wait on child/child
    • in parent: wait on child