Other aspects of an OS

• The shell
  • This is a special purpose program designed to interact with the user.
  • It is sometimes called a command interpreter
• The log in process.
  • Some long running process, called a daemon listens for you to log in.
  • You provide your credentials to this process and it eventually starts your shell.
  • This shell is called a login shell
• If we wish, we can trace back to what happened.
  • Each process has a unique process id
  • This is a positive integer.
  • PID 1 is reserved for a system start up process.
  • ps -ef will display both the pid and ppid
  • pstree will display a tree.
• Linux has a number of shells available
  • The bourne shell (sh) is very old.
  • The c-shell (csh) was constructed to mimic aspects of the C programming language.
  • The korn shell (ksh) combined sh and csh
  • The Bourne again shell (bash) is gnu's implementation of sh.
• Shells also frequently offer command interpreters.
  • The shell usually supports variables, control structures and functions of a full programming language.
  • But they tend to be picky, strange and just a bit hard to use.
  • But they are really great for quick os level tasks.
• Users and groups
  • On a linux system a user is just a number, the UID
    • This is established in /etc/passwd
    • An associated lin in /etc/shadow
    • And, hopefully, at least one directory owned by the user.
  • A group is a another construct
    • A group number
    • A list of members.
    • /etc/group
    • Somewhat weak.
  • The superuser
    • Identified by UID 0
    • Typically called root.
    • Can execute privileged commands.
    • Example: int setuid(uid_t uid);
    • Can send any signal to any process
    • Can write to any file.
    • Discuss using the root account.
    • Discuss sudo
• The file system
  • Is a tree like structure.
  • The "root" of the tree is at the top. (/)
  • relative paths.
  • Absolute paths.
  • Tilde paths.
  • make a symbolic link.
  • make a hard link.
• The file I/O model
  • The principle of universality of I/O means that the same system calls work on all file types.
    • read, write, open, close, seek, ...
• Processes
  • Created via fork
  • Have an effective user id, who the process is running as.
  • Have a parent.
  • Process exit, parents and zombies
  • Init the ultimate zombie hunter.
• Signals
  • Messages sent to processes by the kernel
  • Or possibly other processes
  • Inform the process that a major event has occurred.