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Intro to Unix for Smart People Part I

Intro to Unix for Smart People Part I. Joe Morrison, Lab49. GE 645. (Artist’s rendition). GE 645 Specs. Dual-core .000435 GHz 3 Mb RAM 16 Mb swapping drum 136 Mb HD. Multics achievements. Modular design Memory mapped files Dynamic linking Hot swappable everything

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Intro to Unix for Smart People Part I

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  1. Intro to Unix for Smart People Part I Joe Morrison, Lab49

  2. GE 645 (Artist’s rendition)

  3. GE 645 Specs • Dual-core .000435 GHz • 3 Mb RAM • 16 Mb swapping drum • 136 Mb HD

  4. Multics achievements • Modular design • Memory mapped files • Dynamic linking • Hot swappable everything • Ring oriented security • First OS to have a hierarchical file system • Symlinks • Per-process stacks

  5. Verdict • Only supported 35 users • Performance deemed inadequate • Verdict: Failure • Bell Labs: “We’re out of here.”

  6. Take 2: Unics • Bell Labs pulled out of Multics in 1969 • Started work on a simpler alternative • Unics = Uniplexed Information and Computing System • Also a pun on Eunuchs • Later name simplified to Unix

  7. Unix family tree (simplified) 1970 AT&T Bell Labs UC Berkeley 1 BSD Version 7 1980 Microsoft 4.2 BSD Unix System III Xenix SUN Microsystems Unix System V SUN OS Licenses 4.3 BSD 1990 Linux Novell Solaris SCO Red Hat Caldera 2000 SCO Group Lawsuits

  8. Strange and wonderful: fork() int pid = fork (); if (pid == 0){ /* I am the child process */}else { /* I am the parent process and pid is my new child*/ }

  9. exec () • exec family of system calls • execl, execv, execle, execve, execlp, execvp • Load executable file and replace myself (while keeping same process identity and resources) … /* goodbye, cruel world */ exec (“/bin/foo”); LOAD /bin/foo /* not reached */ /* process continues with code loaded from /bin/foo */ …

  10. Launching a program pid_t pid; if ((pid = fork ()) == 0) { exec (…); /* not reached */ } /* child has been launched – parent can wait if desired */ int status; while (wait (&status) != pid) ;

  11. Orphans and zombies • Zombies (defunct processes) • All processes have entries in a process table • When child terminates and parent calls wait(), entry is removed • If child terminates, but parent does not wait()– zombie (dead but not reaped) • Orphans • If parent dies before child, child becomes an orphan and is adopted by “init” • If child exits and parent dies without calling wait(), child is an orphan zombie • No problem, “init” periodically checks all its adoptees and reaps them when they terminate

  12. Unix system startup /etc/inittab getty fork/exec (respawn) exec login init (pid = 1) exec fork/exec (respawn) sh getty exec login Start other required processes exec sh

  13. Simple structure is very adaptable • Shell is just a program, not a deep component of the OS • Anybody can write one! • Easy to provide computing services to anything that can send/receive chars • For example telnetd • Listen on a port • When an incoming request is received, fork a handler process and exec login

  14. A small but working shell main () { while (1) { printf ("49: "); char line [1024]; if ((fgets (line, sizeof line, stdin) == 0) || (strncmp (line, "quit", 4) == 0)) { exit (0); } char *tokens [256]; tokens [0] = strtok (line, " \n"); int i = 1; while (tokens [i++] = strtok (0, " \n")) { if (i == 255) break; } tokens [i] = 0; for (i=0; i<2; i++) { pid_t pid; int status; if ((pid = fork ()) == 0) { execvp (tokens[0], tokens); perror ("error"); exit (1); } else { while (wait (&status) != pid) ; } } } } 49: ls foo.txt bar.txt baz.txt foo.txt bar.txt baz.txt 49:

  15. A history of shells • Bourne shell (sh) – the classic • Slightly unwieldy for interactive use • ash - lightweight version of sh • csh – better interactivity, crappy for programming • tcsh – fewer bugs, more features • ksh – Korn shell (great but not free) • bash – Bourne again shell (learn this one) • rc – a shell for Plan9 - simple and clean • zsh – another feature-laden shell • es – an enhanced version of rc

  16. Bash shell examples # example 1 foo > bar 2>&1 # example 2 foo 2>&1 > /tmp/out | wc # example 3 mkfifo /tmp/fifo foo > /tmp/fifo 2>&1 & ... cat /tmp/fifo # example 4 for i in `grep -l foo *` do cp $i $i.saved done # example 5 find /home/jdm -type f -iname '*foo*' | xargs grep bar # or even better (handles filenames with spaces) find /home/jdm -type f -iname '*foo*‘ -print0 | xargs -0 grep bar

  17. Closing thoughts • In general Unix desktops are primitive compared to other popular operating systems • (NeXT and Mac being notable exceptions) • But it’s not just about features, ease of use, and stability • (This better be good) • It’s also about internal interfaces • Make OS reusable in more contexts • Create a breeding ground for improvements • For example • Accessibility (rewrite the window manager) • Embedded systems (replace /etc/inittab)

  18. Unix versus Windows • Windows: Best platform for creating end-user applications • Unix: Best set of building blocks for general purpose, secure, multi-user, multi-process computing

  19. Final closing thoughts (really) • Windows • Intelligent design • Unix • Mutation and natural selection

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