Transcript Intro to Unix for Smart People

Intro to Unix for Smart
People Part I
Joe Morrison, Lab49
GE 645
(Artist’s rendition)
GE 645 Specs
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Dual-core .000435 GHz
3 Mb RAM
16 Mb swapping drum
136 Mb HD
Multics achievements
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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
Verdict
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Only supported 35 users
Performance deemed inadequate
Verdict: Failure
Bell Labs: “We’re out of here.”
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
Unix family tree (simplified)
1970
AT&T Bell Labs
UC Berkeley
1 BSD
Version 7
1980
Microsoft
Xenix
Unix System III
4.2 BSD
SUN Microsystems
Unix System V
SUN OS
Licenses
1990
4.3 BSD
Linux
Novell
SCO
Caldera
2000
SCO Group
Lawsuits
Red Hat
Solaris
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*/
}
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
/* not reached */
/bin/foo
/* process continues with code loaded from /bin/foo */
…
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)
;
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
Unix system startup
/etc/inittab
fork/exec
(respawn)
init (pid = 1)
getty
exec
login
exec
sh
fork/exec
(respawn)
getty
exec
login
Start other required processes
exec
sh
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
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:
A history of shells
 Bourne shell (sh) – the classic
 Slightly unwieldy for interactive use
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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
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
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)
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
Final closing thoughts (really)
 Windows
 Intelligent design
 Unix
 Mutation and natural selection