Unit 2: Introduction to UNIX and Linux

CptS 360 (System Programming)Unit 2: Introduction to UNIX and Linux

Bob Lewis

School of Engineering and Applied SciencesWashington State University

Spring, 2018

Bob Lewis WSU CptS 360 (Spring, 2018)

Unit 2: Introduction to UNIX and Linux

Motivation

� APIs have a history: Learn from it.

� What do OSes really do?� What happens when...

� a system boots?� you log in?� you log out?� a system shuts down?

� What general facilities does the OS provide the programmer?

� Note in passing: What influenced OS design decisions?

Bob Lewis WSU CptS 360 (Spring, 2018)

Unit 2: Introduction to UNIX and Linux

References

� Stevens & Rago, Ch. 1 & 2

� http://www.levenez.com/unix

Bob Lewis WSU CptS 360 (Spring, 2018)

Unit 2: Introduction to UNIX and Linux

Logging In and Out

What happens when you log in on a console?

1. init(1) prompts for your login, passing it to...

2. login(1) prompts for your password� if unsuccessful, login(1) exits and control returns to (1)� if successful, login(1)...

� cd’s to your new directory� starts up your shell

What happens when you log in on a display?... and when you log out?

Bob Lewis WSU CptS 360 (Spring, 2018)

Unit 2: Introduction to UNIX and Linux

Contents of /etc/passwd

� name

� password (encrypted – why?)

� UID

� GID

� comment (usually full name, phone, etc.)

� home directory

� login shell

Bob Lewis WSU CptS 360 (Spring, 2018)

Unit 2: Introduction to UNIX and Linux

UNIX Shells

classic shells:� sh

� [Steven]Bourne shell

� Bell Labs

� csh� [Bill Joy] C

shell� UC Berkeley

contemporary shells:� ksh

� [David] Korn shell� AT&T Bell Labs

� bash� “Bourne-again” shell� net-developed

� zsh� ”z shell”� ”...”� huge

� ash� [Kenneth] Almquist shell� teensy� used for diagnostics & small systems

Bob Lewis WSU CptS 360 (Spring, 2018)

aka "dash"

Unit 2: Introduction to UNIX and Linux

Files and Directories

file A named collection of bytes residing in a directory.Under UNIX, it’s always byte-, not record-oriented.

directory A collection of files and other directories.

working directory The directory used to interpret relative directorynames. (aka “current directory” or “current workingdirectory”)

home directory The working directory to which you log in.

root directory The uppermost directory in the directory tree. It isalways named “/”.

Bob Lewis WSU CptS 360 (Spring, 2018)

Unit 2: Introduction to UNIX and Linux

Filenames and Paths

filename a sequence of characters describing a file or directorywithin a directory.Filenames may have restrictions on name lengths andpermittable characters.Q: What two characters cannot appear in a filename?(With one exception.)

path a sequence of one or more filenames joined by “/”s.

absolute path a path that begins with “/” (which is the name of adirectory).

relative path a path that does not begin with “/”.

Q: What’s “.”? Is it absolute or relative?Q: What’s “..”?

Bob Lewis WSU CptS 360 (Spring, 2018)

Unit 2: Introduction to UNIX and Linux

Example: A Small Directory Tree

/

home bin usr etc

bobl bash ls cat lib passwd

Bob Lewis WSU CptS 360 (Spring, 2018)

Unit 2: Introduction to UNIX and Linux

Standard Input/Output

UNIX supports:

� standard input (n = 0)(aka stdin)

� standard output (n = 1)(aka stdout)

� standard error (n = 2)(aka stderr)

These are all part of the“standard I/O” package, whichwe’ll study in an upcoming unit.

What do these mean on the shellcommand line? (n and m arenon-negative integers.)

>

>>

<

n>

n>>

n<

|

n>m

n>&m

Bob Lewis WSU CptS 360 (Spring, 2018)

Unit 2: Introduction to UNIX and Linux

Programs and Processes

program an executable file

process a running program

Big deal in UNIX: process control

Bob Lewis WSU CptS 360 (Spring, 2018)

Unit 2: Introduction to UNIX and Linux

Oh, Say, Can You C?

You can’t talk about UNIX without mentioning C.� A C compiler is a given for all UNIX OSes.� Original C was by Brian Kernighan and Dennis Ritchie (hence

“K & R”) at Bell Labs from 1969 to 1973.� ANSI (in 1989, hence“C89”) and ISO (in 1990, hence “C90”)

standards are identical. “ANSI C” is common usage.� ANSI C New Features:

� function prototypes (borrowed from C++)� generic pointers (void *)� international character sets (ISO 8859)� very portable (arguably more so than C++)

� ISO/IEC 9899:1999 (hence “C99”) added:� inline functions� new types (long long int, complex)� vararg macros� // comments

and finally ...Bob Lewis WSU CptS 360 (Spring, 2018)

Unit 2: Introduction to UNIX and Linux

C11

� alignment control

� _Generic(): selects expressions based on type

� multi-threading support

� better Unicode support

� gets() goes away (and it’s about time!)

� bounds-checking

� static assertions that know about types at compile time

� anonymous structs and unions

Bob Lewis WSU CptS 360 (Spring, 2018)

Unit 2: Introduction to UNIX and Linux

So What’s C Good For?

� not just UNIX

� OS kernels

� drivers

� debuggers

� embedded systems

� compilers (e.g. Haskell, C++ (originally))

� interpreters (e.g. Perl, Python)

� practically anything else, if the design is good

Bob Lewis WSU CptS 360 (Spring, 2018)

Unit 2: Introduction to UNIX and Linux

Other UNIX Entities

� user/group ID’s� positive integers� uniquely identify a user or group

� signals� notify process that some error has occurred� may have “handlers”

� time values� time-of-day� CPU usage� elapsed time (time interval)

Bob Lewis WSU CptS 360 (Spring, 2018)

Unit 2: Introduction to UNIX and Linux

UNIX vs. UNIX-like OSes

� Major UNIX implementations:� SVr4� 4.4BSD� FreeBSD (for Intel)� NetBSD (for all platforms)� OpenBSD (secure)

� Major UNIX-like implementations:� Linux� MacOS X

Darwin = FreeBSD running on Mach microkernel (Which is?)� Solaris

But how many UNIX and UNIX-like OSes do you think there are?

Bob Lewis WSU CptS 360 (Spring, 2018)

Unit 2: Introduction to UNIX and Linux

UNIX and UNIX-like OSes: A Detailed View

(see chart)

Bob Lewis WSU CptS 360 (Spring, 2018)

Unit 2: Introduction to UNIX and Linux

UNIX System Calls

� two kinds of (system) library:� static� dynamic (most system libraries are this)

� POSIX Standard� IEEE Std. 1001.1-1989 (a.k.a. ISO/IEC 9945)� includes commands and the API (threads, real-time, IPC, etc.)� supported just about everywhere, except Windows,

� Interix environment subsystem (up to and including Windows7)

� deprecated in Windows 8� alternatives: Cygwin (separate library), MinGW (built-in)

� this class mostly concerns the API (POSIX.1[abc])

Some common POSIX programming features...

Bob Lewis WSU CptS 360 (Spring, 2018)

Unit 2: Introduction to UNIX and Linux

POSIX Error Handling

If a system call returns a negative value (usually, but not always,-1), something went wrong. Look in the global errno for details.

� To get errno:

#include <errno.h>

� To get the string associated with errno:

#include <string.h>

char *strerror(int ernnum);

� To print an error string:

#include <errno.h>

void perror(const char *msg);

prints “msg : error string” on stderr.

Remember: errno is set when an error occurs, but never cleared.Bob Lewis WSU CptS 360 (Spring, 2018)

RESUME

Unit 2: Introduction to UNIX and Linux

Limits

� Q: How large should I declare an array that will hold a path?Take a look at /usr/include/limits.h.

� Other useful quantities:

CHAR_MIN

CHAR_MAX

INT_MIN

INT_MAX

UINT_MIN

UINT_MAX

LONG_MIN

LONG_MAX

ULONG_MIN

ULONG_MAX

Bob Lewis WSU CptS 360 (Spring, 2018)

Unit 2: Introduction to UNIX and Linux

cpp Feature Test Macros

� _GNU_SOURCE

to enable non-POSIX GNU features:

#define _GNU_SOURCE

#include <whatever>

� _POSIX_SOURCE

to force POSIX compliance:

#define _POSIX_SOURCE

#include <whatever>

� __STDC__

(mainly) to create macros to avoid function prototyping,which you shouldn’t do. You probably won’t use it, but youmight come across it.

Bob Lewis WSU CptS 360 (Spring, 2018)

Unit 2: Introduction to UNIX and Linux

Primitive System Data Types

There are a number of system-defined typedefs (usually) that thePOSIX API uses.

� Example: pid_t� Where is it defined?� (/usr/include is your friend.)� It used to be 16 bits long.� How many processes did that permit?� Why did it change?

� Others:

caddr_t

clock_t

time_t

size_t

off_t

Use these to declare API arguments and return values.

Bob Lewis WSU CptS 360 (Spring, 2018)