CS计算机代考程序代写 file system assembly interpreter 60-256 System Programming: Introduction to Unix

60-256 System Programming: Introduction to Unix

Content

COMP 2560 System Programming:
Introduction to Unix
Courtesy of Dr. B. Boufama
Modified by Dan Wu

School of Computer Science University of Windsor

Instructor: Dr. Dan Wu

Content

1
Introduction

2
A tour of Unix

3
Summary

Introduction A tour of Unix
Summary

Operating Systems

A computer hardware cannot function without software.
In particular, a computer system must possess an operating system.

Example of services provided by an operating system : provide a framework for executing programs,
share system resources (CPU, memory, disk) among programs and users,
allow communication with devices(monitor, keyboard, network, etc.) and other programs,
open a file, read from a file, get the time of the day, etc.

COMP 2560 System Programming
3

Introduction A tour of Unix
Summary

History of Unix
Unix started at Bell Laboratories

1969: Multics −→ Unix by Ken Thomson,
The first version was a primitive single-user one, written in assembly language.
But it was more efficient and faster than Multics.

1970s: B −→ C by Dennis Ritchie.

1973: Unix rewritten in C by Ritchie and Thomson.
1974: Unix Time Sharing System, the first Unix paper was published by Ritchie and Thomson.

COMP 2560 System Programming
Multiplexed Information and Computing Service
4

Introduction A tour of Unix
Summary

Versions of Unix

Two popular versions of Unix

Unix (Thomson & Ritchie) →

System V : from Bell Laboratories (V.2. . . V.4.1).
BSD Unix – Berkeley Standard Distribution (4.2. . . 4.3): introduction of sockets and networking programming.
→ Sun OS (Solaris)…….

5
Unix History Diagram

https://www.levenez.com/unix/
http://www.unix-diagram.org/diagram/unix_diagram.jpg

Introduction A tour of Unix
Summary

Features & philosophy of Unix
Unix features
Unix is available for all platforms: PCs, minis and mainframes.
Unix is among few operating systems that allow more than one user to share a computer system at a time.
Unix is written in C and its source code is freely distributed among the community of users.
Unix is simple and elegant

The philosophy of Unix
Unix has a simple philosophy :
a program(utility) should do one thing and do it well
a complex problem should be solved by combining multiple existing utilities.
→ Unix achieves this goal using pipes.

COMP 2560 System Programming
6

Introduction A tour of Unix
Summary

Unix Layers

Unix employs several layers:

4
The first layer is the kernel. It runs on the actual machine hardware and manages all interaction with it.
The second layer includes all applications and Unix commands which interact with the kernel rather than the hardware itself.
The third layer is called the shell. It interprets commands and manages the interaction between the user, the applications, and Unix commands.
The fourth layer is the windowing system. It usually interacts with the shell, but can also interact directly with applications.

The user
The user interacts with the entire operating system through either the shell or a combination of the shell and the windowing system.

COMP 2560 System Programming
7

https://opensource.com/article/18/5/differences-between-linux-and-unix

https://makelinux.github.io/kernel/map/

COMP 2560 System Programming
8

Introduction A tour of Unix
Summary

Loggin in

login
The login (/bin/login) command is invoked by the system. It is used at the beginning of each terminal session to identify oneself to the system.
login checks our login-name in /etc/passwd and matches our password with the one in the encrypted file
/etc/shadow.

Introduction A tour of Unix
Summary

Example of an entry in /etc/passwd:
oconnel:x:1003:10:David O’Connell, M.Sc.
Student:/users/oconnel:/bin/tcsh
where
oconnel is the login-name(or user-name),
x used to be the encrypted password in the old version of Unix,
1003 is the user ID,
10 is the group ID,
David O’Connel is a comment,
/users/oconnel is the home directory,
/bin/tcsh is the shell program used by this user.
The file /etc/passwd can be read by all users. Example of an entry in /etc/shadow
oconnel:oaIL4MQMGaJBQ:::::::
The file /etc/shadow can only be read by super users.

Introduction A tour of Unix
Summary

User Identification (UID)

User ID
Each user is assigned a user ID, a unique nonnegative integer called uid.The user ID is used by the kernel to check if the user has the appropriate permissions to perform certain tasks like accessing files, . . . etc.

The superuser (root)
The user root (superuser) has uid=0. This user has special privileges, like accessing any file on the system.

getuid()
A process can obtain its uid using the system call getuid().
//getuid.c
#include int main(void){ printf(‘‘hello, my uid is %d\n’’, getuid());
}

COMP 2560 System Programming
11

Introduction A tour of Unix
Summary

Group Identification (GID)
Group ID
The Group ID (gid) is a positive integer allowing to group different users into different categories with different privileges. A group ID is also used by Unix for permissions verifications. Both uid and gid are assigned by the system administrator at the time of the creation of the user account.
There is a group file, /etc/group, that maps group names into numeric IDs.
Example of entries in the /etc/group file:
sysadmin::14:steve,walid,anas,maunzer,mcdade,oracle www::20:steve,walid,maunzer,anas,ai04,moodle,www cs212::1020:danwu,walid,ejelike,chikker,sood8,uddin2 cs140::1021:steve,daemon

Introduction A tour of Unix
Summary

Shells

A shell is a command-line interpreter that reads and executes user commands.
A shell reads user inputs either from a terminal or, from a file called script file.

Example
Some existing shells:
the Bourne shell, /bin/sh the C shell, /bin/csh
the Korn shell, /bin/ksh

Important
Unix is case sensitive.

Introduction A tour of Unix
Summary

Some useful shell commands
man: find and display reference manual pages
man command_name who: who is on the system
ps: give details of user’s processes
date: write the date and time
lpr: print out a file
lpq: list the jobs queued for printing
passwd: change your password
quota: information on a user’s disk space quota and usage
grep: search file for a specified string or expression
more: page through a text file
echo: writes its arguments, separated by blanks and terminated by a newline, to the standard output

Introduction A tour of Unix
Summary

Pipes
What are pipes?
Pipes are a mechanism that allows the user to specify that the output of one program is to be used as the input of another program.
→ several programs can be connected in this fashion to make
a pipeline of data flowing from the first process through the last one.

Example

ps -e | grep netscape | more
where the three commands mean :
ps -e: report status on every process now running
grep: search a file for a pattern
more: page through a text file

Introduction A tour of Unix
Summary

Files and Directories

Unix File System
The Unix file system is a hierarchical arrangement of files and directories.
The root directory is represented by the slash character (/).
A directory is a file that contains entries for files and directories. When a new directory is created, two filenames are automatically created:
. (called dot) that refers to the current directory
.. (called dot-dot) that refers to the parent directory.

pathnames
A pathname is made of filenames separated by slashes. If a pathname starts with a / then it is an absolute pathname, otherwise, it is a relative one.

Example
/export/home/users/zebra/set.txt (absolute)

images/city/park.jpg (relative)
16

Introduction A tour of Unix
Summary

Some useful related shell commands
cd: change the current directory pwd: return working directory name ls: list contents of directory mkdir: create a directory
rmdir: remove a directory
cat: concatenate and display files
cp: copy files
rm: remove files (and directory entries)
mv: move files
find: search for files in a named directory and all its subdirectories
sort: sort a text file

mailx: for sending and receiving mail messages
17

Introduction A tour of Unix
Summary

A bare bones implementation of the ls command.

//myls.c
#include
#include
#include

int main(int argc, char *argv[]){ DIR *dp;
struct dirent *dirp;

if(argc==1)
dp = opendir(“./”); else
dp = opendir(argv[1]);

while ( (dirp=readdir(dp)) != NULL) printf(“%s\n”, dirp->d_name);

closedir(dp); exit(0);

}
18

Introduction A tour of Unix
Summary

Standard Input, Standard Output and Standard Error

stdin, stdout and stderr
Whenever a new program is run, three file descriptors are opened :
the standard input stdin, by default the keyboard, the standard output stdout, by default the monitor, the standard error stderr, by default the monitor.

Redirection
Shells provide means to redirect standard input and standard output. For example:
ls > outputFile.txt, the outputs are stored in the newly created file outputFile.txt instead of the monitor,.
mailx someone@uwindsor.ca < myFile.txt, the inputs for mailx come from the file myFile.txt instead of the keyboard. 19 Note stdin, stdout, and stderr are names used in stdio C library. Copyright @ 2019, 2020, 2021 all rights reserved Introduction A tour of Unix Summary Standard Input, Standard Output and Standard Error #include

int main(int argc, char *argv[]){ FILE *fd;
char c;

if(argc==1) fd=stdin;
else
if((fd = fopen(argv[1], “r”))==NULL){
fprintf(stderr, “Error opening %s, exiting\n”, argv[1]); exit(0);
}
while( (c=getc(fd)) != EOF) putc(c, stdout);

exit(0);

}
20

Introduction A tour of Unix
Summary

Programs and Processes

A program is an executable file residing on a disk.
A process is an executing (running) program, usually with a limited life-time.
Note that sometimes a process is called task.
A process ID (PID) is a unique nonnegative integer assigned by Unix, used to identify a process.

Example
Example of the ps command output:
PID TTY TIME CMD
10258 pts/6 0:01 gs
7478 pts/6 0:06 emacs-20
5598 pts/6 0:01 csh
10184 pts/6 0:01 netscape

ps reports process status.

Introduction A tour of Unix
Summary

Programs and Processes
getpid() and getppid()
A process can obtain its PID by the getpid() system call. A process can also obtain its parent ID PPID by the getppid() system call.
#include int main(void){

printf(“Hello, my PID is %d\n”, getpid()); printf(“Hello, my PPID is %d\n”, getppid()); exit(0);
}

Shell-Prompt> a.out Hello, my PID is 11723 Hello, my PPID is 5598

Introduction A tour of Unix
Summary

Process Control
Comment on page 19
There are three primary functions (system calls) for process control :
fork: allows an existing process to create a new process which is a copy of the caller
exec: allows an existing process to be replaced with a new one.
wait: allows a process to wait for one of its child processes to finish and also to get the termination status value.

Note
The mechanism of spawning new processes is possible with the use of fork and exec.

Introduction A tour of Unix
Summary

Signals

Signals are used to notify a process of the occurrence of some condition.

Example
For example, the following generate signals :
A division by zero: the signal SIGFPE is sent to the responsible process that has three choices. Ignore the signal, terminate the process or, call a function to handle the situation.
The Control-C key: generates a signal that causes the process receiving it to interrupt.
The function kill: a process can send a signal to another process causing its death. Unix checks our permissions before allowing the signal to be sent.

Introduction A tour of Unix
Summary

Unix Time values

The execution time of a process can be measured with three values:
Clock time: amount of time the process takes to run. This is more like the wall clock time for the process.
User CPU time: the time of the CPU used on the process instruction.
System CPU time: CPU time attributed to the kernel, when it executes instructions on behalf of the process, for instance, a disk reading.
The sum of the user CPU time and system CPU time is often called the CPU time.

Introduction A tour of Unix
Summary

The time command

The time command can be used to measure the clock time, the user time and, system time.

Example
/usr/bin/time -p gcc myLs.c -o myLs real 0.60
user 0.14
sys 0.07
In that case, the CPU time is 0.14 + 0.07 = 0.21 second.

Introduction A tour of Unix
Summary

System calls
A user process can invoke either a system call or a library function. Try “man 2 syscalls”
System calls
Operating systems provide entry points for programs to request services from the kernel.
Entry points are called system calls in Unix. In particular :
Each system call in Unix has an interface function, in the C standard library, with the same name that the user process invokes.
The interface function then invokes the appropriate kernel service, using whatever technique is required on the system.

Introduction A tour of Unix
Summary

Library functions

An interface function for a system call cannot be replaced, however, a library function, such as strcpy, can be rewritten by the user.
For our purpose, a system call will be viewed as a regular C function. A library function might invoke a system call.

Introduction A tour of Unix
Summary

Summary

Operating system: Software to manage computer resources, in particular,
it runs a program for a user
it allows communication with devices and processes
A program is a file containing instructions A process is a program being executed Unix is a multi-user operating system
Most of Unix is written in the C language
The Unix philosophy is simple: a program should do one thing and do it well.
Entry points in Unix are called system calls. They allow the user to get services from the kernel.

COMP 2560 System Programming
30

Related Posts