1. UNIX System Overview

2. UNIX Architecture Kernel of os
Software that controls hardware resources & provides environment under which programs can run
System calls - Interface to kernel
Libraries of common functions – built on top of system calls
Shell – interface for running other programs

3. UNIX Architecture

4. Logging In Login name Password file – usually in /etc/passwd
7 colon separated fields
Encrypted password
Numeric user id
Numeric group id
Comment field
Home directory
Shell program

5. Shells Command line interpreter – reads user input & executes commands
Bourne shell – written by Steve Bourne
C shell – written by Bill Joy
Korn shell – written by David Korn
Bourne-again shell (bash) – GNU shell

6. File System Hierarchical arrangement of files & directories
Starts at root – symbolic name is /
Directory – file that contains directory entries
Directory entry – filename & file attribute
Size, owner, permissions, date last modified, ...

7. File Names Slash (/) & null characters – cannot appear in a file name
. (current directory) & .. (parent directory) are automatically created

8. Pathname
Sequence of 1 or more filenames separated by /'s & potentially starting with a /
Start with / - absolute pathname
Otherwise – relative pathname

9. Directories Working directory Current directory Home directory
Each user has home directory – subdirectory of home

10. Input & Output File descriptor – non-negative integer used by kernel
Predefined descriptors – automatically opened whenever new program is run
Standard input
Standard output
Standard error

11. Input & Output Unbuffered I/O
Provided by functions open, read, write, lseek, close
Standard I/O
Buffered interface to unbuffered functions
printf
fgets

12. Programs & Processes Program – executable file on disk
Read into memory & executed by kernel
Process – executing instance of a program
Process id – unique numeric identifier
Process control
Fork
Exec
waitpid

13. Threads Threads within a process share address space Have own stack
Need to synchronize access to shared variables
Each thread – own id

14. Error Handling
System functions – usually return a numeric error flag - <error.h>
Each thread needs its own copy of errno
Errno – not reinitialized each time – only use if an error has occurred
Never set to 0
strerror – returns pointer to error message
perror – creates error message on standard error

15. Error Recovery 2 types of errors: fatal & nonfatal
Fatal error – no recovery possible
Nonfatal – delay & try later

16. User Identification
User ID - numeric value – assigned by system administrator when account is created
root – id 0
Group id – assigned by system administrator
Groups – collection of users with common characteristics
Sharing of resources within group

17. Signals Technique to notify a process that a condition has occurred
Choices for dealing with signal
Ignore
Let default action occur
Provide a function which is called

18. Time Values
Calendar time – number of seconds since Epoch (Jan 1, 1970)
time_t data type
Process time – measures CPU resources used
Measured in terms of clock ticks
clock_t data type

19. Time Values Execution time of a process Wall Clock time
Amount of time for process to run
User CPU time
CPU time for user instructions
System CPU time
CPU time for kernel executing instructions for user
time command

20. System Calls & Library Functions
System calls – functions to request services from kernel
C library function with same name as system call