Download Ch. 14 : UNIX Operating System with Linux

Ch. 14 : UNIX Operating System
with Linux
• History
– UNIX System V
– Linux
• Design Goals
• Memory Management
• Processor Management
• Device Management
• File Management
• Network Management
• Security Management
• User Interface
Understanding
Operating Systems
1
UNIX Runs On All
Sizes of Computers
Advantages:
1. Portable from large to small
systems because written in C.
2. Powerful utilities are brief,
single-operation commands
that can be combined in single
command line.
3. Device independent -- can be
configured to run any device
because it includes device
drivers as part of OS.
Understanding
Operating Systems
Disadvantages
1. No single standardized version
of OS.
– AT&T’s UNIX V standard
(late 1980s).
2. Commands are so brief that
novice users find it unfriendly
2
Year
Internal
release
1971
Version 1
1972
1973
1973
1975
1979
Version 2
Version 3
Version 4
Version 6
Version 7
1980
1981
1983
Release 3.0
Release 4.0
1984
1989
1991
1993?
199x?
Understanding
Operating Systems
External
release
Features
Based on MULTICS; introduced shell concept,
written in assembly language
Added pipes and filters
Kernel and I/O, first version written in C
UNIX V4
UNIX V6
UNIX V7
UNIX System III
UNIX System V
Release 1
UNIX System V
Release 2
UNIX System V
Release 4
Novell’s
Unix Ware
Solaris ?
First version to become commercially available
More powerful shell added: string variables,
structured programming, trap handling
First version used in 16-bit microcomputers
First version available for a mainframe
Added small general-purpose programs
Added features from Berkeley version: shared
memory, more commands, vi editor, termcap
database, flex file names
Open Software Foundation founded
Combined features from BSD, SunOS and Xenix
Novell’s version of System V release 4 designed
to work with NetWare
Sun’s version designed to run to Sun
workstations, derived from AT&T’s UNIX
System V, Release 4
3
UNIX System V
• AT&T entered computer industry by offering line of PCs powered by
UNIX System V.
• 1990 -- +24 versions of UNIX (AT&T’s UNIX System V, A/UX,
Ultrix, Microsoft’s XENIX & University of California at Berkeley’s
UNIX Versions 4.1 bsd, 4.2 bsd, and 4.3 bsd.
• 1991 -- AT&T’s Unix System Labs developed System V release 4
which incorporated almost all features found in System V release 3
plus BSD release 4.3, SunOS, and Xenix.
– Other companies established Open Software Foundation.
• 1993 -- Berkeley released 4.4BSD based on AT&T’s UNIX, requiring
customers to obtain licenses from AT&T to use it.
• Novell acquired UNIX from AT&T & released own version, UNIX
Ware. Designed to interact with Novell’s NetWare system.
Understanding
Operating Systems
4
New Releases of UNIX
• Although newer commands become more difficult to use,
they can be adapted to new situations with relative ease.
• Offer full support for LANs.
• Comply with international OS standards.
• System security has been greatly improved & meets many
of the U.S. government security requirements.
• Most UNIX systems feature Common Desktop
Environment (CDE).
– Uniform GUI designed to give UNIX systems a standard look.
• Many “standards,” each supported by coalition of vendors.
Understanding
Operating Systems
5
Linux
• Developed in Finland, by Linus Torvalds in 1991.
• Linux was based on UNIX, multi-platform OS.
• Brought speed, efficiency & flexibility of UNIX to a PC environment.
• Open-source program -- source code is freely available to anyone for
improvement.
– Under constant development by uncounted people around world.
• Linux is not UNIX, it’s clone.
• Linux’s ncurses (new curses) -- screen handling library that improved
on curses libraries featured in UNIX Release V.
• First used by programmers at universities and research environments,
but becoming widely adopted in commercial & industrial markets.
Understanding
Operating Systems
6
UNIX Design Goals
“OS by programmers for programmers”
1. Develop OS to support software development.
– Utilities for which programmers typically need to write code.
– Each utility was designed for simplicity.
– Designed to be used in combination with each other.
2. Keep algorithms as simple as possible (without becoming
rudimentary).
– UNIX can be mastered by experienced programmers in weeks.
3. Make OS & any application software developed for it,
portable from machine to machine.
– Reduces conversion costs & doesn’t cause application packages to
become obsolete with every change in hardware.
Understanding
Operating Systems
7
Linux Design Goals
Function
Multiple processes &
multiple processors
Multiple platforms
Multiple users
Inter-process
communications
Terminal management
Peripheral devices
Buffer cache
Demand paging memory
management
Dynamic and shared
libraries
Disk partitions
Network protocol
Understanding
Operating Systems
Purpose
Linux can run +1 program at a time using one or several processors.
Operate on Intel’s x86 processors & other platforms including Alpha, Sparc, certain
6800 platforms, certain MIPS machines, and Power PCs.
Allows several users to work on the same machine at same time.
Linux supports pipes, IPC, sockets, etc.
Its terminal management conforms to POSIX standards and it also supports pseudoterminals as well as process control systems.
Supports a wide range of devices including sound cards, graphics interfaces, networks,
SCSI, etc.
Supports a memory area reserved to buffer the input and output from different processes.
Loads pages into memory only when they’re needed.
Dynamic libraries are loaded only when they’re needed & their code is shared if several
applications are using them.
Linux allows file partitions used by file systems such as Ext2 and partitions having other
formats (MS-DOS, ISO 9660, etc.)
It supports TCP/IP and other network protocols.
8
UNIX Memory Management
• Beginning with Version 4, available for multi-users.
• Swapping or demand paging memory management used.
• Best depends on kind of applications that will run.
– Small jobs -- swapping. Many large jobs -- demand paging.
• Swapping requires entire program in RAM before execution.
– Imposes size restriction on programs.
– Uses round robin policy which slows system when traffic is heavy.
• Paging requires more complicated hardware configurations.
– Increases system overhead & under heavy loads,thrashing can occur.
– Advantage of implementing the concept of virtual memory.
Understanding
Operating Systems
9
Typical Memory Layout for Single
User-Memory Part UNIX Image
Stack
Highest Memory
Address
Data
Program
Code
Understanding
Operating Systems
Lowest Memory
Address
• Image -- computer
execution environment
composed of: usermemory part, general
register values, status of
open files, & current
directory.
• Must remain in memory
during execution of a
process
10
Program Code
• Sharable portion of program.
• Because this code will be physically shared by several
processes it must be written in reentrant code.
– Code protected -- instructions not modified during normal execution.
– All data references made without use of absolute physical addresses.
• Memory Manager gives program code special treatment.
– Space allocated to program code can’t be released until all of
processes using it have completed their execution.
– UNIX uses “text table” to track which processes using which code.
– Memory isn't released until the program code is no longer needed.
Understanding
Operating Systems
11
Data Segments & Stack Segments
• Data segment starts after program code & grows toward
higher memory locations as needed by program.
• Stack segment starts at highest memory address & grows
downward as subroutine calls & interrupts add info to it.
• Data & stack are non-sharable sections of memory, so
when original program terminates memory space is
released.
• While each process is in memory, Memory Manager
protects them from each other so they don’t overlap .
Understanding
Operating Systems
12
UNIX Kernel
• Part of OS that implements “system calls” to set up memory
boundaries so several processes coexist in memory.
– Permanently resides in memory
– Processes use calls to interact with File Manager & request I/O.
• Kernel -- set of programs that implements most primitive of
system’s functions.
– Only part of OS to permanently reside in memory.
• Remaining sections of OS handled in same way as any large
program.
– Pages of OS brought into memory on demand.
– Uses least-recently-used (LRU) page replacement algorithm.
Understanding
Operating Systems
13
Linux Memory Management
• Allocates memory space to each process.
– Address space divided : process code, process data,
code & shared library data used by process, & stack
used by process.
• When process begins execution its segments have fixed
size but sometimes process handles variables with
unknown number & size.
– System calls change size of process data segment
(expanding it to accommodate extra data values or
reducing it when certain values positioned at end of
data segment no longer needed).
Understanding
Operating Systems
14
Linux Memory Management - 2
• Memory protection based on type of info stored in each
region belonging to process address space.
– If process modifies access authorization assigned to a
memory region, kernel changes protection info assigned
to corresponding memory pages.
• When process requests pages, Linux loads them into
memory.
• When kernel needs the memory space, pages are released
using a least recently used (LRU) algorithm.
Understanding
Operating Systems
15
Linux Memory Management - 3
• Linux maintains dynamically managed area in memory
(page cache) where new pages inserted or old pages
deleted.
– If necessary, Linux deactivates swap devices without
rebooting.
• To track free/busy pages, uses system of page tables.
• With certain chip architectures, memory access uses
segments.
• Uses VM mechanism provided by processor on which it is
executing, converting virtual addresses into physical
addresses.
Understanding
Operating Systems
16
UNIX Processor Management
•
•
•
•
Allocates CPU, schedules process, & handles process requests.
– Kernel maintains several tables to coordinate execution of
processes & allocation of devices.
Using predefined policy, Process Scheduler selects process from
READY queue & begins its execution for given time slice.
Process scheduling algorithm picks process with highest priority to be
run first; any processes using a lot of CPU time get lower priority.
– System updates compute-to-total-time ratio for each job every
second.
– If processes have same computed priority, handled round-robin.
Effect of negative feedback -- system balances I/O-bound jobs with
CPU-bound jobs.
Understanding
Operating Systems
17
UNIX Processor Management Policies
• To load process from READY queue, Processor Manager chooses
process with longest time spent on secondary storage.
• To select process to temporarily move out, Processor Manager chooses
process that’s either waiting for disk I/O or currently idle.
• If process is waiting for completion of I/O request & isn’t ready to run,
UNIX dynamically recalculates all process priorities to determine
which inactive but ready process will begin when processor available.
• Policies seem to work well & don’t impact on running processes.
• If disk is used for secondary file storage & as “swapping area,” then
heavy traffic significantly slow disk I/O because job swapping may
take precedence over file storage.
Understanding
Operating Systems
18
UNIX Uses Process & User Table to
Keep System Running Smoothly
• Process table always resides in memory.
– Set up when process created; deleted when process terminates.
• Each entry contains: process identification #, user
identification #, process memory/secondary storage
address, process size, & scheduling info.
• For processes with sharable code, process table maintains
text table.
– Contains: memory/ secondary storage address of text segment
(sharable code) & count to track # of processes using this code.
• When count =0, code not needed.
– Table entry & any memory locations allocated to code segment
released.
Understanding
Operating Systems
19
User Table
• User table resides in memory only while process is active.
– Allocated to each active process.
– Kept in transient area of memory.
• Contains info needed when process is running: user &
group identification # to determine file access privileges;
pointers to system’s File Table for every file being used by
process; pointer to current directory; & list of responses for
various interrupts.
• User table, process data segment and code segment
(sharable code, can be swapped into/out of main memory
as needed.
Understanding
Operating Systems
20
Synchronization
• UNIX is true multitasking OS.
– Achieves process synchronization by requiring that
processes wait for certain events.
– Each event is represented by integers that are equal to
address of table associated with event.
• Race occurs if event happens during process’s transition
between deciding to wait for event & entering WAIT state.
– Isn’t a problem in single-processor environments.
– May pose a problem in multiprocessor environments.
Understanding
Operating Systems
21
fork and wait Commands
• fork -- gives user capability of executing 1 program from
another.
– Gives 2nd program all attributes of 1st program.
– Splits program into 2 copies, both running from
statement after fork.
– Process id (pid) generated so each process has unique ID
number.
• wait -- allows programmer to synchronize process
execution by suspending parent until child is finished.
Understanding
Operating Systems
22
exec Commands (execl, execv, execls,
execlp, execvp)
• Used to start execution of new program from another
program.
• Successful exec call overlays 2nd program over 1st.
– Leaves only 2nd program in memory.
– 2nd program considered new process but has pid of 1st program.
• No return from successful exec call.
– Concept of parent-child doesn’t hold.
• Programmer can use fork, wait, & exec commands in this
order to create parent-child relationship & then have child
be overlaid by another program that, when finished,
awakens parent so that it can continue its execution
Understanding
Operating Systems
23
Linux Processor Management
•
•
•
Linux scheduler scans list of processes in READY state &
uses predefined criteria to choose which process to
execute.
3 different scheduling policies: 1 for "normal" processes,
and 2 for "real time" processes.
Each process has associated process type, fixed priority &
variable priority.
– Used by scheduler to determine which scheduling
policy used on processes in READY queue.
Understanding
Operating Systems
24
Process Types in Linux
SCHED_FIFO
for non-preemtible "real time" processes
SCHED_RR
for preemptible "real time" processes
SCHED_OTHER
for "normal" processes
• SCHED_INFO -- executed immediately; scheduler selects
process with highest priority & executes it.
– Non-preemptible; normally runs to completion.
• SCHED_RR -- executed using round robin with small time
quantum.
• SCHED_OTHER -- executed only when "real time" processes do
not exist in READY queue.
– dynamic priorities set by user-specified level using system calls
"nice" and "setpriority," & by factor computed by system.
Understanding
Operating Systems
25
Linux Supports Pipes & Clones
• Linux supports pipes to allow executing processes to
exchange data.
• Extension permits process clones to be created.
– Clone process is created using primitive clone, by duplicating
parent process.
– Allows both processes to share same segment of code & data.
– Any modification of 1 1 visible to other, unlike classical processes.
• Ability to clone processes allows implementation of
servers where several threads may be executing.
– Operations may simply share data, without making use
of inter-process communication mechanisms.
Understanding
Operating Systems
26
Organization of Table of
Processes in Linux
• Each process is referenced by descriptor.
– Approximately 70 fields describing process attributes &
info needed to manage process.
– Kernel dynamically allocates descriptors when
processes begin execution.
– All process descriptors organized in doubly linked list.
– Descriptors of ready/in execution processes put in
another doubly linked list with fields indicating "next
run" and “previously run."
• Several macro instructions used by scheduler to manage &
update process descriptor lists as needed.
Understanding
Operating Systems
27
Process Synchronization
in Linux
• Provides wait queues & semaphores to allow 2 processes
to synchronize with each other.
• Wait queue -- linked and circular list of process
descriptors.
• Semaphores used to solve problems of mutual exclusion
& problems of producers & consumers.
– Contains 3 fields: semaphore counter, number of waiting processes,
& list of processes waiting for semaphore.
– Semaphore counter may contain only binary values.
– Semaphore counter takes value of number of units which are
accessible concurrently.
Understanding
Operating Systems
28
UNIX Device Management :
Device Drivers
• Innovative feature is treatment of devices—device
independent.
– Treats each I/O device as special type of file.
– Every device assigned name & descriptors that identify devices,
contain info about them, & stored in device directory.
• Device drivers -- subroutines working with OS to
supervise transmission of data between main memory &
peripheral unit.
• Incorporation of driver into kernel done during system
configuration.
– Config.c automatically creates conf.c file for any hardware config.
– Contains parameters that control resources & 2 tables (bdevsw,
cdevsw).
29
Each Physical Device Identified By
1. Major device number -- used as index to array to access
appropriate code for specific device driver.
2. Minor device number -- passed to device driver as
argument & is used to access one of several identical
physical devices.
3. Class
• Each class has Configuration Table with array of entry
points into device drivers.
– Only connection between system code & device drivers.
– Allows systems programmers to create new device drivers quickly
to accommodate differently configured systems
Understanding
Operating Systems
30
UNIX Divides I/O System Into
2 Separate Systems
1. Block I/O system (structured I/O system).
–
–
Used for devices addressed as sequence of 512-byte blocks.
Allows Device Manager to use buffering to reduce I/O traffic.
–
UNIX has from 10 to 70 buffers for I/O, & buffer-related info.
2. Character I/O system (unstructured I/O system).
–
–
–
•
Device drivers that implement character lists.
Subroutine puts a character on list, or queue, and another
subroutine retrieves character from list.
E.g., terminal -- 2 input (raw, canonical) & 1 output queue.
Some devices actually belong to both classes.
Understanding
Operating Systems
31
Linux Device Drivers
• Building on UNIX architecture, each device has special
section in kernel (device driver).
– Includes all instructions necessary to communicate with
device.
• When new device is developed, it can be used with Linux
by writing its own device driver.
• Instructions for talking to devices are loaded as needed
(seldom used devices) or can be kept in memory all the
time when OS system boots.
• Although device files may be kept anywhere on file system,
kept in /dev directory.
Understanding
Operating Systems
32
Linux Device Classifications
• Devices defined as either character mode or block mode.
• Information is kept in device file.
• Since there may be +1 device of same type in system, each
device is identified by 2 device numbers.
– Major number -- device driver to be used.
– Minor number -- device number.
– Assigned to ensure each device has unique code.
Understanding
Operating Systems
33
UNIX File Management :
Types of Files
1. Directories -- files used by system to maintain
hierarchical structure of file system.
•
Users can read info, but only system can modify.
2. Ordinary files -- where users store information.
•
Protection is based on user’s requests (read, write, execute, &
delete functions).
3. Special files -- device drivers that provide interface to I/O
hardware.
•
•
•
Appear as entries in directories.
Part of file system & reside in /dev directory.
Name of each special file indicates type of device with which it’s
associated.
Understanding
Operating Systems
34
UNIX File Structure
• Stores files as sequences of bytes.
• Doesn’t impose any structure on them.
– Text files are strings of characters with lines delimited
by line feed, or new line, character.
– Binary files are sequences of binary digits grouped into
words as appear in memory during execution.
• Structure of files is controlled by programs that use them,
not by system.
Understanding
Operating Systems
35
UNIX File Management System
Organizes Disk Into 512-Byte Blocks
•
1.
2.
3.
Disk divided into 4 basic regions:
Address 0 reserved for booting.
Size of disk & boundaries of other regions.
i-list -- list of file definitions using combination of major
& minor device numbers & i-numbers to uniquely identify
file.
4. Free blocks available for file storage -- kept in linked list
where each block points to next available empty block.
– As files grow, noncontiguous blocks linked to chain.
Understanding
Operating Systems
36
UNIX File Storage
• Whenever possible files are stored in contiguous empty
blocks.
• Allocation is very simple.
• No need to compact files until become large & more
dispersed—so that file retrieval becomes cumbersome.
– Compaction to bring retrieval time back to normal.
Understanding
Operating Systems
37
i-list
• Each entry in i-list called i-node with 13 disk addresses.
– 1st 10 addresses point to 1st 10 blocks of a file.
– If file > 10 blocks, 11th address points to block with
addresses of next 128 blocks of file.
• Each i-node contains info on specific file (e.g., , owner’s
identification, protection bits, physical address, file size,
time of creation, last use/last update, number of links, & of
file is directory, ordinary file, or special file).
Understanding
Operating Systems
38
File Names
• UNIX is case sensitive.
• Most UNIX versions allow file names with unlimited
length.
• Some system programs (e.g., compilers) expect files to
have specific “suffixes”.
• UNIX supports a hierarchical tree file structure.
– Root directory is identified by slash (/).
– Names of other directories are preceded by (/) symbol,
which is used as a delimiter.
Understanding
Operating Systems
39
A Few Rules Apply to All Path Names
1. If path name starts with a slash, path starts at root
directory.
2. A path name can be either one name or list of names
separated by slashes.
3. Using 2 periods (..) in path name will move you upward
in hierarchy (closer to root).
4. Spaces are not allowed within path names.
Understanding
Operating Systems
40
File Directories
• Information for each file: access control, number of links,
name of group and owner, byte size of file, date & time of
last modification, and file name.
Access
control
drwxrwxr-x
drwxrwxr-x
-rwxr-xr-x
-rwxr--r--rwx------
Understanding
Operating Systems
No. of
links
2
2
1
1
1
Group
Owner
journal
journal
journal
journal
journal
comp
comp
comp
comp
comp
No. of
bytes
128
128
11904
12556
10362
Date
Time
Jan 10
Jan 15
Jan 6
Jan 20
Jan 17
19:32
09:59
11:38
18:08
07:32
File
name
chap7
chap8
ms-dos
unix
vax
41
Data Structures for Accessing Files
• Info presented in directory isn’t all kept in same location.
• UNIX divides the file descriptors into parts.
– Hierarchical directories contain only name of file & “i-number.”
• All i-nodes are stored in reserved part of device where directory
resides, usually in Block 1.
• Each i-node has room for 13 pointers (0–12).
– 1st 10 block # stored in i-node list relate to 1st 10 blocks of file.
– For files larger than 138 blocks, 12th entry points to a block that
contains a list of 128 indirect blocks.
– For extremely large files of more than 16,522 blocks, 13th entry
points to a “triple indirect block.”
Understanding
Operating Systems
42
Open/Create/Link/Delete Files
• File opened -- its device, i-number, & read/write pointer
are stored in System File Table & indexed by the i-node.
• File created -- i-node is allocated to it, & directory entry
with file name & its i-node number is created.
• File linked -- directory entry is created with new name &
original i-node number, & link-count field in the i-node is
incremented by one.
• Delete shared file -- link-count field in i-node is
decremented by 1.
– When count =0, directory entry is erased & all disk blocks
allocated to file, along with its i-node block, are deallocated.
Understanding
Operating Systems
43
Linux File Management
• All Linux files organized in directories connected to each other in treelike
structure.
• Linux specifies 5 types of files used by system to determine what file is to
be used for.
File type
Directory
Ordinary file
Symbolic link
Special file
Named pipe
Understanding
Operating Systems
File functions
Contains lists of file names.
Contains data or programs belonging to users.
Contains path name of another file that it is linking
to.
Assigned to device controller located in kernel.
When accessed, physical device associated with it is
activated & put into service.
Used as communication channel among several
processes to exchange data. Creation is same as any
sort of file. Carried out via a C function named
mkfifo.
44
Linux File Names & File Directories
• File names can be up to 256 characters long and contain
alphabetic character, underscores, & numbers.
• Linux actually uses only 1st 32 or 64 characters of name.
• File directories contain same info, in same order as UNIX.
• Linux supports—for each type of member, owner, group,
or world—3 types of file permissions: read, write, &
execute.
Understanding
Operating Systems
45
Linux Data Structures
• Linux support several file types.
– Some inherent to this OS, such as Ext2FS.
– Others from other OS such as UNIX System V.
• Kernel has layer of software to maintain interface between system calls
related to files & file management code.
– Virtual File System (VFS).
– Any process-initiated system call to files is directed to VFS.
– Performs file operations independent of format of file system
involved.
– VFS then redirects request to module managing file.
Understanding
Operating Systems
46
UNIX User Interface
Command
(filename)
ls
ls -l
cd
cp
rm
mv
more
lpr
date
date -u
mkdir
grep
cat
format
diff
pwd
Stands for
Run File
List Directory
Long List
Change Directory
Copy
Remove
Move
Show More
Print
Date
Universal
Date/Time
Make Directory
“Global Regular
Expression/Print”
Concatenate
Format
Different
Path Working
Directory
Action to be performed
Run/Execute file with that name.
Show a listing of file names in directory.
Show a comprehensive directory list.
Change working directory.
Copy a file into another file or directory.
Remove/delete a file or directory.
Move or rename a file or directory.
Type the file’s contents to the screen.
Print out a file.
Show date and time.
Show date and time in universal format.
Make a new directory.
Find a specified string in a file.
Create a file or append to existing file.
Format a volume.
Compare two files.
Show the path name of this directory.
47
Command Files (Script Files)
• Used to automate repetitious tasks.
– Each line of file is valid UNIX instruction and can be executed by
user simply by typing sh and the name of the script file.
– E.g., bash (Bourne Again Shell), csh (C shell) & ksh (Korn shell).
• Script files used to automate repetitive tasks & to simplify complex
procedures.
• If script file included in user’s configuration file, automatically
executed every time user logs on.
– Exact name of user configuration file varies from system to system.
– Common names are .profile and .login.
Understanding
Operating Systems
48
Redirection
• To send output to a file or to another device, use symbol >
between command & output destination.
– ls > myfiles
• To concatenate files (copy their contents into file), use cat
file1 file2 > newfile
• Symbol >> appends new file to existing file.
• Reverse redirection takes input for program from existing
file. mail ann roger < memo
Understanding
Operating Systems
49
Pipes
• Pipes & filters allow you to redirect output or input to selected files or
devices based on commands given to command interpreter.
– UNIX does that by manipulating I/O devices as special files.
• Pipe -- open file connecting 2 programs: info written to it by 1
program may be read immediately by other, with synchronization,
scheduling, & buffering handled automatically by system.
– Programs are executing concurrently, not one after other.
– who | sort
• Pipeline -- several programs simultaneously processing same I/O
stream.
– who | sort | lpr
Understanding
Operating Systems
50
Filters
• UNIX has many programs that read some input,
manipulate it in some way, & generate output – filters.
• wc (word count) -- counts lines, words, & characters in file.
• sort -- contents of file sorted & displayed on screen or
accepts input from keyboard & directs output to screen.
–
–
–
–
sort -f sortednames
sort -n sortednums
sort -nr sortednums
sort +2f sortedempl
Understanding
Operating Systems
51
Other Commands
• man – displays on-line manual supplied with OS.
• grep -- looks for specific patterns of characters.
– grep Pittsburgh maillist
– grep -v Pittsburgh maillist
– grep -c Pittsburgh maillist
• pg or more -- displays output one screen at a time.
• nohup -- starts its execution & then log off system without having to
wait for it to finish.
– nohup cp oldlargefile newlargefile &
• nice – lowers priority of a program if it uses large number of resources
& you’re not in hurry for results.
– nice cp oldlargefile newlargefile &
Understanding
Operating Systems
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Linux User Interface
• Linux can be used in command-based mode.
• Also has built-in GUI support for X-Windows developed
at MIT as part of Athena project.
– Windows Application Binary Interface (Wabi)
• Linux tape archive utility is significant improvement over
UNIX tar utility.
– Features built-in error recovery.
– Accommodates both command line interface and
menu-driven GUI.
Understanding
Operating Systems
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