Tm hiu v cc Shell

Tm hiu v cc ShellCam Nht Linh

LOGOMc LcShell l g.1Chc nng ca Shell2Cc Shell thng dng...3Bourne Shell (sh)...4Bourne Again Shell (bash)......7C Shell(csh).........9Korn Shell (ksh)11Z Shell (zsh)...12c im chung14 Cc tnh nng tng tc....15Tng Quan..16Kt Lun..18

MP3 l g?

MP3 l vit tt ca MPEG-1 Audio Layer 3. n gin m ni th MP3 ch l phn m rng ca mt tp tin m thanh. Hay chnh xc hn l mt nh dng m ha m thanh k thut s, c dng to ra cc tp tin vi dung lng c gim i rt nhiu, trong khi cht lng thu c vn gn ging nh nhng g th hin trn mt my nghe a CD.

Cc tp tin MP3 thng c chng ta ti v t internet hoc c to ra t vic s dng nhng chng trnh chuyn dng. N c th c pht bng nhng phn mm nghe nhc thng dng ngay trn my tnh hay a s cc thit b di ng ngy nay.

MPEGl tn vit tt ca hi phim nh th gii (The Moving Picture Experts Group) l mt sn phm nhm mang tnh ISO/IEC c pht trin cho cc thit b m thanh v hnh nh bng cch nn d liu chun. Chun MPEG ln u tin c ra mt vo thng 5 nm 1988 ti Ottawa, Canada. Cho n ngy nay, MPEG pht trin hn 350 thnh vin t cc hi nghi trn tt c cc lnh vc cng nghip, cc khu nghin cu, n cc trng i hc. Tiu chun chnh thc ca MPEG lISO/IEC JTC1/SC29 WG11. MPEG (c pht m l EM-peg) c chun ha theo nh dng nn v cc chun ph thuc nh sau:3MP3 c to ra khi no? Ai l ngi to ra MP3?MP3 c thit k bi Moving Picture Experts Group (MPEG), y l mt tp on chu trch nhim v thit lp tiu chun m ha cho cc loi m thanh hay video.

Tht s rt kh xc nh chnh xc ngy m MP3 c to ra cng nh ngy m n bt u tr nn ph bin trong x hi. Nhng chng ta vn c th bit c rng, cc tp tin MP3 u tin bt u xut hin vo khong nhng nm 90 v c cng b chnh thc vo nm 1993. Khi m Internet v mng ngang hng(peer to peer)ang dn tr nn ph bin th MP3 cng nhanh chng tr thnh nh dng m thanh c la chn nhiu nht bi cht lng tt v kch thc nh.

Step

Ti sao mp3 li ph bin Trong nhng ngy u ca my tnh v truyn thng, khng gian lu tr trong cng cn rt hn ch. Bn cnh , di s xut hin ca Internet th nhu cu chia s thng tin ca con ngi cng c nng cao, nht l nhu cu chia s nhng ni dung gii tr. Ngi dng khng th no gi c mt CD nhc vi dung lng ln n 700 MB qua mng ch bng vic s dng ng truyn Dial-Up 56 Kbps.

Do , cc nh nghin cu v cc t chc ln c gng tm ra nhng nh dng m thanh mi, s dng nhng thut ton ring nhm gim bt dung lng d liu ca cc tp tin m thanh, trong khi vn gi cho m thanh c to ra gn vi m thanh gc nht. Chnh v nhng l do m MP3 ra i, v ng nhin, n tr thnh mt nh dng ph bin trong lnh vc nh dng v m ha m thanh. MP3 chng t mnh l mt s la chn hng u khi c kh nng lm gim kch thc tp tin gp nhiu ln so vi kch thc ban u. Thm vo l vic to ra cc tp tin MP3 cng rt d dng m vn khng lm mt i tnh r rng ca m thanh.

Cu trc MP3Mt tp tin MP3 c to thnh t nhiu flame MP3, trong bao gm mt header v mt data .Cc khi d liu MP3 cha cc thng tin m thanh (nn) v tn s v bin .Cc file MP3 mi nht hin nay chaID3

[12 bit u] Khi ng b (12 bit): L t m ng b , (12 bit ny u l 1).[bit 13] ID (1 bit): Ch r loi MPEG. Nu l 1, th khung c m ha bng MPEG-1 chun, nu l 0, l MPEG-2. Mt s dng khc ch dng 11 bit ng b, thm 1 bit na cho phn ID. Bng di y m t tng h thng nn.[bit 14,15]

[Bit 16]

[Bit 17-20]

[Bit 21,22]

[Bit 23]

[Bit 24] Bit ring t : bit ny cho bn quyn s dng mt s ng dng khc nhng khng c g c bit[Bit 25,26]

[Bit 27,28]

[Bit 29]

[Bit 30] Nguyn bn : 0 l copy t bn gc 1 l bn gc

[Bit 31,32]

Mp3 gim dung lng nh th no?

Mt th d kim nh cht lng ca phng php nn ny (t l nn 6:1 tn hiu Stereo, m ha 16 bit, tn s ly mu 48 Khz), trong cng mt iu kin khng gian nghe rt tt, th chuyn gia giu kinh nghim cng khng th phn bit c s khc nhau gia m thanh s dng k thut nn v m thanh gc ban u. Thut ton nn ny t c hiu qu kh cao l do s dng hiu ng mt n (Masking effect), hiu ng ny li dng hiu qu m thanh khi xut hin mt m thanh c cng rt ln s che lp cc hiu ng m thanh nh hn. Thng k cho thy tai ngi c tnh chn lc tn s t tn s rt b (nh hn 100Hz) ti nhng tn s ln (hn 4Khz). V vy di tn ph m thanh nghe r c th c phn chia thnh nhiu di c bn ph hp vi c tnh phn tch ca tai ngiCc thut ton m ha MP3 lm vic da trn cc quy tc v mc cm nhn m thanh ca tai ngi. Tai con ngi ch c kh nng cm nhn c nhng m thanh nm trong khong tn s t 16 Hz - 20 KHz, m thanh nm ngoi di tn s coi nh v ngha. Hn na ti cng mt thi im, khi nghe nhng m thanh vi m lng ln, ngi dng s kh c th cm nhn c nhng m thanh c m lng rt nh. Bng cch s dng cc thut ton ring gip MP3 c th phn bit c cc on m thanh c tn s qu cao hay qu thp, v loi b nhng tp m khng cn thit.

Ni mt cch khi qut hn l n c th nn cc tp tin bng cch loi b i nhng m thanh d thabao gm c ting n, m tng qu cao hoc qu thp v m nhiu ca sng in do dng c thu m gy ra. y cng l nhng m thanh chim kh nhiu dung lng ca bn nhc.

Mt thng s quan trng nh hng n cht lng m thanh ca tp tin MP3 chnh l bit rate. Lun c mt s lin quan t l thun gia bit rate v kch thc tp tin khi to ra mt file MP3. T l bit rate cng ln cng ng ngha vi vic dung lng ca file MP3 c to ra cng ln. Cc bit rate thng thy trn nhng tp tin Mp3 c gi tr vo khong 128 Kbps, 192 Kbps, 256 Kbps, 320 Kbps. y vn l con s kh thp so vi 1411 Kbps trn cc CD c m thanh chun. Chnh vic gim bit rate xung thp cng gp phn lm gim kch thc ca cc tp tin MP3.

Song, bn cnh qu trnh loi b nhng m thanh khng cn thit, MP3 vn c mt s thut ton b p li nhng ch mt mt m n lc b bng cch to ra nhng m thanh mo m so vi m gc, m vn khng lm gim cht lng cm nhn khi so snh vi cc tp tin cha qua nn MP3. Tuy nhin, vi nhng tp tin c nn vi bit rate cng thp th hin tng mo m m thanh xy ra cng nhiu.

Chnh vic loi b i nhng m thanh tn s cao hoc thp cng lm cho MP3 tr nn yu th hn so vi cc chun nhc lossless. Nht l trong vic th hin m thanh ca nhc c trong cc bui biu din nhc ho tu hay nhc c in v nhng m thanh do nhc c ny pht ra thng rt trm (tn s cc thp) hoc rt bng (tn s cc cao). V khi chng c nn bng MP3, dy tn s ny b loi b hoc iu chnh khc i so vi thc t, i vi mt s ngi th iu ny thc s khng th chp nhn c.TT HN NA VI BITRATEC 3 la chn bitrate nhm ti u mc "nn" file nhc v cho cht lng m thanh tt nht trong kh nng c th. CBR (Constant Bit Rate): s dng bitrate c nh m ho ton b file. y l thit lp mc nh ca hu ht cc my nghe nhc. VBR (Variable Bit Rate): s dng bitrate thay i tu theo yu cu ca tng on m thanh. Thng th VBR cho cht lng tt hn vi kch thc file nh hn CBR nh cp pht bitrate "thng minh" hn. Tt c nh dng "lossless" u c m ho theo VBR, nhiu nh dng "lossy" mi (nh WMA phin bn 9) cng h tr VBR. Nhiu my nghe nhc i mi c h tr VBR nhng nhng my nghe nhc i c c th khng "hiu" ng VBR. ABR (Average Bit Rate): gn ging VBR ngoi tr kch thc file bit trc (vi VBR, kch thc file c khi ln, khi nh tu vo phc tp ca m thanh) nh iu chnh mc thay i quanh gi tr bitrate trung bnh n nh trc.Cng vi bitrate, tn s ly mu (Sampling Frequency hay Sample Rate) cng c nh hng n cht lng file nhc (nhng gn nh khng nh hng n dung lng). Tn s ly mu cng cao th cht lng m thanh cng tt, v cho "m thanh trn vn" th t nht phi gp i tn s cao nht m tai ngi c th cm nhn (20KHz). Hu ht cc CD nhc u c ghi vi tn s ly mu l 44KHz (xp x gp i 20KHz) v y cng l tn s chun cho cc file nhc nn.C mt iu thng gy ng nhn l thit lp knh "stereo" v "mono". Thng ngi ta ngh rng file nhc nn 128kbps knh n (mono) s c kch thc ch bng file 128kbps knh i (stereo). Nhng khng phi vy, m bo s bitrate, file "mono" c m ho vi cht lng gp i file "stereo" (s bitrate thc ca mi knh = s bitrate ca file chia cho s knh).M haTi b m ha, tn hiu m thanh vo c a qua mt lot b lc tch ra cc di tn ph. Dng m thanh vo c xc nh t s tn hiu vi hiu ng mt n trn tng di tn ph. Khi phn phi bit dng t s ny quyt nh cch chia, lng t ha tn hiu di tn ph, ng thi ti thiu ha nhiu lng t. Khi cui cng ly mu lng t v to nh dng s liu thnh dng bit. B gii m s khi phc cc gi tr bng tn ph lng t. Cui cng l khu chuyn i ton b gi tr bng tn ph thnh tn hiu m thanh theo thi gian. Do c mt s yu cu c bit, nn cc s liu ph c gi i cng dng bit m ha.

MP3 gim c bao nhiu dung lng?

Nu mt tp tin MP3 c to ra bng cch s dng bit rate mc 128 kbps vi thi lng 4 pht s c dung lng khong 3,5 MB so vi 40 MB trn cc tp tin gc. iu ny c ngha l dung lng ca n s c gim xung 11 ln. Ni n gin hn l vi nhng tp tin ny, ngi dng c th tng s lng bi ht lu tr trn thit b ln thm 11 ln. Hoc gim thiu thi gian download xung 11 ln so vi vic s dng cc tp tin trn a CD c to ra t ngun m thanh ban u.

Cc loi chun m thanh khcBit rate(kbps)NgunT l nn(so vi CD nhc)S bi nhc (di 3 pht) / CD1411CD nhc1:120192MP3/WMA/AAC cht lng cao7.3:1154128Nhc cho download11:123164Nhc pht trc tuyn22:14628Thoi (voice, radio)176:13.697Bng di s cho ta nim v mi lin h gia bitrate v dung lng.

Phng php nn MPEG-1 c mt s th tc nn tn hiu ti u l: Phng php m hc tm l loi b nhng thnh phn m thanh it ngha i vi cm nhn ca con ngi. Dng cc b lc, bin i Cosin bin i trong phm vi trc tn s v thi gian. Lng t ha gi tr mu, m ha Huffman. C nn c suy hao kt hp vi nn khng suy hao trong qu trnh x l lm gim s liu p ng yu cu nn tn hiu.Phn trn cung cp nhng thng tin c bn nht v cu trc d liu nn MP3. Nh vy, phng php nn MP3 lm gim tc bit ng k, p ng yu cu truyn dn cng nh lu tr d liu mt cch hiu qu.

Cho n thi im ny, MP3 vn l nh dng file nhc c a chung nht (i vi c ngi dng v cc hng sn xut my nghe nhc). D nhin, bn c th gim bitrate ca file nhc MP3 t 128kbps xung 96kbps hay thm ch thp hn gim kch thc file v cha c nhiu file nhc hn trn my nghe nhc ca mnh, min l cht lng m thanh "chp nhn c" theo cm th "tai nghe" ca bn. Mi ngi c cm nhn m thanh khc nhau, khng ai ging ai. "Tai nghe" chuyn nghip c th yu cu bitrate ln n 320kbps (mc bitrate cao nht cho cht lng chnh xc "nh CD"), nhng vi "tai nghe" ch cn giai iu th bitrate 20kbps cng tho mn - nghe c nhiu giai iu hn v kch thc file nh hn. Thng th g ng ca mnh l tt.S thnh cng ca MP3 cng to nn mt "cn c mng" cho ngnh cng nghip m nhc khi cc tp tin ny c chia s mt cch bt hp php. i khi ngi dng chia s cc tp tin MP3 cho nhau nhng h khng h bit rng chnh bn thn h li ang vi phm bn quyn m nhc. V v l , c th s lm mt i v th c tn ca MP3 trong tng lai, khi cc chun nh dng mi ra i. Ngoi vic nn d liu tt hn, cc chun mi s c kh nng chng sao chp. Tuy nhin, vic MP3 c b thay th hay khng l chuyn ca tng lai, cn by gi chng ta vn phi cn n n.

Z Shell (zsh)Cc Shell thng dngBourne Shell (sh)Bourne Again Shell (bash)C Shell(csh)Korn Shell (ksh)

Bourne Shell (sh) Bourne Shell (sh) lUnix shellmc nhcaUnix phin bn 7 pht hnh vo nm 1977.

Pht trin biStephen BournetiBell Labs, l mt thay th choThompso shell.

Bourne Shell cng l Shell u tin tnh nng quy c ca vic s dngm t tp tincho ccthng bo li, cho php kim sot ln hn nhiu chng trnh.

Stephen Bourne

Bourne Shell (Tip) Cc mc tiu chnh ca shell bourne:_ cho php ccshell scriptsc s dng nh cc b lc._ cung cp cho lp trnh bao gm ckim sot dng chyvbin._Kim sot trn tt c cctp tinu vo / u ra._Kim sotx l tn hitrong scripts._Khng c gii hn v di chui khi thng dch shell scripts._Hp l ha v khi qut c ch chui trch dn._Ccc ch mi trng.

Bourne Shell (Tip) Nhiu nm sau khi c pht hnh bourne shell c nhiu ci tin chc nng nhng vn khng th trnh khi nhn ch trch ph bnh:

_Shell Bourne khngthn thin vi ngis dng.Bourne shell khng cung cplch s,b danh,kim sot cng vicv cc tnh nng khc._Mc d phn cn li ca h thng Unix c vit bngC,ng phpca Bourne shelltrng khng c g ging nh C._N thiu mtng php biu hin."Khng ai thc s hiu r ng php ca Bourne shell ,ngay c kim tra ca m ngun cng khng gip c nhiu.-Tom Duff

Bourne-again shell _Bash l mt Unix Shell c vit bi Brian Fox cho d n GNU nh l mt phn mm min ph thay th cho Bourne Shell.

_Pht hnh vo nm 1989, c phn phi rng ri nh Shell cho h iu hnh GNU v l shell mc nh trn Linux v Mac OS.

_Bash l mt b x l lnh cho php ngi s dng g lnh hoc cng c th c lnh t mt file hay script. Cc tnh nng c bn khc ca ngn ng u c sao chp t sh.

_Bash l mt POSIX Shell , nhng vi mt s phn m rng.

Bourne-again shell (Tip)_Brian Fox bt u m ha Bash trn 10 thng 1 nm 1988. Fox thc hin cc cng vic nh l mt nhn vin ca FSF. _Fox pht hnh Bash nh mt beta, phin bn .99, ngy 07 thng 6 nm 1989 v vn duy tr chnh cho n khong gia gia 1992 -1994, khi ng b sa thi t FSF.

_C php lnh ca bash nh l mt tp hp con ca Bourne shell. Phn ln cc script ca Bourne c th c thc hin bi Bash m khng sa i, Bash c php lnh bao gm tng rt ra t Korn shell (ksh) v C shell (csh). _Bash hon thnh h thng l rt linh hot v ty bin v c php c nhiu phn m rng khc m shell Bourne thiu. _Bash h tr cc ti liu nh Bourne shell lun lun c.

Brian Fox C shell _C shell l mt shell Unix c to ra bi Bill Joy khi ng va tt nghip ti i hc California, Berkeley vo cui nm 1970.

_Joy bt u phn phi vo nm 1978.

_C shell l mt b x l lnh cho php ngi s dng g lnh hoc cng c th c lnh t mt file hay script.

_Nhng nm 1980, cc tnh nng tng tc c b sung mt cch tng th. Cc tnh nng mi ca n lm cho n d dng hn v nhanh hn s dng d c hn.

Bill Joy

37C shell (Tip) _Mc d ph bin cho cc tng tc s dng v nhiu tnh nng sng to ca mnh, csh khng ph bin cho script. _C shell cng phi i mt vi nhng li ch trch t nhng ngi khc trn cc khim khuyt b co buc C shell trong c php, tnh nng cn thiu, v thc hin km. _C shell rt thnh cng trong vic gii thiu mt s lng ln cc sng kin . _Tuy nhin, tri ngc vi sh , sn sinh ra mt s lng ln nhn bn c lp pht trin, bao gm c ksh v bash.Writing a Unix-Like Shell for MS-DOS

C shell ang v s l mt ngn ng lp trnh ph bin trong cng ng nghin cu v vn nhn dng ging ni.Korn Shell (ksh)

_KornShell ( ksh ) l mt shell Unix c pht trin bi David Korn Bell Labs trong u nhng nm 1980 v cng b ti USENIX ngy 14 Thng By nm1983.

_KornShell tng thch ngc vi cc Bourne shell.

_KornShell bao gm nhiu tnh nng ca C shell.David Korn _KornShell l c sn nh l mt phn ca AT & T Cng ngh phn mm. KornShell ban u ch c thng qua mt giy php c quyn t AT & T, sau ny c min ph v ngun m c to ra. _KornShell ban u l v mc nh trn AIX t phin bn 4.

Z Shell _Z Shell ( zsh ) l mt Unix shell c th c s dng nh mt thng dch mnh m cho script shell . Zsh c th c coi nh l mt m rng Bourne shell vi mt s lng ln cc ci tin, bao gm mt s tnh nng ca bash , ksh , v tcsh .

_Cc phin bn 1 ca zsh c vit bi Paul Falstad vo nm 1990 khi ng l mt sinh vin ti i hc Princeton . Ci tn zsh xut pht t gio s Yale.

Paul Falstad Z Shell (Tip) Cc tnh nng ng ch bao gm:_Chia s lch s lnh trong tt c cc lp ang chy._Ci thin x l bin / mng._Chnh sa nhiu dng lnh trong mt b m duy nht._Sa li chnh t._Ch tng thch khc nhau, v d nh zsh c th gi v l mt shell Bourne khi chy nh / bin / sh._Themeable nhc nh , bao gm c kh nng a thng tin kp thi pha bn phi ca mn hnh v c n t ng n khi g mt lnh diLoadable m-un, cung cp trong s nhng th khc: y TCP v cm Unix min iu khin, mt FTP client, v cc chc nng ton hc m rng._Hon ton ty chnh.

c im Chungmi trngGii thiuNn tng c lpMc nh ng nhp ScriptGiy phpUnicodeDngchuyn hngNativeCIM/WBEMh trChn cc kch bn unsignedlin kt tnh, tp tin duy nht c lpBourne shell7 Ed.UNIX1977C7 Ed.UNIX7 Ed.UNIXc quyn,CDDLcC (ty FDS)KhngKhngcbash(v4.0)POSIX1989CGNU,h iu hnh Mac OS X10,3 +GNU,Haiku

GPLcC (ty FDS)KhngKhngccshPOSIX1978C??BSDKhngC (stdin, out, out + err)KhngKhngckshPOSIX1983CAIX,HP-UXOpenSolarisC Giy php Cng cngcC (ty FDS)KhngKhngczshPOSIX1990CGrml,GobolinuxGrmlBSD-stylecC (ty FDS)KhngKhngc

Cc tnh nng tng tcHon thnhNgn xp th mcLch s th mc WindowThay i mc Implicitlch s lnhLch s hon thnhKim tra chnh tNgi dng mc nhTu chnh du nhc lnhAliasesBinary tin t k hiuVic kim sotTp lnh khi ngBourne shellKhngKhngKhngKhngKhngKhngKhng$C (bin: $ PS1)KhngKhngK t khi SVR4cbash(v4.0)C (m rng)cKhngC(ty chn)ccKhngbash-C (bin: $ PS1)cKhngcccshC (thng qua phm ESC)cKhngC(ty chn)ccKhng%C (bin: $ du nhccKhngcckshC (m rng)KhngKhngKhngccKhng$C (bin: $ PS1, v nhiu hn na)cKhngcczshC (m rng)cKhngC(ty chn)ccc%C (bin: $ PS1, v nhiu hn na)cKhngcc

Tng Quan

Shell Bourne (sh)Shell nguyn thu c mt trn hu ht cc h thng Unix/LinuxN rt hu dng cho vic lp trnh Shell nhng n khng x l tng tc ngi dung nh cc Shell khc

Bourne Again Shell (bash)y l phn m rng ca sh, n k tha nhng g sh c v ph huy nhng g sh cha cN c giao din lp trnh rt mnh v linh hotCng vi giao din lnh d dungy l Shell c ci t mc nh trn cc h thng Linux.

44Content LayoutsTng QuanShell C (csh)p ng tng thch cho ngi dungN h tr rt mnh cho nhng Programmer Cv vi c tnh t ng hon thnh dng lnh

Shell Korn (ksh)C th ni y l mt Shell tuyt vi, n kt hp tnh nng u vit ca sh v csh

Shell Z (zsh)y l shell c nh gi l mnh nht trong thi gian gn y. H tr rt nhiu nhng tnh nng cc mnh, nhng vic ci t li rt n gin.

Kt lunH thng Linux tn ti rt nhiu loi shell v mi loi shell c c trng ring, c ch ring nn phi chn loi shell ph hp vi mc ch khi lm vic.

Ti liu v shell rt phc tp v kh tm ,khng c ti liu bin dch sang ting vit nn s thiu st nhiu thng tin.

Thank You!

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