UNIX环境高级编程1-5从标准输入读命令并执行

//program1-5proc/shell1.c

#include"apue.h"

#include<sys/wait.h>

intmain(void)

{

charbuf[MAXLINE];/*fromapue.h*/

pid_tpid;

intstatus;

printf("%%");/*printprompt(printfrequires%%toprint%)*/

//WeusethestandardI/Ofunctionfgetstoreadonelineatatimefromthe

//standardinput.Whenwetypetheend-of-filecharacter(whichisoften

//Control-D)asthefirstcharacterofaline,fgetsreturnsanullpointer,theloop

//stops,andtheprocessterminates.InChapter18,wedescribeallthespecial

//terminalcharacters--endoffile,backspaceonecharacter,eraseentireline,and

//soon--andhowtochangethem.

//Becauseeachlinereturnedbyfgetsisterminatedwithanewlinecharacter,

//followedbyanullbyte,weusethestandardCfunctionstrlentocalculatethe

//lengthofthestring,andthenreplacethenewlinewithanullbyte.Wedothis

//becausetheexeclpfunctionwantsanull-terminatedargument,nota

//newline-terminatedargument.

//Wecallforktocreateanewprocess,whichisacopyofthecaller.Wesaythat

//thecalleristheparentandthatthenewlycreatedprocessisthechild.Then

//forkreturnsthenon-negativeprocessIDofthenewchildprocesstotheparent,

//andreturns0tothechild.Becauseforkcreatesanewprocess,wesaythatitis

//calledonce--bytheparent--butreturnstwice--intheparentandinthechild.

//Inthechild,wecallexeclptoexecutethecommandthatwasreadfromthe

//standardinput.Thisreplacesthechildprocesswiththenewprogramfile.The

//combinationofforkfollowedbyexeciscalledspawninganewprocesson

//someoperatingsystems.IntheUNIXSystem,thetwopartsareseparatedinto

//individualfunctions.We'llsayalotmoreaboutthesefunctionsinChapter8.

//Becausethechildcallsexeclptoexecutethenewprogramfile,theparent

//wantstowaitforthechildtoterminate.Thisisdonebycallingwaitpid,

//specifyingwhichprocesstowaitfor:thepidargument,whichistheprocessID

//ofthechild.Thewaitpidfunctionalsoreturnstheterminationstatusofthe

//child--thestatusvariable--butinthissimpleprogram,wedon'tdoanything

//withthisvalue.Wecouldexamineittodeterminehowthechildterminated

//Themostfundamentallimitationofthisprogramisthatwecan'tpass

//argumentstothecommandweexecute.Wecan't,forexample,specifythename

//ofadirectorytolist.Wecanexecutelsonlyontheworkingdirectory.Toallow

//argumentswouldrequirethatweparsetheinputline,separatingthearguments

//bysomeconvention,probablyspacesortabs,andthenpasseachargumentasa

//separateparametertotheexeclpfunction.Nevertheless,thisprogramisstilla

//usefuldemonstrationoftheUNIXSystem'sprocesscontrolfunctions.

while(fgets(buf,MAXLINE,stdin)!=NULL)

{

if(buf[strlen(buf)-1]=='\n')

{

buf[strlen(buf)-1]=0;/*replacenewlinewithnull*/

}

if((pid=fork())<0)

{

err_sys("forkerror");

}

elseif(pid==0)

{/*child*/

//iftheexeclpexecutesuccessfully,execlpwillnotreturn

//ifitexecutefailed,return-1,andtheerrornumberstoreinerrnovariable

execlp(buf,buf,(char*)0);

err_ret("couldn'texecute:%s",buf);

exit(127);

}

/*parent*/

if((pid=waitpid(pid,&status,0))<0)

{

err_sys("waitpiderror");

}

printf("%%");

}

return0;

}