golang 提供了package bufio。bufio.NewReader()创建一个默认大小的readbuf，当然，也可以bufio.NewReaderSize。

funcNewReader(rdio.Reader)*Reader
NewReaderreturnsanewReaderwhosebufferhasthedefaultsize(4096).


funcNewReaderSize(rdio.Reader,sizeint)*Reader
NewReaderSizereturnsanewReaderwhosebufferhasatleastthe
specifiedsize.Iftheargumentio.ReaderisalreadyaReaderwithlarge
enoughsize,itreturnstheunderlyingReader.

bufio

func(b*Reader)ReadByte()(cbyte,errerror)
ReadBytereadsandreturnsasinglebyte.Ifnobyteisavailable,returnsanerror.

func(b*Reader)ReadBytes(delimbyte)(line[]byte,errerror)
ReadBytesreadsuntilthefirstoccurrenceofdelimintheinput,returningaslicecontainingthedatauptoandincludingthedelimiter.
IfReadBytesencountersanerrorbeforefindingadelimiter,itreturns
thedatareadbeforetheerrorandtheerroritself(oftenio.EOF).
ReadBytesreturnserr!=nilifandonlyifthereturneddatadoesnot
endindelim.Forsimpleuses,aScannermaybemoreconvenient.

func(b*Reader)ReadString(delimbyte)(linestring,errerror)
ReadStringreadsuntilthefirstoccurrenceofdelimintheinput,returningastringcontainingthedatauptoandincludingthe
delimiter.IfReadStringencountersanerrorbeforefindingadelimiter,itreturnsthedatareadbeforetheerrorandtheerroritself(often
io.EOF).ReadStringreturnserr!=nilifandonlyifthereturneddata
doesnotendindelim.Forsimpleuses,aScannermaybemore
convenient.

ReadByte这个接口，和C语言中fgetc很接近，每次读取一个字节。ReadBytes和ReadString都可以实现逐行读取，只要delim设置为'\n'.

packagemain
import"fmt"
import"os"
import"io"
import"flag"
import"bufio"

varnum_flag=flag.Bool("n",false,"numeachline")

funcusage(){
fmt.Printf("%s%s\n",os.Args[0],"filename")
}



funccat(r*bufio.Reader){
i:=1
for{
//buf,err:=r.ReadBytes('\n')
buf,err:=r.ReadString('\n')
iferr==io.EOF{
break
}

if*num_flag{
fmt.Fprintf(os.Stdout,"%5d%s",i,buf)
i++
}else{
fmt.Fprintf(os.Stdout,"%s",buf)
}

}
return
}


funcmain(){

flag.Parse()
if(flag.NArg()==0){
cat(bufio.NewReader(os.Stdin))
}

fori:=0;i<flag.NArg();i++{
f,err:=os.OpenFile(flag.Arg(i),os.O_RDONLY,0660)
iferr!=nil{
fmt.Fprintf(os.Stderr,"%serrreadfrom%s:%s\n",flag.Arg(0),err)
continue
}

cat(bufio.NewReader(f))
f.Close()
}
}

用scaner逐行读取

funccat(scanner*bufio.Scanner)error{

forscanner.Scan(){
fmt.Println(scanner.Text())
//fmt.Fprintf(os.Stdout,"%s\n",scanner.Text())
}

returnscanner.Err()
}

注意，为啥执行Scan，Text()函数就能返回下一行呢？因为默认的分割函数就是ScanLines.如你有特殊的需求来分割，func (s *Scanner) Split(split SplitFunc)

这个函数可以制定SplitFunc。你可以定制自己的分割函数。

需要注意的是，Scan会将分割符号\n去除，如果Fprintf输出的话，不添加\n打印，会出现没有换行的现象，如下所示

fmt.Fprintf(os.Stdout,scanner.Text())

manu@manu-hacks:~/code/go/self$gorunmycat_v2.gotest.txt
thisistestfilecreatedbygoifnotexisted,pleasecreatethisfileifexisted,Pleasewriteappendhelloworld,hellogothisistestfilecreatedbygoifnotexisted,hellogomanu@manu-hacks:~/code/go/self$cattest.txt
thisistestfilecreatedbygo
ifnotexisted,pleasecreatethisfile
ifexisted,Pleasewriteappend
helloworld,hellogo
thisistestfilecreatedbygo
ifnotexisted,hellogo

调用部分的代码如下：

f,0660)
...
error:=cat(bufio.NewScanner(f))
iferr!=nil{
fmt.Fprintf(os.Stderr,flag.Arg(i),error)
}