dd的作用是转换和拷贝文件,我们可以利用它来分割文件,相关的选项如下:

  • if=filename:输入的文件名
  • of=finename:输出的文件名
  • bs=bytes:一次读写的字节数,默认是512bytes
  • skip=blocks:拷贝前,跳过的输入文件的前blocks块,块的大小有bs决定
  • count=blocks:只拷贝输入文件的前blocks块

例如,现在有一个文件file,大小为116616字节:

[root]# du -b file  
116616  file  

将其分割为两文件file1和file2,那我们就设置每块为1024字节,将file的前60块放入file1,余下的放入file2:

[root]# dd if=file bs=1024 count=60 skip=0  of=file1  
[root]# dd if=file bs=1024 count=60 skip=60 of=file2  

然后用cat将两个文件合并为file.bak,要注意文件的顺序:

[root]# cat file1 file2 > file.bak  

可以用md5sum验证一下file和file.bak:

[root]# md5sum file  
3ff53f7c30421ace632eefff36148a70  file  
[root]# md5sum file.bak  
3ff53f7c30421ace632eefff36148a70  file.bak  

可以证明两个文件时完全相同的。

为了方便分割、合并文件,我写了两个脚本:

ddf.sh

#ddf.sh:分割文件,分割后的文件以数字结尾,例如file分割为两个文件:file1和file2  
#!/bin/sh  

#使用脚本是第一参数是要分割的文件名  
Filename=$1     
Filesize=0  

Path=`pwd`  

#验证文件名是否正确,然后计算文件的大小  
if [ -z $Filename ];then  
    echo "Error:The file name can not be empty"  
    exit  
fi  
if [ -e $Filename ];then  
    Filesize=`du -b $Filename | awk '{print $1}'`  
    if [ $Filesize == 0 ];then  
        echo "Error:The File size is zero!"  
        exit  
    fi  
    echo "The file size is $Filesize Byte"  
    echo "Plese enter the subfile size(KB):"  
else  
    echo "Error:$Filename does not exist!"  
    exit   
fi  

#输入分割后每个文件的大小,单位是KB  
read Subfilesize  
if [ -z $Subfilesize ];then  
    echo "Error:Input can not be empty"  
    exit  
fi  

echo $Subfilesize | grep '^[0-9]\+$' >> /dev/null  
if [ $? -ne 0 ];then  
    echo "Error:The Input is not a number!"  
    exit  
elif [ $Subfilesize -eq 0 ];then  
    echo "Error:The Subfile size is zero!"  
    exit  
fi  

#计算需要分割为几个文件  
SubfileByte=`expr $Subfilesize \* 1024`  
Subfilenum=`expr $Filesize / $SubfileByte`  
if [ `expr $Filesize % $Subfilesize` -ne 0 ];then  
    Subfilenum=`expr $Subfilenum + 1`  
fi  

#将文件分割  
echo "$Filename will be divided into $Subfilenum"   
i=1  
skipnum=0  
while [ $i -le $Subfilenum ]  
do  
    echo "$Filename$i"  
    dd if=$Filename of="$Path/$Filename$i" bs=1024 count=$Subfilesize skip=$skipnum  
    i=`expr $i + 1`  
    skipnum=`expr $skipnum + $Subfilesize`  
done  
echo "$Filename has been divided into $Subfilenum"  
echo "Done !"  

caf.sh

#caf.sh:合并文件,需要合并的文件要放在一个文件夹里  
#       文件名分为两个部分,第一部分都相同,第二部分必须是从1开始的连续数字,例如file1,file2,file3  
#       合并后的文件名为file.bak  
#!/bin/sh  

#输入文件名的第一部分  
echo "Please enter file name:"  
read Filename  
if [ -z $Filename ];then  
    echo "Error:File name can not be empty"  
    exit  
fi  

#输入待合并文件的个数  
echo "Please enter the number of subfiles:"   
read Subfilenum  
if [ -z $Subfilenum ];then  
    echo "Error:The number of subfiles can not be empty"  
    exit  
fi  
echo $Subfilenum | grep '^[0-9]\+$' > /dev/null  
if [ $? -ne 0 ];then  
        echo "Error:Input must be a number"  
exit  
fi  
if [ $Subfilenum -eq 0 ];then  
        echo "Error:The number of subfiles can not be zero"  
exit  
fi  


#合并文件  
i=1  
Newfile=$Filename\.bak  
while [ $i -le $Subfilenum ]  
do  
    Subfilename=$Filename$i  
        if [ -e $Subfilename ];then  
            echo "$Subfilename done!"  
            cat $Subfilename >> $Newfile  
            i=`expr $i + 1`  
        else  
            echo "Error:$Subfilename does not exist"  
            rm -rf $Newfile  
            exit  
        fi  
done  


echo "Subfiles be merged into $Newfile"  
echo "Success!"  

用这两个脚本完成对file的分割、合并:

[root]# ./ddf.sh file   
The file size is 116616 Byte  
Plese enter the subfile size(KB):  
60  
file will be divided into 2  
file1  
记录了60+0 的读入  
记录了60+0 的写出  
61440字节(61 kB)已复制,0.0352612 秒,1.7 MB/秒  
file2  
记录了53+1 的读入  
记录了53+1 的写出  
55176字节(55 kB)已复制,0.0316272 秒,1.7 MB/秒  
file has been divided into 2  
Done !  
[root]# ls  
caf.sh  ddf.sh  file  file1  file2  
[root]# ./caf.sh   
Please enter file name:  
file  
Please enter the number of subfiles:  
2  
file1 done!  
file2 done!  
Subfiles be merged into file.bak  
Success!  
[root]# ls  
caf.sh  ddf.sh  file  file1  file2  file.bak
Li Shaocheng. Published under BY-NC-SA
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