二进制到十进制的转换

本文是一个把二进制数转换为十进制数功能的shell脚本实例。这也是一个用expr命令进行数学运算的例子：

文件b2h.sh

#!/bin/sh

# vim: set sw=4 ts=4 et:

help()

{

echo "b2h -- convert binary to decimal"

echo "USAGE: b2h [-h] binarynum"

echo "OPTIONS: -h help text"

echo "EXAMPLE: b2h 111010 wiil be 58"

exit 0

}

error()

{

# print an error and exit

echo "$1"

exit 1

}

lastchar()

{

# return the last character of a string in $rval

if [-z"$1" ]; then

# empty string

rval=""

return

fi

# wc puts some space behind the output this is why we need sed:

numofchar=`echo -n "$1" | wc -c | sed 's/ //g' `

# now cut out the last char

rval=`echo -n "$1" | cut -b $numofchar`

}

chop()

{

# remove the last character in string and return it in $rval

if [-z"$1" ]; then

# empty string

rval=""

return

fi

# wc puts some space behind the output this is why we need sed:

numofchar=`echo -n "$1" | wc -c | sed 's/ //g' `

if [ "$numofchar" = "1" ]; then

# only one char in string

rval=""

return

fi

numofcharminus1=`expr $numofchar "-" 1`

# now cut all but the last char:

rval=`echo -n "$1" | cut -b 1-${numofcharminus1}`

}

while [-n"$1" ]; do

case $1 in

-h)help;shift 1;; # function help is called

-*)error"error: no such option $1. -h for help";;

*) break;;

esac

done

# The main program

sum=0

weight=1

# one arg must be given:

[ -z "$1" ] && help

binnum="$1"

binnumorig="$1"

while [ -n "$binnum" ]; do

lastchar "$binnum"

if [ "$rval" = "1" ]; then

sum=`expr "$weight" "+" "$sum"`

fi

# remove the last position in $binnum

chop "$binnum"

binnum="$rval"

weight=`expr "$weight" "*" 2`

done

echo "binary $binnumorig is decimal $sum"

该脚本使用的算法是利用十进制和二进制数权值 (1,2,4,8,16,..)，比如二进制"10"可

以这样转换成十进制：

0 * 1 + 1 * 2 = 2

为了得到单个的二进制数我们是用了lastchar 函数。该函数使用wc –c计算字符个数，

然后使用cut命令取出末尾一个字符。Chop函数的功能则是移除最后一个字符。

rval=`echo -n "$1" | cut -b $numofchar`

这里cut的意思是提取位置$numofchar的这个字符，字符为位置是从1开始计算的

rval=`echo -n "$1" | cut -b 1-${numofcharminus1}`

这里cut的意思是提取从位置1到位置${numofcharminus1}个字符，该语句是闭包含。字符位置是从1开始计算的。

另外，"shift 1"是把脚本文件传入参数编号起点从左向右移一位的意思。

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