Shell脚本编程学习入门：Shell编程基础(1)(2)

二进制到十进制的转换

脚本 b2d 将二进制数 (比如 1101) 转换为相应的十进制数。这也是一个用expr命令进行数学运算的例子：

#!/bin/sh
# vim: set sw=4 ts=4 et:
help()
{
   cat << HELP
   
b2d -- convert binary to decimal
 
USAGE: b2d [-h] binarynum
 
OPTIONS: -h help text
 
EXAMPLE: b2d 111010
will return 58
HELP
   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" | sed 's/ //g' | wc -c `
   # 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 "$1" | cut -b -$numofcharminus1` 
   #原来的 rval=`echo -n "$1" | cut -b 0-${numofcharminus1}`运行时出错.
   #原因是cut从1开始计数，应该是cut -b 1-${numofcharminus1}
}
 
while [ -n "$1" ]; do
case $1 in
   -h) help;shift 1;; # function help is called
   --) shift;break;; # end of options
   -*) 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shell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。