5.4bash算术运算、位置参数和read

ARITHMETIC EVALUATION  算术运算
   The  shell  allows  arithmetic  expressions  to  be  evaluated, under certain circumstances (see the let and
   declare builtin commands and Arithmetic Expansion).  Evaluation is done  in  fixed-width  integers  with  no
   check for overflow, though division by 0 is trapped and flagged as an error.  The operators and their prece-
   dence, associativity, and values are the same as in the C language.  The  following  list  of  operators  is
   grouped into levels of equal-precedence operators.  The levels are listed in order of decreasing precedence.

   id++ id--
          variable post-increment and post-decrement
   ++id --id
          variable pre-increment and pre-decrement
   - +    unary minus and plus
   ! ~    logical and bitwise negation
   **     exponentiation
   * / %  multiplication, division, remainder
   + -    addition, subtraction
   << >>  left and right bitwise shifts
   <= >= < >
          comparison
   == !=  equality and inequality
   &      bitwise AND
   ^      bitwise exclusive OR
   |      bitwise OR
   &&     logical AND
   ||     logical OR
   expr?expr:expr
          conditional operator
   = *= /= %= += -= <<= >>= &= ^= |=
          assignment
   expr1 , expr2
          comma

   Shell variables are allowed as operands; parameter expansion is performed before the  expression  is  evalu-
   ated.   Within  an  expression,  shell  variables may also be referenced by name without using the parameter
   expansion syntax.  A shell variable that is null or unset evaluates to 0 when  referenced  by  name  without
   using the parameter expansion syntax.  The value of a variable is evaluated as an arithmetic expression when
   it is referenced, or when a variable which has been given the integer attribute using declare -i is assigned
   a value.  A null value evaluates to 0.  A shell variable need not have its integer attribute turned on to be
   used in an expression.

   Constants with a leading 0 are interpreted as octal numbers.  A leading 0x or 0X denotes hexadecimal.   Oth-
   erwise,  numbers  take  the  form [base#]n, where base is a decimal number between 2 and 64 representing the
   arithmetic base, and n is a number in that base.  If base# is omitted, then base 10  is  used.   The  digits
   greater than 9 are represented by the lowercase letters, the uppercase letters, @, and _, in that order.  If
   base is less than or equal to 36, lowercase and uppercase letters may be used interchangeably  to  represent
   numbers between 10 and 35.

   Operators  are evaluated in order of precedence.  Sub-expressions in parentheses are evaluated first and may
   override the precedence rules above.

sum=5
sum=$[$sum+8]  同等  let sum+=8
[root@linux_basic ~]#sum=5
[root@linux_basic ~]#sum=$[$sum+8]
[root@linux_basic ~]#echo $sum
13
[root@linux_basic ~]#let sum+=7
[root@linux_basic ~]#echo $sum
20
练习：
a.计算100之内全部正整数之和
b.分别计算100之内全部偶数之和和奇数之和；
c.计算当前系统全部用户的ID之和；
a.
sum=0
for n in {1..100}
do
  sum=$[$sum+$n]
done
 
echo "sum=$sum"

bash下用来测试脚本
bash -n scriprs.sh
    用来单步执行脚本，用来调试的
    bash -x scripts.sh
从1开始步进为2直到10结束  能够获得10之内的全部奇数   
[root@linux_basic scripts]#seq 1 2 10
1
3
5
7
9
从0开始步进为2知道10结束  能够获得10之内的全部偶数
[root@linux_basic scripts]#seq 0 2 10
0
2
4
6
8
10

b.
declare -i sum1=0
declare -i sum2=0
for n in $(seq 0 2 100)
do
    let sum+=$n
done

for n in $(seq 1 2 100)
do
    sum=$[$sum+$n]
done

echo "sum1=$sum1,sum2=$sum2"

c.
sum=0
for n in $(cut -d: -f3 /etc/passwd)
do
    let sum+=$n
done

echo "sum=$sum"

wc命令的使用
[root@linux_basic scripts]#type wc
wc is hashed (/usr/bin/wc)
[root@linux_basic scripts]#wc --help
Usage: wc [OPTION]... [FILE]...
  or:  wc [OPTION]... --files0-from=F
Print newline, word, and byte counts for each FILE, and a total line if
more than one FILE is specified.  With no FILE, or when FILE is -,
                                  后面跟文件，或者没有文件时，读取标准输入
read standard input.
  -c, --bytes            print the byte counts  打印字节数
  -m, --chars            print the character counts 打印字符数
  -l, --lines            print the newline counts  打印行数
      --files0-from=F    read input from the files specified by
                           NUL-terminated names in file F;
                           If F is - then read names from standard input
  -L, --max-line-length  print the length of the longest line  打印最长行的长度
  -w, --words            print the word counts  打印单词数
 
[root@linux_basic scripts]#wc -m /etc/rc.d/rc.sysinit /etc/init.d/functions /etc/issue
19914 /etc/rc.d/rc.sysinit
19295 /etc/init.d/functions
   47 /etc/issue
39256 total

练习：
a.计算/etc/rc.d/rc.sysinit,/etc/init.d/functions和/etc/issue三个文件的字符数之和；
b.新建用户tmpuser1-tmpuser10,并计算他们的id之和；
a.
sum=0
for n in /etc/rc.d/rc.sysinit /etc/init.d/functions /etc/issue
do
   num=$(wc -m $n|cut -d' ' -f1)
   sum=$[$sum+$num]
done

echo "sum=$sum"  

b.
sum=0
for n in {1..10}
do
    useradd tmpuser$n
    num=$(grep "^tmpuser$n:" /etc/passwd | cut -d: -f3)  或者使用  num=$(id -u tmpuser$n)
    let sum+=$num
done

echo "sum=$sum"

知识点：位置参数
    位置参数：   
    ./scripts.sh  arg1 arg2 arg3
         $0:脚本自身名字  scripts.sh
         $1:脚本的第一个参数  arg1
[root@linux_basic scripts]#chmod +x pos.sh
[root@linux_basic scripts]#bash -n pos.sh
[root@linux_basic scripts]#./pos.sh 23 9
The sum in:32
[root@linux_basic scripts]#cat pos.sh
#!/bin/bash
#
echo "The sum in:$[$1+$2]"
    特殊变量：
       $#:位置参数的个数
       $@、$*：引用全部的位置参数
[root@linux_basic scripts]#./pos.sh 12 5
The sum in:17
2
12 5
12 5
[root@linux_basic scripts]#cat pos.sh
#!/bin/bash
#
echo "The sum in:$[$1+$2]"
echo "$#"
echo "$*"
echo "$@"

知识点：交互式脚本
[root@linux_basic scripts]#type read
read is a shell builtin
[root@linux_basic scripts]#help read
read: read [-ers] [-a array] [-d delim] [-i text] [-n nchars] [-N nchars] [-p prompt] [-t timeout] [-u fd] [name ...]
    Read a line from the standard input and split it into fields.
   
    Reads a single line from the standard input, or from file descriptor FD
    if the -u option is supplied.  The line is split into fields as with word
    splitting, and the first word is assigned to the first NAME, the second
    word to the second NAME, and so on, with any leftover words assigned to
    the last NAME.  Only the characters found in $IFS are recognized as word
    delimiters.
经常使用选项
-p prompt     输出prompt字串的内容后，可在后面接着输入
   output the string PROMPT without a trailing newline before
   attempting to read
-t timeout    在等待输入的超时时间
    time out and return failure if a complete line of input is
        not read withint TIMEOUT seconds.  The value of the TMOUT
        variable is the default timeout.  TIMEOUT may be a
        fractional number.  If TIMEOUT is 0, read returns success only
        if input is available on the specified file descriptor.  The
        exit status is greater than 128 if the timeout is exceeded  

[root@linux_basic scripts]#read a b
12 10 23
[root@linux_basic scripts]#echo $a
12
[root@linux_basic scripts]#echo $b
10 23
[root@linux_basic scripts]#read a b
12 36
[root@linux_basic scripts]#echo $a
12
[root@linux_basic scripts]#echo $b
36

知识点：给变量以默认值
    varname=${varname:-value}
       若是varname不空，则其值不变；不然，varname值为value       
[root@linux_basic scripts]#b=22
[root@linux_basic scripts]#echo $b
22
[root@linux_basic scripts]#b=${b:-90}
[root@linux_basic scripts]#echo $b
22
[root@linux_basic scripts]#unset b
[root@linux_basic scripts]#b=${b:-90}
[root@linux_basic scripts]#echo $b
90       

[root@linux_basic scripts]#cat read.sh
#!/bin/bash
#
read -t 5 -p "Enter a number:" num

num=${num:-9}
echo $num       
[root@linux_basic scripts]#./read.sh
Enter a number:9   等待了5秒都没有給值  没有給值，默认是不会换行
[root@linux_basic scripts]#./read.sh
Enter a number:90  在5秒以内给了值
90

练习： 经过键盘给定一个文件的路径，来判断文件内容的类型； read -p "Enter a file name:" filename file $filename 经过键盘给定一个目录的路径，没有给定则默认为‘/’,来判断文件内容的类型； read  -t 5 -p "Enter a directory path:" pathname pathname=${pathname:-/} for name in $(ls $pathname) do     file $pathname$name done