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文件暂存 SNMP学习总结（1）

[题目4-8]
The functions pushd and popd implement a stack of directories that enable you to move to another directory temporarily and have the shell remember where you were. Implement them as shell functions.

实现pushd和popd函数

[解答]

DIR_STACK="" # stack定义
export DIR_STACK # 输出到环境变量

function pushd
{
     dirname=$1
     DIR_STACK="$dirname ${DIR_STACK:-$PWD' '}" # 将新目录附加到栈顶，若栈空，则先压入当前目录
     cd ${dirname "missing directory name."}           # 进入目录，无参数则报错退出
     echo "$DIR_STACK"                                             # 打印当前栈
}

function popd
{
     DIR_STACK=${DIRSTACK#* }   # 首段删除一个目录（目录间以空格分隔）
     cd ${DIR_STACK%% *}             # 进入下一个目录（尾端多次删除，取栈顶的目录）
     echo "$PWD"                          # 打印当前目录
}

[题目4-5]

The file used in Task 4-1 is actually a report derived from a bigger table of data about albums. This table consists of several columns, or fields, to which a user refers by names like "artist," "title," "year," etc. The columns are separated by vertical bars (|, the same as the UNIX pipe character). To deal with individual columns in the table, field names need to be converted to field numbers.Suppose there is a shell function called getfield that takes the field name as argument and writes the corresponding field (or column) number on the standard output. Use this routine to help extract a column from the data table.

Task4-1扩展，album数据文件含有许多列如作者，题目，年代等等，每一项数据以 | 分隔。 想处理数据的话，需要将列名转化为列数。假设这里有一个函数 getfiled用来将列名转换为列数，请用这个函数帮助你提取数据表中的某一列。

[解答]

#! /bin/bash

fieldname=$1
cut -f$(getfield $fieldname) -d\| album

[题目4-6]

Send a mail message to everyone who is currently logged in.

给当前系统中登录的每一个人发一封邮件。

[解答]

mail $(who | cut -f1 -d' ')

[题目4-7]

The ls command gives you pattern-matching capability with wildcards, but it doesn't allow you to select files by modification date. Devise a mechanism that lets you do this.

ls命令允许使用通配符进行匹配查询，但是它不支持通过修改日期查询。设计一种机制可以让你按修改日期查询。

[解答]

function lsd
{
date=${1 'no date found'}
ls -l | grep -i "$date"
}

[题目4-2]

You are writing a graphics file conversion utility for use in creating a web page. You want to be able to take a PCX file and convert it to a JPEG file for use on the web page.[7]

你准备写一个图形文件转换工具，用来制作网页。你想把PCX格式的文件转换为可以在网页上显示的JPEG文件。

[解答]

#字符串尾部模式替换

outfile=${infile%@(.pcx|.PCX)}.jpg 

#截图如下

[题目4-3]

You are implementing a filter that prepares a text file for printer output. You want to put the file's name without any directory prefix on the "banner" page. Assume that, in your script, you have the pathname of the file to be printed stored in the variable pathname.

将一个完整路径名缩短为只剩文件名(pathname to basename)

[解答]

basename=${pathname##*/}

#截图如下

[题目4-4]

The directories in PATH can be hard to distinguish when printed out as one line with colon delimiters. You want a simple way to display them, one to a line.

打印出环境变量PATH中的目录时，所有目录显示在一整行中并且是以冒号区分的。现在你想一行显示一个目录。

[解答]

echo -e ${PATH//:/'\n'}    # -e 让echo开启转义字符

#截图如下

［题目］

You have a large album collection, and you want to write some software to keep track of it. Assume that you have a file of data on how many albums you have by each artist. Lines in the file look like this:

你有一本大型画册，并且你想写一些软件来统计管理它。假设这个画册有一个数据文件，上面有每位画家的作品数目，文件格式如下：

5        Depeche Mode
2        Split Enz
3        Simple Minds
1        Vivaldi, Antonio

Write a program that prints the N highest lines, i.e., the N artists by whom you have the most albums. The default for N should be 10. The program should take one argument for the name of the input file and an optional second argument for how many lines to print.

写一个程序，打印出你所拥有作品数量最多的前N个画家及其作品数 ，N的默认值为10。这个程序的第一个参数应当是数据文件的名称，第二个参数（可选参数）指定打印多少行信息（指定N）

［思考］

（1）数据文件不一定有序，需要排序

使用sort，对文件中每行数据进行排序，参数选用 -n 以数字为序，-r 逆序排列

（2）打印文件信息时需要截取

使用head，截取输出的起始部分，参数选用 -n 来指定截取的行数

（3）默认值 N＝10

将第二参数作默认值处理 ${2:-10}，即如果没有指定第二个参数直接指定其值为10（这里不能用${2:=10}，因为$2有可能不存在）

 ［解答］

建立task4-1.sh文件，脚本如下：

#! /bin/bash

file=${1 "no file found"}
howmany=${2:-10}
echo
echo "============"
echo "top $howmany :"
sort -nr $file | head -n $howmany
echo "finished"
echo "============"
echo

运行过程如截图所示