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       /var/tmp/exercise-two/Alice/likes/an-ox             <---- a file
       /var/tmp/exercise-two/Emily/has-bought/a-horse      <---- a file
       /var/tmp/exercise-two/Gary/has-bought/a-rhino       <---- a directory
       /var/tmp/exercise-two/Gary/has-bought/a-rhino/less  <---- a file
       /var/tmp/exercise-two/Gary/has-bought/a-rhino/is    <---- a file
       /var/tmp/exercise-two/Gary/has-bought/a-rhino/more  <---- a file
       /var/tmp/exercise-two/Mary/is/a-snake               <---- a file
       

       ~/unix-1031/exer2/
            |---Alice/
            |   `---likes/
            |       `---an-ox
            |---Emily/
            |   `---has-bought/
            |       `---a-horse
            |---Gary/
            |   `---has-bought/
            |       `---a-rhino/
            |           |---is
            |           |---less
            |           `---more
            `---Mary/
                `---is/
                `---a-snake
       

• 若目錄或檔名開頭為 ex-, 則新目錄名或檔名為移除 ex- 後剩餘部分.
• 若為 file, 內容必須一樣.
• 若為 hard link, 則新建出的檔案間的關係也是 hard link.
• 若為 soft link, 則新建出的檔案間的關係也是 soft link, 而 且 link file 的 內容也必須一樣.

• 用 at 在 11/07 01:00 至 03:00 間執行一個程式. 時間自選, 作一次即可. 輸出的第一行要包含當時執行的時間.
• 設定 crontab, 在一小時內執行三次程式, 時間任選, 每次 run 的程式不一定都要一樣. 第一次的輸出會新增或蓋掉既有的檔案, 第二次及第三次的輸出則 會接到前一次的後面. 每次輸出的第一行要包含當時執行的時間.
• at 及 crontab 所需的程式及輸出檔案皆放在 ~/unix-1031/exer4 下.
• 請參閱 README-exer4.

      ===prompt===> cat IN-5
      klim is good.
      But oak is unknown.
      Really? How about redcow? 
      Drinking klim makes you healthy.
      So? 
      At least you drink much milk.
      Hmm.
      klim is not milk.
      ===prompt===> ./ex5.sh < IN-5
      milk is good.
      Drinking milk makes you healthy.
      At least you drink much klim.
      milk is not klim.
      ===prompt===> 
      

      $ cat IN-7 
      # sample input file of exercise 7
                                      # this is an blank line.
      1234   Alice                    # Alice wins 1234
      999    Bob                      # Bob   wins 999
      Alice  200                      # Alice spends 200
      Bob    100    Alice             # Bob loses 100 to Alice
      4000   Bob    250    Carol      # Bod wins 4000 and loses 250 to Carol
      3000   Carol  300               # Carol wins 3000 and spends 300
      $ ./ex7.awk IN-7 
      Carol has 2950
      Bob has 4649
      Alice has 1134
      $ 
      

• 寫一個 awk script, 讀進一個 file. 其中 # 以後為 comment. 不是 comment 的那些行包含兩個 patterns, 若第一行含 A1 B1, 第二 行含 A2 B2, 則 awk 會產出二個檔, 一為 grep 用的 pattern file, 一為 sed 用的 command file. 若將 grep 執行的結果用 pipe 送給 sed, 則會取出 stdin 含 A1 或 A2 的那幾行, 並將 A1 換成 B1, A2 換成 B2.
• 寫一個 shell script, 判斷是否有一個參數. 若有, 則將其視為 awk 的 input. 呼叫 awk, 並結合 grep 與 sed 以達成上述功能.

      $ cat IN-data-8 
      This is klim speaking.
      klim is good, klim is healthy, and klim is not milk.
      Haha, it is funny.
      $ cat IN-pat-8 
      # Each line is composed of two strings as the following shows.
      klim toostrong
      milk klim
      toostrong milk
      $ cat IN-data-8 | ./ex8.sh          
      ./ex8.sh pattern-file
      $ cat IN-data-8 | ./ex8.sh IN-pat-8 
      This is milk speaking.
      milk is good, milk is healthy, and milk is not klim.
      $ 
      

      $ ./ex9.sh 
      ./ex9.sh name [file1 file2] ....
      $ ./ex9.sh cow
      moo 1234 9833
      cow 200 400
      klim 200
      cow 1000
      cow has total 1600 dollars.
      $ cat Week-1
      # first week
      klim 120 100 900   #not much
      milk 98 33 100 120
      moo  10000 200 20 
      cow  2000 10 100000  # Wow!
      $ cat Week-2
      moo 20 100 20 1
      cow 299 44 22 
      cow 333 22 90
      klim 49 22
      moo 333
      $ ./ex9.sh cow Week-1
      cow has total 102010 dollars.
      $ ./ex9.sh cow Week-2
      cow has total 810 dollars.
      $ ./ex9.sh cow Week-1 Week-2
      cow has total 102820 dollars.
      $ cat Week-1 Week-2 | ./ex9.sh cow
      cow has total 102820 dollars.
      $