shell腳本處理大數據系列之(二)使用函數返回值

轉自http://longriver.me/?p=80

隨着shell腳本行數的增大，爲了編寫和維護的須要，想把一些特定的功能的代碼抽取出來，組成函數的形式，放到一個叫作functions.sh中這樣未來使用的時候只須要在腳本中source functions.sh 一下，而後就能夠方便使用，處理大數據的時候，上篇博文提到過使用多線程處理是重要的手段，因此想到了寫一個lock()函數，使得多個進程之間能夠異步的工做。

函數的返回值有兩種用途
1，返回函數結束的狀態（能夠是int，也能夠是string）
2，共享一些變量

如下代碼來自

(EDIT: This is also true for some other shells...)
1. echo strings

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lockdir= "somedir" testlock(){      retval= ""      if mkdir "$lockdir"      then # directory did not exist, but was created successfully           echo >&2 "successfully acquired lock: $lockdir"           retval= "true"      else           echo >&2 "cannot acquire lock, giving up on $lockdir"           retval= "false"      fi      echo "$retval" } retval=$( testlock ) if [ "$retval" == "true" ] then       echo "directory not created" else       echo "directory already created" fi

2. return exit status

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lockdir= "somedir" testlock(){      if mkdir "$lockdir"      then # directory did not exist, but was created successfully           echo >&2 "successfully acquired lock: $lockdir"           retval=0      else           echo >&2 "cannot acquire lock, giving up on $lockdir"           retval=1      fi      return "$retval" }   testlock retval=$? if [ "$retval" == 0 ] then       echo "directory not created" else       echo "directory already created" fi

3. share variable

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lockdir= "somedir" retval=-1 testlock(){      if mkdir "$lockdir"      then # directory did not exist, but was created successfully           echo >&2 "successfully acquired lock: $lockdir"           retval=0      else           echo >&2 "cannot acquire lock, giving up on $lockdir"           retval=1      fi }   testlock if [ "$retval" == 0 ] then       echo "directory not created" else       echo "directory already created" fi

【NOTICE】關於shell函數的一個陷阱，我曾經深嘗苦果，shell函數內部定義的變量，不是局部變量，這就意味着變量不會隨着函數的結束而消失，這個特色能夠被利用，因此shell函數中的變量要初始化，每次執行的時候注意從新賦值。