shell脚本处理大数据系列之(二)使用函数返回值

转自http://longriver.me/?p=80

随着shell脚本行数的增大，为了编写和维护的须要，想把一些特定的功能的代码抽取出来，组成函数的形式，放到一个叫作functions.sh中这样未来使用的时候只须要在脚本中source functions.sh 一下，而后就能够方便使用，处理大数据的时候，上篇博文提到过使用多线程处理是重要的手段，因此想到了写一个lock()函数，使得多个进程之间能够异步的工做。

函数的返回值有两种用途
1，返回函数结束的状态（能够是int，也能够是string）
2，共享一些变量

如下代码来自

(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】关于shell函数的一个陷阱，我曾经深尝苦果，shell函数内部定义的变量，不是局部变量，这就意味着变量不会随着函数的结束而消失，这个特色能够被利用，因此shell函数中的变量要初始化，每次执行的时候注意从新赋值。