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Accepted
Zoltan K.
Asked: 2019-10-26 07:02:09 +0800 CST

bash 链式逻辑运算符执行顺序，惰性求值

  • 772

我不明白：

脚本：WORKDIR/sh/script.sh

[ -e filename ]          \
&& echo filename         \
|| [ -e ../filename ]    \
&& echo ../filename      \
|| { echo 'ERROR: failed to find "filename"' 1>&2 ; exit -1; }

输出：

$ cd WORKDIR/sh
$ ./script.sh
../filename

$ cd WORKDIR
$ sh/script.sh
filename
../filename      # <---- WHY????

我的想法：

1

[ -e filename ]          \ -> false
&&                         -> skip this, it is already false
   echo filename         \ -> don't even try
|| [ -e ../filename ]    \ -> true
&& echo ../filename      \ -> true
||                         -> already true, skip the rest
   { echo 'ERROR: failed to find "filename"' 1>&2 ; exit -1; }

2

[ -e filename ]          \ -> true
&& echo filename         \ -> true
||                         -> already true, skip the rest
   [ -e ../filename ]    \
&& echo ../filename      \
|| { echo 'ERROR: failed to find "filename"' 1>&2 ; exit -1; }

版本：

bash --version
GNU bash, version 4.4.12(1)-release (x86_64-pc-linux-gnu)
bash

2 个回答

  1. Best Answer

    &&并且||具有相同的优先级，所以：

    当一个命令通过时，它会寻找下一个&&并执行它，即使它不是直接相邻的操作符。您永远不应在单个命令列表中使用多个这些运算符。如果需要多个，则使用 if/then 构造。

    $ true && true || echo yes && echo no
no

    这与以下有很大不同：

    if true; then 
  true 
else 
  echo yes && echo no
fi

    $ if true; then true; else echo yes && echo no; fi
$

    或者：

    $ true && false || echo yes && echo no
yes
no
$ if true; then false; else echo yes && echo no; fi
$

    我会把你的构造写成：

    if [ -e filename ]; then
    echo filename
elif [ -e ../filename ]; then
    echo ../filename
else
    echo 'ERROR: failed to find "filename"' >&2
    exit -1
fi
    • 9

  2. 我认为您希望 bash 这样做：

    (A && B) || (C && D) || E

    但它实际上是这样做的

    (A && B || C) && D || E

    如果 B 或 C 成功，则 D执行。

    添加更多分组：

    [ -e filename ] && echo filename || {
    [ -e ../filename ] && echo ../filename || { 
        echo 'ERROR: failed to find "filename"' 1>&2 
        exit -1
    }
}

    或者

    { [ -e filename ]    && echo filename   ; } || 
{ [ -e ../filename ] && echo ../filename; } || 
{ echo 'ERROR: failed to find "filename"' 1>&2 ;  exit -1; }

    if-elif-else或者使用@Jesse_b 展示的非常清晰易读的风格。

    一步一步，这正在发生：

    1. 文件名在当前目录中：

      [ -e filename ]    \   # test succeeds, status is now 0
&&                     # status is zero, will execute this branch
echo filename      \   # echo succeeds, status is now 0
||                     # status is zero, do not execute
[ -e ../filename ] \   # not executed
&&                     # status is zero, will execute this branch
echo ../filename   \   # echo succeeds, status is now 0
||                     # status is zero, do not execute
{ echo; exit; }        # not executed

    2. 文件名在父目录中：

      [ -e filename ]    \   # test fails, status is now 1
&&                     # status is non-zero, do not execute this branch
echo filename      \   # not executed
||                     # status is non-zero, will execute this branch
[ -e ../filename ] \   # test succeeds, status is now 0
&&                     # status is zero, will execute this branch
echo ../filename   \   # echo succeeds, status is now 0
||                     # status is zero, do not execute
{ echo; exit; }        # not executed
    • 6

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