bash 链式逻辑运算符执行顺序，惰性求值

Question

我不明白这个：

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)

Answer 1

&&并||具有相同的优先级，因此：

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

这与以下非常不同：

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

Answer 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

其中d执行，如果任一B或C成功。

添加更多分组：

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

或者

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

或者使用if-elif-else@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