Question

我有一个变量，我试图设置这样的东西：

#!/bin/sh

found=0
id=1
echo "Hello" |
while [ $id != 5 ]
do
      id=`expr $id + 1`
      echo $id
      found=1
done

echo "found = $found" // I expect this to be 1

为什么以及如何设置此值？我被迫使用这样（管道），因为生产环境中的实际代码是：

found=0
id=1
my_mount_name="/opt/insiteone/fuse-mount"
echo "select file_system_id, mount_name from SystemTable" | mysql ifm -uroot -pinsite3 |
while read file_system_id mount_name
do
   if [ "$id" == "$file_system_id" -a "$my_mount_name" == "$mount_name" ]; then
      echo "Match found for file system ID and mount name"
      found=1
   fi
done
echo "found = $found" // I expect this to be 1, when a match, but does not

Answer 1

管道在子壳中运行。你可以做一些事情来使它工作，最简单的是：

found=0
id=1
my_mount_name="/opt/insiteone/fuse-mount"
echo "select file_system_id, mount_name from SystemTable" | 
mysql ifm -uroot -pinsite3 | {
while read file_system_id mount_name
do
   if [ "$id" == "$file_system_id" -a "$my_mount_name" == "$mount_name" ]; then
      echo "Match found for file system ID and mount name"
      found=1
   fi
done
echo "found = $found"; }
# Note the enclosing {}. Inside the black, the variable $found is set.
# After this comment, it will be zero.

这种技术可能要求封闭块相当大，因此您可能需要重构脚本的其余部分以使其可用。另一种选择是使用fifo或将echo / mysql管道放入进程替换中。（后者不是可移植的，但在bash中工作可能就足够了。）但是，在这种特殊情况下，最好做一些类似的事情：

found=0
id=1
my_mount_name="/opt/insiteone/fuse-mount"

echo "select file_system_id, mount_name from SystemTable" |
mysql ifm -uroot -pinsite3 | {
while read file_system_id mount_name
do
   if [ "$id" == "$file_system_id" -a "$my_mount_name" == "$mount_name" ]; then
      echo "Match found for file system ID and mount name"
      exit 0  # Exit the subshell succesfully
   fi
done
exit 1; } && found=1

Answer 2

您可以将变量传递给子shell的environemnt：

foo=1
bar=2

echo "sds" | foo="$foo" bar="$bar" while ...

Answer 3

正如@fedorqui评论的那样，bash将管道组件放在不同的子shell中，因此当子shell退出时，对变量的更改会消失。

有两种策略可以解决这个问题：

仅在同一子shell中使用已更改的变量

echo "Hello" |
{
    while [ $id != 5 ]
    do
        ((id++))
        echo $id
        found=1
    done
    echo "found = $found" // I expect this to be 1
}

如果你有很多依赖这些变量的代码

用流程替换替换管道。这意味着while循环不在子shell中执行，它在当前shell中运行：

while [ $id != 5 ]
do
    ((id++))
    echo $id
    found=1
done < <(echo "Hello")
echo "found = $found" // I expect this to be 1

这可能会导致可读性差，但您可以根据需要在<(...)内放置尽可能多的换行符。

您的生产代码使用流程替换（和bash / ksh条件语法）重写：

found=0
id=1
my_mount_name="/opt/insiteone/fuse-mount"

while read file_system_id mount_name; do
    if [[ $id == $file_system_id ]] && [[ $my_mount_name == $mount_name ]]; then
        echo "Match found for file system ID and mount name"
        found=1
    fi
done < <(
    echo "select file_system_id, mount_name from SystemTable" | 
    mysql ifm -uroot -pinsite3 
)
echo "found = $found"

为什么shell不设置管道变量？

3 个答案: