variable in while loop between ksh vs. bash

最新推荐文章于 2025-07-21 18:17:30 发布

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最新推荐文章于 2025-07-21 18:17:30 发布

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分类专栏： Shell

本文链接：https://blog.youkuaiyun.com/HiHui/article/details/52877839

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本文通过示例对比了Bash和Ksh两种Shell环境下变量的作用域差异，指出Bash中循环内的变量更改不会保留到子Shell外部，而Ksh则保持变量更改。演示了不同Shell对变量处理的不同行为。

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In BASH, a variable is changed in a loop, but is kept untouched when quit loop; i.e, the new value set in loop is lost.

function showme
{
   echo -n "Loop=$linecount"
   sh -c 'echo " PID=$PPID"'
}

echo "========1======="
linecount=0


showme
echo "111 222 333" | tr ' ' '\n' | while read LINE
do
  (( linecount = linecount + 1 ))
  showme
done
showme

echo "========2======="
linecount=0
showme
while read LINE
do
  (( linecount = linecount + 1 ))
  showme
done <<-EOF
  $(echo "111 222 333" | tr ' ' '\n')
EOF
showme

The execute output:

$ bash t.sh
========1=======
Loop=0 PID=2620
Loop=1 PID=2647
Loop=2 PID=2647
Loop=3 PID=2647
Loop=0 PID=2620
========2=======
Loop=0 PID=2620
Loop=1 PID=2620
Loop=2 PID=2620
Loop=3 PID=2620
Loop=3 PID=2620

the same script when execute using ksh, the output is different:

$ ksh t.sh  
========1=======
Loop=0 PID=12090
Loop=1 PID=12090
Loop=2 PID=12090
Loop=3 PID=12090
Loop=3 PID=12090
========2=======
Loop=0 PID=12090
Loop=1 PID=12090
Loop=2 PID=12090
Loop=3 PID=12090
Loop=3 PID=12090

This is because the while in BASH is loop executed in a subshell. So any changes you do for the variable will not be available once the subshell exits.
From the PID value, we can found the loop are in process, PID=2647 while script itself PID=2620

The same script in KSH environment use same PID=12090 in all cases.