在Bash循环内递增变量 [英] Incrementing a variable inside a Bash loop

问题描述

我正在尝试编写一个小的脚本，该脚本将对日志文件中的条目进行计数，并且在循环完成后递增要使用的变量(USCOUNTER).

I'm trying to write a small script that will count entries in a log file, and I'm incrementing a variable (USCOUNTER) which I'm trying to use after the loop is done.

但是在那一刻，USCOUNTER看起来是0，而不是实际值.知道我在做什么错吗?谢谢！

But at that moment USCOUNTER looks to be 0 instead of the actual value. Any idea what I'm doing wrong? Thanks!

FILE=$1

tail -n10 mylog > $FILE

USCOUNTER=0

cat $FILE | while read line; do
  country=$(echo "$line" | cut -d' ' -f1)
  if [ "US" = "$country" ]; then
        USCOUNTER=`expr $USCOUNTER + 1`
        echo "US counter $USCOUNTER"
  fi
done
echo "final $USCOUNTER"

它输出:

US counter 1
US counter 2
US counter 3
..
final 0

推荐答案

您在子Shell中使用USCOUNTER，这就是为什么变量未显示在主Shell中的原因.

You are using USCOUNTER in a subshell, that's why the variable is not showing in the main shell.

仅执行while ... done < $FILE，而不是cat FILE | while ....这样，您可以避免我在管道循环中设置变量的常见问题.为什么它们在循环终止后消失了?或者，为什么我不能通过管道读取数据?:

Instead of cat FILE | while ..., do just a while ... done < $FILE. This way, you avoid the common problem of I set variables in a loop that's in a pipeline. Why do they disappear after the loop terminates? Or, why can't I pipe data to read?:

while read country _; do
  if [ "US" = "$country" ]; then
        USCOUNTER=$(expr $USCOUNTER + 1)
        echo "US counter $USCOUNTER"
  fi
done < "$FILE"

请注意，我还用$()替换了"表达式.

Note I also replaced the `` expression with a $().

我也用while read country _替换了while read line; do country=$(echo "$line" | cut -d' ' -f1).这样您可以说出while read var1 var2 ... varN，其中var1包含该行中的第一个单词，$var2，依此类推，直到$varN包含其余内容.

I also replaced while read line; do country=$(echo "$line" | cut -d' ' -f1) with while read country _. This allows you to say while read var1 var2 ... varN where var1 contains the first word in the line, $var2 and so on, until $varN containing the remaining content.

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