hadoop - 混淆了awk中的[01459]，什么意思？

Question

#!/usr/bin/env bash
for year in all/*
do
    echo -ne `basename $year .gz`"\t"
    gunzip -c $year | \
    awk '{ temp = substr($0, 88, 5) + 0;
          quality = substr($0, 93, 1);
          if (temp !=9999 && quality ~ /[01459]/ && temp > max) max = temp }
        END { print max }'
done

我正在学习 Hadoop 权威指南第 4 版中的练习。我已经完成了第一个练习，并且有效，但是我不明白quality ~/[01459]/ 的含义。

Answer 1

根据 http://www.math.utah.edu/docs/info/gawk_5.html

exp ~ /regexp/

This is true if the expression exp (taken as a string) is matched by regexp.

和

[...]

This is called a character list. It matches any one of the characters that are enclosed in the square brackets.

因此 quality ~/[01459]/ 是 true 如果 quality 包含任何 1, 4, 5, 9