写的比较详细的一个Linux下引号的区别，Mark一下。

http://www.opsers.org/linux-home/base/shell-in-single-quotes-double-quotes-backticks-use-a-backslash.html

顺便加上自己写的一个小程序，做个示例。

 1 #! /bin/bash -
 2 
 3 #@Author:           Liang Feng
 4 #@Create Time:      2011-11-26
 5 #@Function:         cut qrels into different levels, i.e. 0, 1, 2
 6 ##                  1 means relevance, 2 means high relevance
 7 
 8 root_dir="/home/liangfeng/Project/TREC2011/Utility/qrel"
 9 allrel_dir="$root_dir/allrel"
10 highrel_dir="$root_dir/highrel"
11 zerorel_dir="$root_dir/zerorel"
12 qrel_file="$root_dir/microblog11-qrels"
13 
14 counter=1
15 while [ $counter -le 50 ]
16 do
17     all_output_file="$allrel_dir/$counter.ans"
18     high_output_file="$highrel_dir/$counter.ans"
19     zero_output_file="$zerorel_dir/$counter.ans"
20     awk '($1=='"$counter)"' && ($4 == 0) {print $3}' $qrel_file | sort -r > $zero_output_file
21     awk '($1=='"$counter)"' && ($4 == 1) {print $3}' $qrel_file | sort -r > $all_output_file
22     awk '($1=='"$counter)"' && ($4 == 2) {print $3}' $qrel_file | sort -r > $high_output_file 
23     counter=`expr $counter + 1`
24 done