Bash error這個：命令未找到

Question

我剛開始學習bash，並試圖編寫一個接受兩個單詞作爲參數的腳本，並打印出找到第一個單詞的行和第第二個字被發現，例如，如果我的test.txt文件看起來像：

This is a line of text 
This is another line of text 
this is the third one 
Another one right here 

而且我運行./prog.bash test.txt This one程序應該返回線1,2和3，因爲我們發現，在一行單詞本，並在一個字第3行。我當前的腳本如下所示：

#!/bin/bash 
filename=$1 
a=$(grep -n -m1 "$2" "$filename" | cut -f 1 -d':') #find first occurence of word1 and use that to cut the line number of word 
b=$(grep -n -m1 "$3" "$filename" | cut -f 1 -d':') #find first occurence of word2 and use that to cut the line number of word 
lines=$(($b-$a)+1) 
thing= $(head -n"${b}" "$filename" | tail -n"${lines}") #Print out b lines from beginning and then print $(lines) lines from bottom of that 
echo $a 
echo $b 
echo $lines 
echo $thing 
exit 

現在我得到一個錯誤說

./prog.bash：6號線：此：找不到命令

由於該行是head和tail我看着在我的路徑是唯一的命令是和他們在那裏位於，我想出了：

$PATH = 
/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games 

head = /usr/bin/head 

tail = /usr/bin/tail 

Answer 1

6號線應該是

thing=$(head -n"${b}" "$filename" | tail -n"${lines}") #Print out b lines from beginning and then print $(lines) lines from bottom of that 

沒有空間後=