BASH: Read a file line by line

假設 $file 的變數值是一個檔案名稱，該檔案內容為：

a 1
b 2
c 3
d 4
e 5

則以下方式可以將該檔案一行一行讀入，並將讀入的每一行存在 $line 變數裡面：

Method 1

Code:

number=0
while read -r line
do
 number=$(($number + 1))
done < <(cat $file)
echo "# of lines = $number"

Output:

# of lines = 5

Note:
除了 cat $file 以外，也可以使用其他指令，不一定要從檔案讀入。

Method 2

Code:

number=0
while read -r line
do
 number=$(($number + 1))
done < $file
echo "# of lines = $number"

Output:

# of lines = 5

Method 3

Code:

number=0
exec 9< $file
while read -u 9 -r line
do
 number=$(($number + 1))
done
echo "# of lines = $number"

Output:

# of lines = 5

Note:
exec 後面的數字是 descriptor 代號，和 read -u 用一樣的就可以了。

Method 4

Code:

number=0
cat $file | while read -r line
do
 number=$(($number + 1))
done
echo "# of lines = $number"

Output:

# of lines = 0

Note:
最後得出來的結果會是 0 。