linux笔记

• cat /dev/null > ./null.txt 清空文件内容

shell 按行读取文件 ，不过for 是按字符串读取，行中无空格等与while相同读取整行

#!/bin/bash
PATH=/bin:/sbin:/usr/bin:/usr/sbin:/usr/local/bin:/usr/local/sbin:~/bin
export PATH
方法1
while read line
do
    echo $line
done<$1

 方法2
cat $1 |while read line
do 
    echo $line
done

方法3
for line in `cat $1`
do 
    echo $line
done

read line 好像readline() 快一点

with open(fr) as frr:
           for line in frr:
                  print(line)
real    0m30.935s
user    0m0.523s
sys 0m0.385s

----------------------
with open(fr) as frr:
  7         for line in frr:
  8                 line = frr.readline()
  9                 print(line)

real    0m14.752s
user    0m0.306s
sys 0m0.210s
______________
with open(fr) as frr:
  7         for line in frr.readlines():
  8                 print(line)

real    0m31.145s
user    0m0.544s
sys 0m0.415s

逐行读取文件，将每行中以分隔符分割的最后一列赋值给变量a，变量a可能会含有换行符，或者在原文件中最后一列后增加一列，读取倒数第二列时就不包含换行符了。