Linux Shell date command pit (date 1 month ago strange phenomenon)

The first and last days of this month:

Echo 'date -- Date "$1" + "% Y-% m-01 "'

 

Year = 'date-d "$1" + "% Y "'
Month = 'date-d "$1" + "% m "'
Last_day = 'cal $ month $ year | xargs | awk '{print $ NF }''

Echo $ year-$ month-$ last_day

 

Calculate the first and last days of the previous month:

#! /Bin/sh
# Obtain the first day of the previous month
Echo 'date -- Date "1 month ago $1" + "% Y-% m-01 "'

 

But problems found during the test:

[Lix @ cq01-crm-lin1rd10..com temp] $ echo 'date -- Date "1 month ago 20130302" + "% Y % m "'
201302

[Lixia@cq01-crm-.com temp] $ echo 'date -- Date "1 month ago 20130330" + "% Y % m "'

201303

[Lixia1@cq01-crm-.com temp] $ echo 'date -- Date "1 month ago 20130329" + "% Y % m "'

201303
[Li@cq01-crm-lin1rd10.com temp] $ echo 'date -- Date "1 month ago 20130229" + "% Y % m "'

201301

 

Why is this strange phenomenon? Because the 1 month ago of 20130330 should be 20130230? However, if there is no 30 on January 1, February, it will automatically change to 20130301. Then, % Y % m will get 201301.

 

There is another solution: Get the first day of the month, and then subtract one day, that is, get the last day of the previous month. Set the last value to 01 to get the first day of the previous month.

You can also obtain the first day of the month and the first day of the previous month.

 

Refer:

Http://blog.csdn.net/yifanernei/article/details/5112232

Http://www.cnblogs.com/billyxp/archive/2013/01/31/2886733.html