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translate done: 20170820 How To Display Date And Time In History Command.md

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How To Display Date And Time In History Command
让 History 命令显示日期和时间
======
We all are familiar with History command. It stores/keep all the commands executed by bash in terminal into `.bash_history` file, which will help us to recheck the previously executed commands by user for further investigation.
我们都对 History 命令很熟悉。它将终端上 bash 执行过的所有命令存储到 `.bash_history` 文件中，来帮助我们复查用户之前执行过的命令。
By default history command shows only the commands executed by users and it doesn't print the date and time but it logs the time when you ran a command.
默认情况下 history 命令直接显示用户执行的命令而不会输出运行命令时的日期和时间，即使 history 命令记录了这个时间。
Whenever you run history command, it looks for an environment variable called `HISTTIMEFORMAT`, which tells how to format date & time with history command.
运行 history 命令时，它会检查一个叫做 `HISTTIMEFORMAT` 的环境变量，这个环境变量指明了如何格式化输出 history 命令中记录的这个时间。
If the value is null or not set then it will shows default results like how most of the systems shows (Without date and time).
若该值为 null 或者根本没有设置，则它跟大多数系统默认显示的一样，不会现实日期和时间。
HISTTIMEFORMAT takes values from strftime (strftime - convert date and time to a string). When you have date and time in history command output, it might help you to track the issue easily.
`HISTTIMEFORMAT` 使用 strftime 来格式化显示时间 (strftime - 将日期和时间转换为字符串)。history 命令输出日期和时间能够帮你更容易地追踪问题。
* **%T :** Replaced by the time ( %H : %M : %S ).
* **%F :** Equivalent to %Y - %m - %d (the ISO 8601:2000 standard date format).
* **%T：** 替换为时间 ( %H：%M：%S )。
* **%F：** 等同于 %Y-%m-%d (ISO 8601：2000 标准日期格式)。
See below default history command output.
下面是 history 命令默认的输出。
```
# history
1 yum install -y mysql-server mysql-client
@ -52,57 +49,55 @@ See below default history command output.
```
We can set this environment variable in three ways based on our requirements.
根据需求，有三种不同的方法设置环境变量。
* Temporarily to current user
* Permanently to current/other user
* Permanently to all users
* 临时设置当前用户的环境变量
* 永久设置当前/其他用户的环境变量
* 永久设置所有用户的环境变量
**注意：** 不要忘了在最后那个单引号前加上空格，否则输出会很混乱的。
### 方法 -1：
**Note :** Don 't forget to add space before the last single-quotes, otherwise the output would be messy.
### Method-1 :
Run the following command to set HISTTIMEFORMAT variable temporarily to current user. This will go away after reboot.
运行下面命令为为当前用户临时设置 HISTTIMEFORMAT 变量。这会一直生效到下次重启。
```
# export HISTTIMEFORMAT='%F %T '
```
### Method-2 :
### 方法 -2：
Append following HISTTIMEFORMAT variable to `.bashrc` or `.bash_profile` file to make it permanent for every user.
将 HISTTIMEFORMAT 变量加到 `.bashrc` 或 `.bash_profile` 文件中，让它永久生效。
```
# echo 'HISTTIMEFORMAT="%F %T "' >> ~/.bashrc
or
或
# echo 'HISTTIMEFORMAT="%F %T "' >> ~/.bash_profile
```
Run the following command to to effect the changes made to the file.
运行下面命令来让文件中的修改生效。
```
# source ~/.bashrc
or
或
# source ~/.bash_profile
```
### Method-3 :
### 方法 -3：
Append following HISTTIMEFORMAT variable to `/etc/profile` file to make it permanent to all users.
将 HISTTIMEFORMAT 变量加入 `/etc/profile` 文件中，让它对所有用户永久生效。
```
# echo 'HISTTIMEFORMAT="%F %T "' >> /etc/profile
```
Run the following command to to effect the changes made to the file.
运行下面命令来让文件中的修改生效。
```
# source /etc/profile
```
See the sample output.
输出结果为。
```
# history
1 2017-08-16 15:30:15 yum install -y mysql-server mysql-client