Linux command type:
Built-in command (shell built-in): cd is shell builtin
External command: command is / usr / bin / command, a command name corresponding to the path of the executable file in a file system
type: Displays the command specifies what type of command belongs
type cd
The results showed that the cd command is built-in command
type date
The results show date command is an external command, that is a date the executable file / usr / bin / directory
Environment: Environment variables are named memory space
Variable assignment:
NAME=Jerry
System environment variable PATH: the use of colon-separated path, the Linux commands printenv can print system environment variables, any executable program is executed to find the PATH from the inlet, the inlet as JAVA code program as the main method;
Implementation of hash command to view the command path and the current use of cache hits
date: Time Management
Windows system time management tool is NTP (Network Time Protocol);
Linux system clock management tool is the RTC (Real Time Clock), that is, the real-time clock chip, the use of spare lithium batteries in the event of power failure continues to record time, so as to ensure the accuracy of time.
The expansion of knowledge:
Linux systems have points of the system time and hardware time:
System Time: Generally the date command execution time is what we see, all the time calling linux system (except for the command to directly access the hardware time) this time are used.
Hardware time (hwclock): motherboard BIOS time, the battery from the motherboard to maintain the operation, this time to read system boot, and to set it in accordance with the system time (Note: The system is set according to the hardware at system startup time when the process in terms of the time zone may be present, depending on the particular system and related setting).
Execution date command to obtain the system time
Hwclock command execution, the time available hardware, ie hwclock Hardware Clock, This command can be executed as root
Execute man date, the user can view the order date command, or use the date --help help documentation
date +% D, displays the current date in the format year month day
date +% T, the current time
date +% Y, displays the current year
date +% y, displays the current year Acronym
Application practice:
1, view the current system time date:
2, the system time provided time date -s +
3, when setting the system time, if required to set the date, the date and time values will have wrapped in double quotes
4. Check the hardware time with hwclock command:
5, the hardware time is written to the system time, execution hwclock -s, and then use the date to see if there is time and hardware time consistent:
6, the system time to hardware time to write, perform hwclock -w:
7, time synchronization, Linux systems need to install ntpdate
Check whether the installation ntpdate: rpm -qa | grep ntpdate
If it is not installed, run the following command: yum install ntpdate
Time synchronization command: ntpdate www.pool.net.org, which www.pool.net.org is the Internet standard time servers
8, contab command
Through vi etc / crontab command to see / write timing of the task format
[root@surfer ~]# vi/etc/crontab
SHELL = / bin / bash #SHELL variable specifies which shell to use the system
PATH = / sbin: / bin: / usr / sbin: / usr / bin #PATH variable specifies the path to the system to execute a command
MAILTO = root #MAILTO variable specifies crond task execution information will be sent to the root user via e-mail, if the MAILTO variable is null, then does not transmit information to the user task execution
References: https: //blog.csdn.net/qq_20663229/article/details/80812215
https://blog.csdn.net/qq_20663229/article/details/80812215