Use the df command to view the disk capacity, using the du command to view the directories capacity
du -h #同--human-readable 以K，M，G为单位，提高信息的可读性。
du -a #同--all 显示目录中所有文件的大小。
du -s #同--summarize 仅显示总计，只列出最后加总的值。
ddCommands for converting and copying files, but it's different from the copy cp. Mentioned before a very important point, namely all documents about Linux, on Linux, hardware device drivers (such as hard disk) and a special device files (such as /dev/zeroand /dev/random) is like an ordinary file, only to realize in the respective driver corresponding features, dd files can be read or written to the file. In this way, ddit may also be used in the boot sector of the backup hardware, obtaining a certain number of random data or null data tasks. ddCan process both data replication, e.g. endian conversion, or interchanged between ASCII and EBCDIC encoding.
Help command under Linux: help, man, info
Built-in command shell is actually part of the program, which contains some of the more simple Linux system commands that are written in bash source builtins inside and completed within the shell program run by the shell program to identify, usually Linux shell was loaded and reside in the system memory to load the runtime system. And resolve internal command shell do not need to create a child process, so its implementation faster than external commands. For example: history, cd, exit, and so on.
External command is a utility part of the Linux system, because the utility's usually more powerful, so it contains the amount of the program will be great, when the system is not loaded with the system is loaded into memory, but in need when it is transferred to memory. Although it is not included in the shell, but the command execution process which is controlled by a shell program. External commands are additionally installed outside the Bash, usually placed in / bin, / usr / bin, / sbin, / usr / sbin like. For example: ls, vi, etc.
Linux crontab plan tasks
crontab command read instruction from the input device, and stored in crontab file, for later read and executed. Typically, the instructions are stored crontab daemon activated, crond its daemon, crond often run in the background every minute will check whether there is a need to perform a scheduled job.
By crontab command, we can execute the specified system command or shell script script at fixed intervals. Unit time interval can be minutes, any combination of hours, days, months, weeks