Linux history, tab, alias, command execution sequence, pipe character and exit

parameter	effect
-c	Clear history command
-w	Write the historical commands in the cache to the historical command save file. If you do not manually specify the history command save file, put it into the default history command save file ~/.bash_history

Modify the default number of recorded historical commands

[root@localhost ~]# vim /etc/profile

Case

Case 1 --- Display the top 10 highest occurrences in history records

 [root@localhost ~]# history | tr -s " " | cut -d " " -f3 | sort | uniq -c |sort -nr | head -10

浏览命令 | 压缩为1个空格 | 截取以空格为分割的第3部分 | 排序 | 统计并去重 | 降序数字排序| 显示前10个

Case 2 --- Add time information displayed in history

[root@localhost ~]# vim ~/.bashrc    # 定位最后一行增加一行内容
export  HISTTIMEFORMAT="%Y-%m-%d %H:%M:%S: "

[root@localhost ~]# source ~/.bashrc    # 刷新配置
[root@localhost ~]# history

Command and file name completion --- tab

command alias

Format

alias alias=original command

Case

[root@localhost ~]# alias hi=history

[root@localhost ~]# hi

Notice

Aliases have higher priority than commands

第一顺位：执行用绝对路径或相对路径执行的命令。
第二顺位：执行别名。
第三顺位：执行 Bash 的内部命令。
第四顺位：执行按照 $PATH 环境变量定义的目录查找顺序找到的第一个命令。

In order to make this alias permanent, you can write the alias into the environment variable configuration file "~/.bashrc

[root@localhost ~]# vim ~/.bashrc 

# 在最下面增加

Command execution order

sequential execution

command 1; command 2

[root@localhost ~]# date ; ls -l /etc/passwd

If the previous command is not executed successfully, the following command will not be executed.

Command 1 && Command 2

[root@localhost ~]# mkdir /mnt/iso && mount /dev/sr0 /mnt/iso

If the previous command is successful, it will not be executed later. If the previous command is unsuccessful, it will be executed later.

Command 1 || Command 2

[root@localhost ~]# mkdir tt || ls /

[root@localhost ~]# mkdir tt || ls /    # 可以再次执行

pipe character

When a pipe is set between two commands, the output of the command on the left of the pipe symbol | becomes the input of the command on the right. As long as the first command writes to standard output and the second command reads from standard input, the two commands can form a pipe

symbol

Command 1 | Command 2

Case

Extract the IP address of the system network card

[root@localhost ~]#  ip  a  |  grep  ens160 | grep  inet | tr -s " " | cut -d  / -f1 | cut -d " " -f3

显示IP信息 | 行向过滤包含ens60 | 行向过滤inet | 压缩为一个空格 | 以/为分割依据，取第一部分 | 以空格为分割依据,取第3部分

Display the remaining memory capacity of the host

[root@localhost ~]# free -h |grep Mem | tr -s " " | cut -d " " -f4

exit exit program

effect

Terminate the execution of the shell program

Format

exit status code

status code

This parameter is an integer value, its value range is 0~255

Notice:

The exit status code of the Shell program is stored in the system variable $?. Therefore, the user can obtain the exit status code returned by the Shell program to the parent process through this variable.

Common status codes

	0----------------命令运行成功
    1----------------通知未知错误
    2----------------误用shell命令
    126-------------命令不可执行
    127-------------没有找到命令
    128-------------无效退出参数
    128+x-----------linux信号x的严重错误
    130--------------命令通过Ctrl+C终止
    255--------------退出状态码越界

Case

Demonstrates that the program returns different status codes under different circumstances

[root@localhost ~]# echo "china"
[root@localhost ~]# echo $?

[root@localhost ~]# ehco "china"
bash: ehco: command not found...
Similar command is: 'echo'
[root@localhost ~]# echo $?