8.1 shell introduction

What is shell?

• The shell is a command interpreter that provides interaction between users and machines (command line terminals)
  • A script shell is a manifestation
  • The last section of passwd is the user's shell
• shell supports specific grammars, such as logical judgement, loops
• Each user can have his or her own specific shell
• The default shell for CentOS 7 is bash (Bourne Again Shell)
• And zsh ksh and so on. Generally the same, the details are different.
  • No installation by default

[root@lixiang01 ~]# yum list |grep zsh
autojump-zsh.noarch                     22.3.0-3.el7                   epel     
zsh.x86_64                              5.0.2-25.el7_3.1               updates  
zsh-html.x86_64                         5.0.2-25.el7_3.1               updates  
zsh-lovers.noarch                       0.9.0-1.el7                    epel     
[root@lixiang01 ~]# yum list |grep ksh
ksh.x86_64                              20120801-26.el7                base     
mksh.x86_64                             46-5.el7                       base       

8.2 Order History

[root@ax-01 ~]# history

• History command, record history command
• bash_history in the user's home directory, recording the command history after the last normal exit
  • Abnormal exit incomplete preservation

Modify the number of command history saved

[root@ax-01 ~]# echo $HISTSIZE // Maximum default command history 1000
1000
[root@ax-01 ~]# Vim/etc/profile//where you can adjust environmental variables

[root@ax-01 ~]# source !$
source /etc/profile
[root@ax-01 ~]# echo $HISTSIZE
5000

• The environment variable controls $HISTSIZE to save the maximum save command. Runtime memory can be saved more than 1000 pieces
• History-c clears the command history in memory
• Save to. bash_history after exiting the terminal

Modify the command history display format

[root@ax-01 ~]# HISTTIMEFORMAT="%Y/%m/%d %H:%M:%S"
[root@ax-01 ~]# history
    2  2017/08/30 09:51:25echo $HISTSIZE
    3  2017/08/30 09:51:44vim /etc/profile
    4  2017/08/30 09:53:37echo $HISTSIZE
[root@ax-01 ~]# !vim
vim /etc/profile 

[root@ax-01 ~]# source /etc/profile
[root@ax-01 ~]# !echo
echo $HISTTIMEFORMAT
%Y/%m/%d %H:%M:%S

• HISTTIMEFORMAT="%Y/%m/%d%% H:%M:%S"// / Note that the case modification profile is permanently valid

Permanent Preservation of Order History

[root@ax-01 ~]# chattr +a .bash_history 
[root@ax-01 ~]# lsattr .bash_history 
-----a-------e-- .bash_history

• Not affected by environmental variable HISTSIZE

Common usage of exclamation marks

[root@ax-01 etc]# cd /etc/yum.repos.d/
[root@ax-01 yum.repos.d]# cd ..
[root@ax-01 etc]# !! Execute the last command again.
cd ..
[root@ax-01 /]#
[root@ax-01 /]# cd
[root@ax-01 ~]# history | tail -n4
 1051  cd /etc/yum.repos.d/
 1052  cd ..
 1053  cd
 1054  history | tail -n4
[root@ax-01 ~]# ! 1051 // / Run as history number
cd /etc/yum.repos.d/

• !!// Last command
• ! n // n is the number that runs according to history number
• xx // Execute the last command that started with xx characters

8.3 Command Completion and Aliases

• tab command completion,
  • Knock and auto-complete if subsequent characters are unique
  • If it's not unique, tap twice to see the optional follow-up characters

Parameter Completion,

[root@ax-01 ~]# Yum install-y bash-completion// installation parameter completion package, need to restart
[root@ax-01 ~]# reboot
[root@ax-01 ~]# systemctl restart network
network-online.target  network.service

• Completion of support parameters newly added to CentOS 7. Very practical

Alias alias

[root@ax-01 ~]# alias renet='systemctl restart network'
[root@ax-01 ~]# echo "alias renet='systemctl restart network'" >> .bashrc
[root@ax-01 ~]# unalias renet cancelled
[root@ax-01 ~]# Vim. bashrc deletes definition aliases

• Some system aliases are in files in the / etc/profile.d / directory
• Each user has its own alias profile

8.4 wildcards

• ls *.txt // / denotes arbitrary
• Ls?. TXT // / denotes any one
• ls [0-9].txt// any one in the range of representation
• ls {1,3}.txt // / ibid.

Experiment

[root@lixiang01 ~]# ls *txt*
1_heard.txt.bak  1.txt  22.txt  2.txt.bak  test.txt  word1.txt  Record.txt
[root@lixiang01 ~]# ls 1*
123.tar.bz2  1_heard.txt.bak  1.txt

111:
22.txt
[root@lixiang01 ~]# ls ?.txt
1.txt
[root@lixiang01 ~]# touch 2.txt
[root@lixiang01 ~]# touch 3.txt
[root@lixiang01 ~]# ls ?.txt
1.txt  2.txt  3.txt
[root@lixiang01 ~]# ls [0-3].txt
1.txt  2.txt  3.txt
[root@lixiang01 ~]# ls [123].txt
1.txt  2.txt  3.txt
[root@lixiang01 ~]#  ls [0-9].txt
1.txt  2.txt  3.txt
[root@lixiang01 ~]#  ls [0-9a-zA-Z].txt
1.txt  2.txt  3.txt
[root@lixiang01 ~]# touch d.txt
[root@lixiang01 ~]#  ls [0-9a-zA-Z].txt
1.txt  2.txt  3.txt  d.txt
[root@lixiang01 ~]# ls {1,3,d}.txt
1.txt  3.txt  d.txt

8.5 Input and output redirection

> Output redirection
 "Append"
2 > Error redirection, support additional 2 >
&> Full redirection, support for additions &>>

Distinguish between correct and wrong output information

[root@ax-01 ~]# cat /etc/passwd nosuchthing.txt > right.log 2> err.log
[root@ax-01 ~]# cat err.log 
cat: nosuchthing.txt: No such file or directory