Shell programming 01_ Shell foundation

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shell Programming 01_Shell Basics

shell command need shell command interpreter

 

common shell Interpreter
/bin/bash
/bin/sh
/bin/csh
/bin/tcsh

Interpreter ： Translate user instructions into instructions recognized by the kernel
adopt usermod、chsh You can change the login shell

When creating temporary users , If we don't specify an interpreter ,
The default interpreter is /bin/bash

[email protected]:/home/ubuntu/songweibin/shell# useradd test1
[email protected]:/home/ubuntu/songweibin/shell# grep test1 /etc/passwd
test1:x:1001:1001::/home/test1:/bin/sh

usermod  chsh To modify the interpreter

adopt /etc/shells
You can see the support shell Interpreter

[email protected]:/home/ubuntu/songweibin/shell# cat /etc/shells
# /etc/shells: valid login shells
/bin/sh
/bin/bash
/usr/bin/bash
/bin/rbash
/usr/bin/rbash
/bin/dash
/usr/bin/dash

We can also install more shell Interpreter

[email protected]:/home/ubuntu/songweibin/shell# apt-get install tcsh
Reading package lists... Done
Building dependency tree       
Reading state information... Done
The following packages were automatically installed and are no longer required:
  linux-headers-5.11.0-27-generic linux-hwe-5.11-headers-5.11.0-27
  linux-image-5.11.0-27-generic linux-modules-5.11.0-27-generic
  linux-modules-extra-5.11.0-27-generic
Use 'apt autoremove' to remove them.
The following NEW packages will be installed:
  tcsh
0 upgraded, 1 newly installed, 0 to remove and 90 not upgraded.
Need to get 427 kB of archives.
After this operation, 1,363 kB of additional disk space will be used.
Get:1 http://cn.archive.ubuntu.com/ubuntu focal/universe amd64 tcsh amd64 6.21.00-1 [427 kB]
Fetched 427 kB in 7s (61.1 kB/s)                                              
Selecting previously unselected package tcsh.
(Reading database ... 227919 files and directories currently installed.)
Preparing to unpack .../tcsh_6.21.00-1_amd64.deb ...
Unpacking tcsh (6.21.00-1) ...
Setting up tcsh (6.21.00-1) ...
update-alternatives: using /bin/tcsh to provide /bin/csh (csh) in auto mode
Processing triggers for man-db (2.9.1-1) ...

Review the supported shell Interpreter

[email protected]:/home/ubuntu/songweibin/shell# cat /etc/shells
# /etc/shells: valid login shells
/bin/sh
/bin/bash
/usr/bin/bash
/bin/rbash
/usr/bin/rbash
/bin/dash
/usr/bin/dash
/bin/tcsh
/usr/bin/tcsh

test1 The user defaults to shell The interpreter is /bin/bash

Modify the default shell Interpreter method
usermod -s /bin/tcsh test

[email protected]:/home/ubuntu/songweibin/shell# usermod -s /bin/tcsh test1
[email protected]:/home/ubuntu/songweibin/shell# grep test1 /etc/passwd
test1:x:1001:1001::/home/test1:/bin/tcsh

have access to chsh -s /bin/bash  test1   Modify user default shell Interpreter
chsh：change shell It means

[email protected]:/home/ubuntu/songweibin/shell# chsh -s /bin/bash test1
[email protected]:/home/ubuntu/songweibin/shell# grep test1 /etc/passwd
test1:x:1001:1001::/home/test1:/bin/bash

Why does the default support /bin/bash Well ？
because Bash Many functions and features

Shortcut key Tab Key completion
Shortcut key ：
Ctrl + a The front of the command line
Ctrl + e At the end of the command line
Ctrl + C The currently executed command is revoked
Ctrl + l Clear the screen
Tab Key completion ： Command-Line Completion Options can be supplemented

Command history （history）
Read the history through the up and down buttons

Alias of command (aliase)
ll= ls-l

Redirection of standard input and output （>、 >>、2>、2>>、&>、 &>>）

> : Correct information redirection
2>: Wrong message redirection
&>: Both correct information and wrong information

[email protected]:/home/ubuntu/songweibin# ls
42fputcandwrite  84fatherandsoncommunication    fl
82testpipe       85brotherscommunication        shell
83mypipe         87fileforprocesscommunication  test
[email protected]:/home/ubuntu/songweibin# ls > a.txt
[email protected]:/home/ubuntu/songweibin# cat a.txt
42fputcandwrite
82testpipe
83mypipe
84fatherandsoncommunication
85brotherscommunication
87fileforprocesscommunication
a.txt
fl
shell
test

Why do you still have a.txt How about myself ？

ls > a.txt Commands are created first a.txt file
And then ls Enter the results of into a.txt in

If you use two greater than signs , Yes means to add
It is to add content on the basis of the original content

[email protected]:/home/ubuntu/songweibin# ls >>a.txt
[email protected]:/home/ubuntu/songweibin# cat a.txt 
42fputcandwrite
82testpipe
83mypipe
84fatherandsoncommunication
85brotherscommunication
87fileforprocesscommunication
a.txt
fl
shell
test
42fputcandwrite
82testpipe
83mypipe
84fatherandsoncommunication
85brotherscommunication
87fileforprocesscommunication
a.txt
fl
shell
test
[email protected]tu-virtual-machine:/home/ubuntu/songweibin# 

Use 2> Export the error information to the specified file

[email protected]:/home/ubuntu/songweibin# ls song.txt
ls: cannot access 'song.txt': No such file or directory
[email protected]:/home/ubuntu/songweibin# 

Use > Only correct information can be output .

[email protected]:/home/ubuntu/songweibin# ls song.txt > a.txt
ls: cannot access 'song.txt': No such file or directory
[email protected]:/home/ubuntu/songweibin# cat a.txt

Use 2> Export the error information to the specified file

[email protected]:/home/ubuntu/songweibin# ls song.txt 2> a.txt
[email protected]:/home/ubuntu/songweibin# cat a.txt
ls: cannot access 'song.txt': No such file or directory

You can also use &> ( Also recreate the file , Will be covered ) It can output correct and wrong information at the same time

single >: All are re created output files , So it will cover
double >： All are additions

The Conduit (|)

The output of the first command can be used as the input of the second command

Search for installed software ：

[email protected]:/home/ubuntu/songweibin# apt-cache search all|grep tcsh
tcsh - TENEX C Shell, an enhanced version of Berkeley csh
[email protected]:/home/ubuntu/songweibin# 

apt-get install ksh
Add a new interpreter

[email protected]:/home/ubuntu/songweibin# cat /etc/shells
# /etc/shells: valid login shells
/bin/sh
/bin/bash
/usr/bin/bash
/bin/rbash
/usr/bin/rbash
/bin/dash
/usr/bin/dash
/bin/tcsh
/usr/bin/tcsh
/usr/bin/ksh2020
/usr/bin/rksh2020

  adopt ksh Use it directly ksh Interpreter
 
 ksh Only through clear Lai Qingping
  Unable to get Ctrl+l
 
 
 ksh Can't use direction keys So by default /bin/bash
 

# ^[[C^[[C^[[C^[[C^[[C^[[D^[[D^[[D^[[D^[[A^[[A^[[A^[[B^[[B

 
ksh adopt exit Return to the last default interpreter

# exit
[email protected]:/home/ubuntu/songweibin# 

Shell The manner in which the order is executed

1. Interactive （ Command line ）
Artificial intervention
Explain and execute one by one Low efficiency

2. Non interactive （ Script ）
It needs to be designed in advance
Batch execution Efficient