Lesson 3 shell syntax

An introduction to
shell It is the language that we communicate with the operating system through the command line .

shell Scripts can be executed directly on the command line , You can also organize a set of logic into a file , Convenient reuse .
AC Terminal The command line in can be regarded as a “shell The script executes line by line ”.

Linux Common in shell There are many kinds of scripts , Common are ：

Bourne Shell(/usr/bin/sh or /bin/sh)
Bourne Again Shell(/bin/bash)
C Shell(/usr/bin/csh)
K Shell(/usr/bin/ksh)
zsh
…
Linux Generally, the system uses... By default bash, So let's talk about bash Chinese grammar .
The beginning of the file needs to be written #! /bin/bash, To specify bash For the script interpreter .

Script example
Create a new one test.sh file , The contents are as follows ：

#! /bin/bash
echo "Hello World!"

Pay attention to save after writing

Operation mode
As an executable

[email protected]:~$ chmod +x test.sh  #  Make the script executable 

Be careful +x Must be written together

Defining variables        

The general definition is string

Variable type
1, Custom variable （ local variable ）
Variables that cannot be accessed by child processes
2, environment variable （ Global variables ）
Variables accessible to child processes

Enter a bash, Enter subprocess , The previous process went into sleep ,exit sign out

character string
Strings can be in single quotes , You can also use double quotes , You can also use no quotes .

The difference between single quotation marks and double quotation marks ：

• The contents in single quotation marks will be output as is , Not execute 、 Will not take variables ;
• The contents in double quotation marks can execute 、 Variable can be taken ;

Be careful ： Use of escape characters , The effect of not adding quotation marks is the same as that of double quotation marks

Extract substring

name="hello, yxc"
echo ${name:0:5}  #  Extract from 0 At the beginning 5 Characters 

Pay attention to subscripts from 0 Start  

  Other parameters and related variables

Parameters        explain

 $*          A space separated string consisting of all parameters , As in the above example, the median value is "$1 $2 $3 $4"
[email protected]          Each parameter is a string enclosed in double quotation marks , As in the above example, the median value is "$1" "$2" "$3" "$4"

Be careful ：

When $* and [email protected]  Not double quoted " " When surrounded , There's no difference between them , Each parameter received is regarded as a piece of data , They are separated by spaces .

But when they're double quoted " " Inclusion time , There will be a difference ：

• "$*" All parameters will be regarded as one piece of data as a whole , Instead of treating each parameter as a piece of data .
• "[email protected]" Still treat each parameter as a piece of data , They are independent of each other .

For example, it was delivered 5 Parameters , So for "$*" Come on , this 5 The parameters will be combined to form a data , They are inseparable ; And for "[email protected]" Come on , this 5 The parameters are independent of each other , They are 5 Copy of the data .

If you use echo Direct output "$*" and "[email protected]" comparing , There is no difference ; But if for Loop to output data one by one , You can see the difference immediately .

Parameters         explain

$?            Exit status of the last command （ Notice that it's not stdout, It is exit code）.0 Indicates normal exit , Other values indicate an error  

such as C++/C Medium return 0

Parameters                         explain

 $(command)          return command This order is stdout（ Can be nested ）
`command`            return command This order is stdout（ Cannot be nested ）

give an example ： 

String expressions

length STRING
return STRING The length of
index STRING CHARSET
CHARSET in arbitrarily Single character in STRING in front Character position of , Subscript from 1 Start . If in STRING It doesn't exist at all CHARSET in The characters of , Then return to 0.
substr STRING POSITION LENGTH
return STRING String from POSITION Start , Maximum length LENGTH The string of . If POSITION or LENGTH It's a negative number ,0 Or non numeric , Returns an empty string .

Example ：

str="Hello World!"

echo `expr length "$str"`  # `` Not single quotes , Indicates that the command is executed , Output 12
echo `expr index "$str" aWd`  #  Output 7, Subscript from 1 Start 
echo `expr substr "$str" 2 3`  #  Output  ell

Be careful ： The reason $str Put it in quotation marks , Because Hello World There's a space in the middle , The default before the space is a string . So enclose it in quotation marks to ensure its integrity , That is, all are one string .

for…in…do…done

Example 3, Output 1-10

for i in $(seq 1 10)
do
    echo $i
done

Example 4, Use {1..10} perhaps {a..z}

for i in {a..z}
do
    echo $i
done

Be careful :

(1)seq It means traversing a sequence , You can write directly in the terminal

(2){} You can't write directly in the terminal