Shell Programming--variables

First, define variables

Define variable names without a $ (unlike PHP, as with Python), with no spaces between variable names and "=" (special, unlike other languages)

Example: x= "Hello World"

Note: The naming of variable names should follow the following rules:

• The first character must be a letter (a-z,a-z).
• You can use an underscore (_) without spaces in the middle.
• Punctuation cannot be used.
• You can't use the keywords in bash (you can see the reserved keywords using the help command).

Referencing a defined variable requires the addition of a $ character

Such as:

x= "Hello World"

Echo ${x}

Second, read-only variables

Defined with the ReadOnly command, such as:

x= "Hello World"

ReadOnly x

The value of x cannot be changed at this time

Third, delete the variable

Use the unset command to delete a variable, but the unset command cannot delete a read-only variable

Iv. getting the string length

string= "ABCD"

echo ${#string} #输出 4

Five, find sub-characters

String="Runoob is a great company"

echo ' expr index ' $string ' is ' # output 8

Six, extracting substrings

The following instance intercepts 4 characters starting with the 2nd character of the string:

String="Runoob is a great site"

echo ${string:1:4} # output Unoo

Seven, reading the array

Valuen=${array_name[n]}

Read all the elements in the array:

Use the @ sign, echo ${array_name[@]}

Shell Programming--variables