Shell string

Single quotation marks

• Any character in a single quotation mark will be output as is , Variables in a single quoted string are invalid ;
• A single quotation mark cannot appear in a single quotation mark string （ You can't escape a single quotation mark ）, But in pairs , Use as string concatenation .

Double quotes

• You can have variables in double quotes
• Escape characters can appear in double quotes  

yours="runoob"
strs="Hello,I know you are \"$yours\"!"
echo $strs

  Concatenate characters ( Double quotes 、 Single quotation marks )

// Double quotes 
name="runoob"
string="hello,"$name"!"
string_2="hello,${name} !"
echo $string $string_2

// Single quotation marks 
name="runoob"
string='hello,'$name'!'
string_2='hello,${name} !'
echo $string $string_2

Get string length

string="abcd"
echo ${#string}