Shell Scripting Tutorial [1] - Shell Variables

Directory : https://blog.csdn.net/shn111/article/details/131590488

Reference tutorial : https://www.runoob.com/linux/linux-shell.html

Online editor : https://www.runoob.com/try/runcode.php?filename=helloworld&type=bash

Shell variables

Variable definitions

name="shn"

Note: There can be no spaces between the variable name and the equal sign

Variable names can only use English letters, numbers and underscores, and the first character cannot start with a number
Keywords in bash cannot be used (use the help command to view reserved keywords)

In addition to explicit direct assignment, you can also use statements to assign values to variables

name=$(ls)
name=`ls`

variable use

To use a defined variable, just add the dollar sign $ before the variable name

name="shn"
echo $name	
echo ${name}

The curly braces outside the variable name are optional here. The curly braces are added to distinguish the boundaries of variables. It is recommended to add curly braces to all variables used

echo "I am ${name}hahaha"

If the above situation is written without curly braces, echo "I am $namehahaha"the variable name cannot be distinguished

Defined variables can be redefined, no need to add $ sign when redefining, only needed when using variables

your_name="tom"
echo $your_name
your_name="alibaba"
echo $your_name

read-only variable

readonlyVariables can be defined as read-only variables using the command

The value of a read-only variable cannot be changed, and an error will be reported when executing the following script

your_name="tom"
readonly your_name
your_name="alibaba"
# script.sh: line 3: your_name: readonly variable

delete variable

unsetVariables can be deleted using the command

After a variable is deleted it cannot be used again. The unset command cannot delete read-only variables.

Executing the following command will not produce any output

your_name="tom"
unset your_name
echo ${your_name}

shell string

String representation can use single quotes, double quotes, or no quotes

apostrophe

name='bob'
str='this is a string'
echo ${str}
# this is a string
str2='hello ${name}'
echo ${str2}
# hello ${name}
str3='this is a \'string'
echo ${str3}
# unexpected EOF while looking for matching `''
str4='this is ''a string'
echo ${str4}
# this is a string

Any characters in single quotes will be output as they are, and variables in single quote strings are invalid
A single single quote cannot appear in a single quote string (even if it is escaped), but it can appear in pairs and used as a string concatenation
To include single quotes enclose it inside a double-quoted string

Double quotes

name="bob"
str="this is a string"
echo ${str}
# this is a string
str2="hello ${name}"
echo ${str2}
# hello bob
str3="this is a \"string"
echo ${str3}
# this is a "string
str4="this is ""a string"
echo ${str4}
# this is a string
str5="this is a\tstring"
echo -e ${str5}
# this is a		string

There can be variables in double quotes
Escape characters can appear in double quotes
echo -e interprets escape characters such as \t \a \b \n

concatenate string

your_name="bob"
# 使用双引号拼接
greeting="hello, "$your_name" !"
greeting_1="hello, ${your_name} !"
echo $greeting  $greeting_1
# hello, bob ! hello, bob !

# 使用单引号拼接
greeting_2='hello, '$your_name' !'
greeting_3='hello, ${your_name} !'
echo $greeting_2  $greeting_3
# hello, bob ! hello, ${your_name} !

get string length

use #get

When the variable is a string, ${#string} is equivalent to ${#string[0]}

string="abcd"
echo ${
    
    #string}
# 4
string="abcd"
echo ${
    
    #string[0]}   
# 4

extract substring

string:n:m
extracts the m characters starting from the nth character in the string string

Note: the index of the first character is 0

string="helloworld"
str2=${string:2:4}
echo ${str2}
# llow

find substring

Find the position of the character i or o (whichever letter comes first is counted)

string="runoob is a great site"
echo `expr index "$string" io`
# 4