Shell learns 37 days----references

Reference

Case , If I want to output an asterisk (*), How do i use echo ?

echo *

This is certainly not possible , need to transfer the * , that is : Echo \*

This leads to the concept of references . referenced by , is used to prevent Shell to interpret something you want to mean differently. . If you want something that might be Shell things that are treated as individual parameters remain as a single parameter , then you have to refer to it .

Three ways to refer to :

Anti-Slash escape

The character's forward backslash (\) is used to tell The shell that the character is literally meaning .

Single quotation marks

Single quotation marks (' ... ') force the shell to nonalphanumeric all the words between a pair of quotes as their literal meaning . The shell script removes both quotes , leaving only the complete text enclosed :

Command : Echo ' Here is some character *? ' $ \ '

Output :here are some character * ?  '  $ \

You cannot enclose a single quotation mark in a single quoted string . even if the backslash , there's no special meaning in single quotes. ( in some systems , commands like echo ' A\TB ' look like the Shell's special handling of backslashes , not actually , this is Echo The command itself has a special way of handling )

If you want to mix single and double quotes , You can use a backslash escape and a connection with a different reference string to do this :

Command : Echo ' He said ' how ' \ ' s tricks? '

Output : He said "How ' s tricks?"

Command "echo" she replied, \ "Movin ' along\" "

Output : She replied, "Movin ' Along"

Regardless of how it is handled , This combination is always difficult to read .

Double quotes

Double quotation marks ("...") are like single quotes , and the enclosed text is treated as a single string . just , double quotes will handle the exact characters and variables enclosed in the text , arithmetic , command Substitution :

X=hahaha

echo "$x"

Output : Hahaha

In double quotes character $, ", '

When single quotes are enclosed in double quotes, there is no special meaning , and they do not have to be accepted or transferred .

Command : echo "Hahah "

Output : Hahah '

In general , the time to use single quotes is where you want to be completely out of the process . otherwise , when you want to treat multiple words as a single string , but also need Shell doing something for you. , use double quotation marks at this time , For example, a variable value is linked to another variable value :

Oldvar= "$oldvar $newvar"

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Shell learns 37 days----references