Shell Learning Note 1

Brief description of single quotes, double quotes, and unquoted

Name	Explain
Single quotation marks	What you see is what you get: Everything inside a single quote is output as it is, or what you see in single quotes.
Anti-Quote	Command substitution, which refers to the ability of the shell to interpolate the standard output of a command anywhere on a command line.
Double quotes	Output all the contents of the double quotation marks, and if there are commands in the content (to be reversed), variables, special escape characters, etc., the variables, commands are parsed out, and then the final content is output.
No quotation marks	Output of the content, the string containing the space as a whole output, if the contents of the command, variables, etc., will first parse the variables, commands, and then output the final content, if the string with a space and other special characters, you can not completely output, need to add double quotation marks, general continuous string, number, The path can be without any quotation marks, but it is best to use double quotes instead of quotation marks.
Tips	The conclusion here is only empirical, and may not be appropriate for some languages, for example: there is a peculiarity within awk.

Example:

[Email protected] ~]# x=*

[Email protected] ~]# echo $x

Public template video Picture document download music Desktop

[Email protected] ~]# echo ' $x '

$x

[Email protected] ~]# echo "$x"

*

Example of anti-single quotes:

echo "Date +%y%m%d" double quotes

echo "date +%y%m%d" double quotes

Date +%y%m%d

echo ' date +%y%m%d ' single quote

Date +%y%m%d

echo ' Date +%y%m%d ' inverted single quote

20160811

Shell Learning Note 1