Conditional judgment and comparison of shell scripts

common syntax for conditional testing:

Syntax format	Description
[< test expression >]	[] has at least one space between the boundary and the content
[[ < Test expression > ]]	[[] There is at least one space between the boundary and the content

Note that the:&&, | |, >, < operators can be applied to [[]], but not to [], and in [] generally use-A,-O,-GT (for integers) instead of the above operators

Commonly used file-judging operators

-D files, all of which are spelled as directory	The file exists and is true for the directory, that is, the test expression is set
-f file, F's full spell file	File exists and is normal file True, that is, the test expression is set
-e file, the full spell for exist	The existence of the file is true, that is, the test expression is established. -E does not distinguish between a directory or a file
-r/w/x file, r/w/x's full spell is read/write/executable	File exists and readable/writable/executable, i.e. test expression established
-S file, S is fully spelled as size	True if the file exists and the file size is not 0
-l file, L's all spelled as Link	The file exists and is true for the linked file
F1-nt F2,nt's full spell is newer than	File F1 newer than file F2 true
F1-ot F2,ot's full spell is older than	File F1 is true compared to file F2 old

Commonly used string test expressions

-N "String"	True if the length of the string is not 0
-Z "string"	True if the length of the string is not 0

operator comparison

Comparison symbols used in test and []	Comparison symbols used in (()) and [[]]	Description
-eq	= = or =	Equals
-ne	!=	Not equal to
-gt	>	Greater than
-ge	>=	Greater than or equal
-lt	<	Less than
-le	<=	Less than or equal
-A	&&	And
-O	||	Or
!	!	Non -

Conditional judgment and comparison of shell scripts