Linux Shell Programming (4): Logical operators, logical expressions

The logical operators of the shell involve the following types, so as long as the appropriate choice, can solve our many complex judgments, to achieve a multiplier effect.

First, the logical operator

Logical Volume Label	Express meaning
1.	About the file and directory of the detection of Logical volume label!
-F	Common! Detect if "Archive" exists eg:if [-f filename]
-D	Common! Detects if the directory exists
-B	Detects if it is a "block file"
-C	Detects if it is a "character file"
-S	Detects if it is a "socket tag file"
-L	Detects if it is a "symbolic Link's profile"
-E	Detect if "something" exists!
2.	About the logical volume label of the program!
-G	Detect whether the program owned by the GID
-O	Detection is owned by the program executed by the UID
-P	Detects if a name pipe or FIFO is being transmitted between programs (to be honest, this is not a good idea!). ）
3.	About the property detection of the file!
-R	Detect if a property is readable
-W	Detects if a property can be written
-X	Detect whether the property is executable
-S	Detects if it is a "non-blank file"
-U	Detect if a property with "SUID" is
-G	Detect if a property with "SGID" is
-K	Detect if a property with "sticky bit" is
4.	judgments and comparisons between two files ; for example [Test file1-nt file2]
-nt	The first file is newer than the second one.
-ot	The first file is older than the second one.
-ef	The first file is the same file as the second file (link and other files)
5.	Logical AND (and) "or (OR)"
&&	The meaning of the logical and
||	The meaning of a logical OR

Operation symbols	Representative meaning
=	Equal to: Integer or string comparison if in [], only the string
!=	Not equal to: integer or string comparison if in [], only the string
<	Less than applied: integer comparison in [], cannot use the representation string
>	Greater than applied: integer comparison in [], cannot use the representation string
-eq	equals applies To: integer comparison
-ne	Not equal to: integer comparison
-lt	Less than applied to: integer comparison
-gt	Greater than applied to: integer comparison
-le	Less than or equal to: integer comparison
-ge	Greater than or equal to: integer comparison
-A	Both sides established (and) logical Expressions –a logical expressions
-O	Single-sided (or) logical expression –o logical expression
-Z	Empty string
-N	Non-empty string

Ii. Logical Expressions

• Test command

How to use:test EXPRESSION

Such as:

[[email protected] ~]# Test 1 = 1 && echo ' OK '
Ok

[[email protected] ~]# test-d/etc/&& echo ' OK '
Ok

[[email protected] ~]# Test 1-eq 1 && echo ' OK '
Ok

[[Email protected] ~]# if test 1 = 1; Then echo ' OK '; Fi
Ok

Note: All characters are separated from the logical operators directly with "spaces" and cannot be joined together.

• Thin expressions

• [] Expression

[[Email protected] ~]# [1-eq 1] && echo ' OK '
Ok

[[Email protected] ~]# [2 < 1] && echo ' OK '
-bash:2: No such file or directory

[[Email protected] ~]# [2 \< 1] && echo ' OK '

[[Email protected] ~]# [2-GT 1-a 3-lt 4] && echo ' OK '

OK

[[Email protected] ~]# [2-GT 1 && 3-lt 4] && echo ' OK '
-bash: [: Missing '] '

Note: In the [] expression, the common >,< need to add an escape character, which represents the string size comparison, as compared to the Acill code position. Do not directly support the <> operator, there are logical operators | | && it needs to be represented by-a[and]–o[or]

• [[]] expressions

[[Email protected] ~]# [1-eq 1] && echo ' OK '
Ok

[[Email protected] ~]$ [[2 < 3]] && echo ' OK '
Ok

[[Email protected] ~]$ [[2 < 3 && 4 > 5]] && echo ' OK '
OK

Note: the [[]] operator is only an extension of the [] operator. The ability to support the <,> symbol operation does not require an escape character, it is also a string comparison size. Logical operators are supported inside: | | &&

Linux Shell Programming (4): Logical operators, logical expressions