Linux shell 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: | | &&

Third, performance comparison

There are three almost equivalent symbols and commands in the conditional expression of bash: test,[] and [[]]. Usually, everyone is accustomed to using if [];then this form. The appearance of [[]], according to ABS, is to be compatible with operators such as ><. The following is a comparison of their performance, found [[]] is the fastest.

$ time (for M in {1..100000}; do test-d.; Done;)
Real 0m0.658s
User 0m0.558s
SYS 0m0.100s

$ time (for M in {1..100000}; do [-D.]; Done;)
Real 0m0.609s
User 0m0.524s
SYS 0m0.085s

$ time (for M in {1..100000}; do [[-D.]]; Done;)
Real 0m0.311s
User 0m0.275s
SYS 0m0.036s

Regardless of the low version of Bash and the compatibility of SH, with [[]] is strong compatibility, and performance is relatively fast, in the case of a conditional operation, you can use the operator.