Linux shell logical operators, logical expressions detailed introduction _linux shell

One, logical operators

Logical Volume Label	Express meaning
1.	About the files and directories of the detection of logical labels!
-F	Common! Detect the existence of "archives" eg:if [-f filename]
-D	Common! Detect if "directory" exists
-B	Detect whether it is a "block file"
-C	Detect whether it is a "character file"
-S	Detect whether it is a "socket tag file"
-L	Detect if it is a "symbolic link file"
-E	Detect if "something" exists!
2.	About the program's Logical volume label!
-G	Detects whether the program performed by the GID has
-O	Detection is owned by the program executed by the UID
-P	Name pipe or FIFO that detects whether to send information between programs (frankly, this is not very understanding!) ）
3.	About the properties of the file detection!
-R	Detect whether the property is readable
-W	Detect if it is a property that can be written
-X	Detect if it is an executable property
-S	Detection is "not a blank file"
-U	Detect if there is a "SUID" property
-G	Detect if there is a "SGID" property
-K	Detect if a property with "sticky bit"
4.	judgements and comparisons between two files ; for example [Test file1-nt file2]
-nt	The first file is a new one than the second.
-ot	The first file is older than the second file.
-ef	The first file is the same file as the second file (link file)
5.	Logical and (and), or (or)
&&	The meaning of the logical and
||	The meaning of the logical OR

Operational symbols	Representative meaning
=	equals apply to: integer or string comparison if in [], it can only be a string
!=	Not equal to: integer or string comparison if in [], it can only be a 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 apply to: integer comparison
-ne	Not equal to: integer comparison
-lt	Less than applied to: integer comparison
-gt	Greater than applies to: integer comparison
-le	Less than or equal to: integer comparison
-ge	Greater-than or equal to: integer comparison
-A	Both sides set up (and) logical Expressions –a logical expressions
-O	Unilateral establishment (or) logical expression –o logical expression
-Z	Empty string
-N	Non-empty string

second, the logical expression

Test command

How to use: Test EXPRESSION

Such as:

Copy Code code as follows:

[root@localhost ~]# Test 1 = 1 && echo ' OK '
Ok
[Root@localhost ~]# test-d/etc/&& echo ' OK '
Ok
[Root@localhost ~]# Test 1-eq 1 && echo ' OK '
Ok
[Root@localhost ~]# if test 1 = 1; Then echo ' OK '; Fi
Ok

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

Thin expression

Copy Code code as follows:

[] An expression
[Root@localhost ~]# [1-eq 1] && echo ' OK '
Ok
[Root@localhost ~]# [2 < 1] && echo ' OK '
-bash:2: No such file or directory
[Root@localhost ~]# [2 \< 1] && echo ' OK '
[Root@localhost ~]# [2-gt 1-a 3-lt 4] && echo ' OK '
Ok
[Root@localhost ~]# [2-gt 1 && 3-lt 4] && echo ' OK '
-bash: [: Missing '] '

Note: In the [] expression, the common >,< needs to be preceded by an escape character that represents a string size comparison and a Acill code position. <> operator not directly supported, and logical operators | | && it needs to be represented by-a[and]–o[or]

[[]] An expression

Copy Code code as follows:

[Root@localhost ~]# [1-eq 1] && echo ' OK '
Ok
[Root@localhost ~]$ [[2 < 3]] && echo ' OK '
Ok
[Root@localhost ~]$ [2 < 3 && 4 > 5]] && echo ' OK '
Ok

Note: the [[]] operator is just an extension of the [] operator. The ability to support <,> symbolic operations does not require an escape character, it is also a string comparison size. Inside supports logical operators: | | &&

Third, performance comparison

Bash has three almost equivalent symbols and commands in the conditional expression: test,[] and [[]]. Usually, it is customary to use the form of if [];then. and [[]] the appearance, according to ABS, is to be compatible with the operators such as ><. The following is a comparison of their performance and found that [[]] is the fastest.

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

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

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

Without considering the compatibility of low version bash and SH, using [[]] is strong compatibility and performance is faster, you can use this operator when doing conditional operations.