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.