Provides an operational expression in the following format: $ ((expression))
$ echo $ ((5* (3+3))
30
$ result = $ (($myvar-10))
The shell provides a convenient number of conversions between:
$ echo $ ((013)) #八进制
$ echo $ ((0xa4)) #十六进制
You can also specify any system from 2 to 64 using the following format:
$ ((base#number))
echo $ ((8#377))
echo $ ((16#D8))
Another trick to make a conversion in a Shell is to use BC, an arbitrary precision operating language that most UNIX installers provide. Because it allows you to specify the output, this is a good technique when you need to export to a decimal system.
The special variables ibase and obase of BC contain the values for inputs and outputs, respectively. By default, all are set to 10. To perform a transform, you need to change one or two of these values, and then provide a number.
Copy Code code as follows:
$ Echo ' obase=16; 47 ' | Bc
2F
$ Echo ' obase=10; ibase=16; A03 ' | Bc
2563
With these basic features in view, the logical operators and logical expressions are looked at below, and the basic +–*/% is no longer to repeat.
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:
[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
[] 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
[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.
Iv. bitwise operator Operators
| Operator |
Name |
Example |
To interpret value |
| << |
Move left |
Value=4>>2 |
4 Left 2 digits, value 16 |
| >> |
Move right |
Value=8<<2 |
8 Right 2 bit, value 2 |
| & |
Bitwise AND |
Value=8&&4 |
8 bitwise AND 4,value value is 0 |
| | |
by bit or |
Value=8|4 |
8 bitwise OR 4,value value is 12 |
| ~ |
Bitwise NON |
Value=~8 |
Bitwise non-8,value value is-9 |
| ^ |
Per-bitwise XOR OR |
Value=10^3 |
10 bitwise XOR or 3,value value of 9 |
Note: For bitwise non, if "~a" then the result is-(a+1) Give a detailed example: Ask "~8" analysis because the computer usually
Use complement for symbolic operation, [[x] complement] =[x] so
Then 8 binary is 00001000 to 11110111.
The reverse code is 1001000 for the complement of 1001001 so the final result is 1001001,~8-9.
