Shell's If statement

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.

Four, bitwise operator

Operator	Name	Example	Interpret value
<<	Move left	Value=4>>2	4 Shift left 2 bits, value 16
>>	Move right	Value=8<<2	8 Move right 2 bit, value is 2
&	Bitwise-AND	Value=8&&4	8 Bitwise AND 4,value values of 0
|	Bitwise OR	Value=8|4	8 bitwise OR 4,value value of 12
~	Bitwise non-	Value=~8	Bitwise non-8,value value is-9
^	Bitwise XOR OR	Value=10^3	10 bitwise XOR or 3,value value is 9

Note: For bitwise non, if the "~a" result is-(a+1) to give a detailed example: "To" analysis because the computer is usually
Sign operation with complement, [[[x] complement] complement =[x] so
Then 8 of the binary is 00001000 for non-11110111
The negation code is 1001000 for the complement of 1001001 so the final result is 1001001,~8-9.

Note: spaces are required within [] in the IF statement

If [$num-eq 100];then #错误

If [$num-eq100];then #错误

If [$num-eq100];then #错误

If [$num-eq];then #正确

If single Branch

[email protected] ~]$ cat test3.sh
#!/bin/bash

Read-p "Enter your score:" num
If [$num-eq];then
echo "Congratulations on your perfect score."
Fi
[Email protected] ~]$

[Email protected] ~]$./test3.sh
Enter your score: 100
Congratulations on getting full marks.
[Email protected] ~]$

If dual Branch

[email protected] ~]$ cat test4.sh
#!/bin/bash

Read-p "Enter your score:" num
If [$num-ge];then
Echo, "you passed."
Else
Echo, "You failed."
Fi
[Email protected] ~]$
[Email protected] ~]$./test4.sh
Enter your score: 70
You passed the exam.
[Email protected] ~]$./test4.sh
Enter your score: 59
You didn't pass.
[Email protected] ~]$

If multi-branch

[email protected] ~]$ cat test5.sh
#!/bin/bash

Read-p "Enter your score:" num
If [$num-ge 60-a $num-lt];then
Echo, "you passed."
elif [$num-eq];then
echo "Congratulations on your perfect score."
elif [$num-lt 60-a $num-ge 0];then
Echo, "You failed."
Else
echo "Please re-enter"
Fi
[Email protected] ~]$

[Email protected] ~]$./test5.sh
Enter your score: 100
Congratulations on getting full marks.
[Email protected] ~]$./test5.sh
Enter your score: 99
You passed the exam.
[Email protected] ~]$./test5.sh
Enter your score: 59
You didn't pass.
[Email protected] ~]$./test5.sh
Enter your score:-1
Please re-enter

This article is from the "Xiao Mo" blog, please be sure to keep this source http://xiaofengmo.blog.51cto.com/10116365/1744961

Shell's If statement