Shell script structure, date command, variable

20.5 logical judgments in shell scripts

Logical Expressions

In the [] bracket:

-lt:=little than less than
-le:=little && equal less than equals
-eq:=equal equals
-ne:=no Equal Not equal to
-gt:=greater than greater than
-ge:=greater && equal greater than or equal
In (()) parentheses:
<,<=,==,!=,>,>=

Note: Use double brackets to enclose

Format 1

if condition; then Commond;fi

eg

#!/bin/bash
A=5
If [$a-GT 3]
Spaces within the #注意 []
Then
echo "OK"
Fi
Format 2

If condition 1;then commond1;else commond2;fi

eg

#!/bin/bash
A=5
If [$a-GT 3]
Spaces within the #注意 []
Then
echo "OK"
Else
echo "Fault"
Fi
Format 3

If condition 1;then commond1;elif condition 2;then commond2;else commond3;fi

eg

#!/bin/bash
A=5
If [$a-lt 3]
Spaces within the #注意 []
Then
echo "A<3"
elif [$a-GT 6]
Then
echo "A>6"
Else
echo "Out of the zone"
Fi
Relationship

The relationships between the various conditions can use logical connectors:

Condition a&& condition B: and
Condition a| | Condition B: or

20.6 File Directory property judgment

The IF in a shell script is often used to determine the properties of a document, such as whether it is a normal file or a directory file, to determine if the file has read, write, execute permissions, and so on. The If common options are the following:

-E: Determine if a file or directory exists
-D: Determine if the directory file is not present
-F: Determine if the file is not normal and whether it exists
-r: Determine if Read permission is available
-W: Determine if Write permission is available
-X: Determine if EXECUTE permission is available
Format

If a file exists:

If [-e filename]
Then
Commond
Fi
The above commands can be simplified to:

[-e filename] && commond
&& before and after the life is executed

Or:
[-e FileName] | | Commond
When | | The previous execution succeeds before executing the subsequent command
If a file does not exist:

if [!-e FileName]
Then
Commond
Fi
“！ "is equivalent to taking the reverse.

20.7 If special usage

If [-Z ' $a]: Indicates what happens when the value of variable A is empty
If [-n ' $a]: Indicates what happens when the value of variable A is not empty
-Z and-N are the opposite two counter-conditions.

eg

#!/bin/bash
n=wc -l /tmp/test.txt
If [$n-gt 20]
Then
Echo 1
Else
Echo 0
Fi
There is no syntax error in this script, only we preset/tmp/test.txt is present, if the file does not exist, the script will be executed with an error:

[Email protected] sbin]# sh if.sh
WC:/tmp/test.txt: No file or directory
if.sh: Line 3rd: [:-gt: Expecting unary expression
Therefore, in order to avoid this error, you need to write a more rigorous script, you need to do "if [$n-gt 20]" before the implementation of the file "/tmp/test.txt" to confirm the existence of:

#!/bin/bash
n=wc -l /tmp/test.txt
If [-Z $n]
Then
echo "Error"
Exit
elif [$n-lt 20]
Then
Echo 1
Else
Echo 0
Fi
That is, if the variable n is empty, the error is displayed and the script exits.

[Email protected] sbin]# sh if.sh
WC:/tmp/test.txt: No file or directory
Error
That is, when the file does not exist, it exits execution without prompting for a syntax error. (There is a logic error in the script, only the effect is shown)

Note: Use double quotation marks when referencing variables in this expression.

If grep ' 123 ' Test.txt;then: Indicates what happens when the test.txt contains 123
eg

To determine the existence of a parameter:
[Email protected] sbin]# vim if1.sh
#!/bin/bash
If
Grep-wq ' user1 '/etc/passwd
Then
echo "User1 exist."
Fi
[Email protected] sbin]# sh if1.sh

To determine that a parameter does not exist:
[Email protected] sbin]# vim if2.sh
#!/bin/bash
If
! Grep-wq ' user1 '/etc/passwd
Then
echo "No User1"
Fi
[Email protected] sbin]# sh if2.sh
No User1
Description: The-w option =word in grep, which means filtering a word;-Q, which means that the results of filtering are not printed. Use when judging that a parameter does not exist! Represents the inverse.

20.8-20.9 Case Judgment

Format:

Case variable name in
value1)
Commond1
;;
value2)
Commod2
;;
VALUE3)
Commod3
;;
Esac
In a case, you can use a "|" in a condition to denote or mean, such as:

2|3)
Commond
;;
eg

[Email protected] sbin]# vim case1.sh
#!/bin/bash
Read-p "Please input a number:" N
If [-Z "$n"]
Then
echo "Please input a number."
Exit 1
# "Exit 1" indicates the return value after executing the partial command
#即, use the value of ECHO $ after the command has finished executing
Fi

n1=echo $n|sed ‘s/[0-9]//g‘
#判断用户输入的字符是否为纯数字
#如果是数字, it is replaced with a null, assigned to $N1
If [-N "$n 1"]
Then
echo "Please input a number."
Exit 1
#判断 $n 1 is not empty (that is, $n is not a pure number) again prompts the user to enter a number and exit
Fi

If the user enters a pure number, execute the following command:
If [$n-lt] && [$n-ge 0]
Then
Tag=1
elif [$n-ge] && [$n-LT 80]
Then
tag=2
elif [$n-ge] && [$n-LT 90]
Then
Tag=3
elif [$n-ge] && [$n-le 100]
Then
Tag=4
Else
Tag=0
Fi
#tag的作用是为判断条件设定标签, conveniently referenced later
Case $tag in
1)
echo "Not OK"
;;
2)
echo "OK"
;;
3)
echo "Ook"
;;
4)
echo "Oook"
;;
*)
echo "The number range is 0-100."
;;
Esac
Description: The purpose of this script is to enter test scores and determine the grade of test scores.

Execution Result:

[Email protected] sbin]# sh case1.sh
Please input a number:90
Oook
[Email protected] sbin]# sh case1.sh
Please input a number:80
Ook
[Email protected] sbin]# sh case1.sh
Please input a number:60
Ok
[Email protected] sbin]# sh case1.sh
Please input a number:55
Not OK

Shell script structure, date command, variable