The third day of linux Shell Study Notes

Day 3: Select Conditions

Outline

Application Instance Analysis

Conditional Test

If... Else... Fi

Case... In... Esac

Implementation menu:

After the script is executed

Press 1 to display the current time

Press 2 to display the CPU load

Press 3 to display the remaining memory

Press 0 to exit the script.

Press other characters to exit after the selected range is exceeded.

Analysis Steps.

# Date + % T

Uptime awk Interception

Free-m

Conditional test format

# Test-option obj

# [-Option obj]

Returned results

1. The expression content test result is true.
2. The expression content test result is false.

Test object category

Execution result (successful or failed)

File (whether the file exists)

Text (consistency)

Number (numerical comparison)

Option for conditional test

Option	Function
-D	Directory
-E	Exist?
-F	Whether it is a common file
-S	Whether the file size is equal to 0
-R	Readable or not
-W	Writable or not
-X	Executable or not

Logical operation symbol

Option	Function
-	And operations
-O	Or operation
!	Non-operation

Instance:

# Test-e/etc/passwd-a-e/etc/shadow is in the middle of a and operation, then 0 is required

# If test-e/etc/passwd-o-e/etc/groups is an o or operation in the middle, the true result is 0.

String Operators

= Two strings are equal

! = Two strings are not equal

-Z empty string

-N non-null string

Instance:

# Test-z $ LOGNAME

# Echo $ LOGNAME

# Echo $?

Numeric comparison operator

Symbol	Description
-Eq	Equal
-Ne	Not equal
-Gt	Greater
-Lt	Less
-Ge	Greater than or equal
-Le	Less than or equal

If... Else... Fi Condition Selection

The basic format of the if control structure:

If condition # It can be a command or a test statement.

Then # execute if the condition is true and the negative value is true.

Command 1 # Run Command 1

Else # execute if the condition is false anti-value false 1

Command 2 # Run Command 2

Fi # End of judgment

Instance (if... Else... Fi) 1

Inputtest. sh

#! /Bin/bash

# Input test

Echo-n "Enter your name :"

Read name

# Did the user just hit return

If ["$ {name}" = ""]

Then

Echo "You did not enter any information"

Else

Echo "Your name: $ {name }"

Fi

Instance (if... Else... Fi) 2

Filecopy. sh

#! /Bin/bash

# File copy

If cp/etc/passwd. bak 2>/dev/null 2>/dev/null discard error message

Then

Echo "Good Copy !"

Else

Echo "'basename $ 0': error, cocould not copy"

Fi

If... Else... Nested fi (two-layer nesting)

If condition 1; then

If condition 2; then

Command 1

Else

Command 2

Else

If Condition 3; then

Command 3

Else

Command 4

Fi

Case... In... Esac Condition Selection (more flexible)

Case statements are mostly used for judgment of multiple branches.

Case format: (multiple modes match only the same pattern as variable)

Case variable in

Mode 1) command 1 ...;;

Mode 2) command 2 ...;;

Esac

Matching Mode

* Match any character

? Match any single character

[] Matching character range

Case... In. esac instance 1

#! /Bin/bash

# Case select

Echo-n "enter a number from 1 to 5 :"

Read NUM

Case $ NUM in

1) echo "you select 1 ″;;

2) echo "you select 2 ″;;

3) echo "you select 3 ″;;

4) echo "you select 4 ″;;

5) echo "you select 5 ″;;

*) Echo "basename $ This is not between 1 and 5 ″

Esac

Case... In. esac instance 2

The question is: the student's score is 0-. If the score is above 85, you must be prompted that you are the best !, Show you get a good mark in 70-84! , Show come on at 60-74 !, You must study hard is displayed below 60 points!

#! /Bin/bash

Echo-n "please input your mark :"

Read mark

Case $ mark in

100 | 9 [0-9] | 8 [5-9]) echo "you are the best !";; 100, 90-99, 85-89

8 [0-4] | 7 [0-9]) echo "you get a good mark !";; 80-84, 70-79

7 [0-4] | 6 [0-9]) echo "come on !";; 70-74, 60-69

[0-5] [0-9]) echo "You must study hard !";; 00-59

Esac

Solve today's problems

Use if... Else... Fi is used to determine input variables.

Execute the corresponding statement on each judgment branch.

Menu. sh

#! /Bin/bash

Clear

Echo "------- menu ------"

Echo "1) Show Time"

Echo "2) CPU load"

Echo "3) Memory free"

Echo "0) Exit"

Echo "---------------"

Echo-n "Enter you chose [0-3]:"

Read NUM

If [$ {NUM}-lt 0-o $ {NUM}-gt 3]

Then

Echo "This is not between 0-3 ."

Else

If ["$ {NUM}" = "1"]

Then

Echo "'date + % t '"

Else

If ["$ {NUM}" = "2"]

Then

Echo "'uptime | awk-F' [,:]'' {print $7 }''"

Else

If ["$ {NUM}" = "3"]

Then

Echo "'free-m | awk' $1 =" Mem: "{print $4 }''"

Else

Exit

Fi

Fi

Fi

Fi

Review this lesson:

Type of the condition test

File Test

Text Test

Numerical Test

Logic Test

If... Else... Fi Condition Selection Structure

Case... In... Esac

Test after class

1. Modify menu. sh to use a non-menu style. The parameter transmission mode is used for selection. For example, #./menu. sh 1 Output Time

2. Use case to select the menu

Sudu answer: (it is inevitable that there will be errors, but it can be implemented successfully)

1. After menu. sh is modified

#! /Bin/bash

If [$1-lt 0-o $1-gt 3]

Then

Echo "This is not between 0-3 ."

Else

If ["$1" = "1"]

Then

Echo "'date + % t '"

Else

If ["$1" = "2"]

Then

Echo "'uptime | awk-F' [,:]'' {print $7 }''"

Else

If ["$1" = "3"]

Then

Echo "'free-m | awk' $1 =" Mem: "{print $4 }''"

Else

Exit

Fi

Fi

Fi

Fi

2 ,#! /Bin/bash

Clear

Echo "------- menu ------"

Echo "1) Show Time"

Echo "2) CPU load"

Echo "3) Memory free"

Echo "0) Exit"

Echo "---------------"

Echo-n "Enter you chose [0-3]:"

Read NUM

Case $ NUM in

1) date + % T ;;

2) uptime | awk-F' [,:] ''{print $7 }';;

3) free-m | awk '$1 = "Mem:" {print $4 }';;

0) exit ;;

*) Echo "This is not between 0-3 .";;

Esac

There are still many gains today. The course lasted for nearly three hours.

Although if... Else... Fi is easier to understand, but it is much easier to use case.

It seems that it is enough to read a tutorial every day. It will also be dizzy if you look too much. Haha. Continue Learning ~