Shell Script Learning Note eight: Process Control

First, if else// If the Else branch does not have a statement execution, do not write this else

1. If statement

If condition

Then

Command1

Command2

Command3

...

Fi

Write a line:

if [$ (ps-ef | grep-c "SSH")-gt 1]; then echo "true"; Fi

2. If Else

If Condition

Then

Command1

Command2

...

Else

Command

Fi

#!/bin/bashnum1=num2; if test $[num1]-eq $[num2]then    echo" two numbers equal " ; Else    Echo " two numbers are not equal " ; fi outputs The following result: two numbers equal

3. If else-if else

If condition

Then

Command1

Command2

Elif condition

Then

Command

Else

Command

Fi

#!/bin/Basha=Tenb= -if[$a = =$b] Then    Echo "a equals b"elif[$a-GT $b] Then    Echo "a greater than B"elif[$a-LT $b] Then    Echo "A is less than B"Else    Echo "there are no conditions to meet"fioutputs The following result: A is less than B

Second, for Loop

For Var in item1 item2 ... itemn

Do

Command1

Command2

...

Done

Write a line

For Var in item1 item2 ... itemn; Do Command1; Command2; ... done;

#!/bin/Bash forLoopinch 1 2 3 4 5 Do    Echo "$loop"; Doneoutput The following results:1 234 5 forStrinch "This is a string" Do     Echo "$str"; Doneoutputs the following results: Thisisastring

Third, while statement

While condition

Do

Command1

Done

 while 5 ]do    echo  $cnt    'cnt++'done     echo" press <CTRL-D> exit "

　The while loop can be used to read keyboard information. In the following example, the input information is set to the variable film, press <Ctrl-D> to end the loop

echo "Press <CTRL-D> exit"

Echo-n "Enter your favorite movie name:"

While Read FILM

Do

echo "Yes! $FILM is a good movie. "

Done

Four, Infinite cycle

1.

While:

Do

Command

Done

2.

While True

Do

Command

Done

3.

for ((;;))

V. until cycle

• Until Loop executes a series of commands until the condition is true stops
• The Until loop and the while loop are just the opposite of the processing mode.
• The condition can be any test condition, and the test occurs at the end of the loop, so the loop executes at least once
• The general while loop is better than the until loop, but at some point-and in rare cases, the until loop is more useful

Until condition

Do

Command

Done

Vi. case

Case value in

Mode 1)

Command1

Command2

...

;;

Mode 2)

Command1

Command2

...

;;

Esac

#!/bin/BashEcho "Enter a number from 1 to 4"Echo "the number you entered is:"Read Anum Case$aNuminch     1)      Echo "you chose 1."      ;; 2)          Echo "you chose 2."          ;; 3)          Echo "you chose 3."          ;; 4)          Echo "you chose 4."          ;;Esac

Seven, break

Jump out of all loops (terminates all loops after execution)

     while :     Do        Echo-N"Please enter a number from 1 to 5:"Read Anum
         Case$aNuminch            1|2|3|4|5)                Echo "the number you entered is: $aNum"                ;; *)                Echo "the number you entered is not between 1 and 5! End Game"Break ;; Esac     Done

Eight, continue

Jump out of this cycle

Shell Script Learning Note eight: Process Control