Process Control statements in shell programming

1. Conditional selection: If statement

① Single Branch

if judgment condition; then condition is true Branch code FI

② Dual Branch

if judgment condition; Then condition for true Branch code else condition for false Branch code FI

③ Multi-Branch

if judgment condition 1; Then condition is true branch code elif judging condition 2; Then condition is true branch code elif judging Condition 3; Then condition is true branch code else above condition is false Branch code FI

Example: Please user input score: 80-100 for excellent, 60-79 for pass, other elimination.

#!/bin/bash read-p "Pelease input your score:" Score if ["$score"-ge 80-a "$score"-le];then echo "excellent" Eli f ["$score"-lt 80-a "$score"-ge];then echo "eliminated" through "else echo" fi

2. Condition Judgment: Case statement

Syntax: Case variable reference in PAT1) branch 1;;    PAT2) branch 2;;        .. *) default branch;; Esac

Case supports GLOB-style wildcard characters:
*: Any character of any length
?: any single character
[]: Any single character within the specified range
A|b:a or B

。。。

Example: Write a script that prompts the user to enter Yes or no case-insensitive and to determine whether the user is entering yes or no or other information.

#!/bin/bashread-p "Please input yes or no:" ynans=$ (Echo $yn |tr "[[: Upper:]]" "[[: Lower:]]") case $ans Iny|yes) echo y es;; N|no) echo no;; *) echo Yes/no?esac

3. For loop

Syntax: for variable name in list; Do
Loop body
Done

Execution mechanism: Assigns the element in the list to the "variable name" in turn; The loop body is executed once each assignment; The loop ends until the elements in the list are exhausted.

List Generation Method:
(1) Give the list directly
(2) List of integers:
(a) {start: End
(b) $ (SEQ [start [step]] end)
(3) command to return a list
$ (COMMAND)
(4) using glob, such as: *.sh
(5) variable reference;
[email protected], $*

Example: Print a 99 multiplication table with a for loop.

#!/bin/bashfor i in {1..9};d O to J in $ (seq $i);d o echo-ne "$j * $i =$[$i * $j]\t" Done Echodone

Special format for loop:

For (control variable initialization; conditional judgment expression; control variable correction expression))
Do
Loop body
Done

• Control variable initialization: Executes only once when running to a loop code snippet

• Correction expressions for control variables: The control variable correction operation is performed at the end of each cycle, then the condition is judged.

  Example: Print a 99 multiplication table with a for loop special format.

#!/bin/bashfor ((i=1;i<=9;i++)); Do for ((j=1;j<=i;j++)); Do echo-ne ' $j * $i =$[$j * $i]\t "Done Echodone

4. While loop

Syntax: while CONDITION; Do
Loop body
Done

CONDITION: cyclic control conditions; Before entering the cycle, make a judgment; once each loop is judged again; the condition is "true", then a loop is executed until the condition test state is "false" to terminate the loop
Therefore: Condtion generally should have a cyclic control variable, and the value of this variable will be continuously corrected in the loop body
Enter condition: Condition is true
Exit Condition: Condition is false

Example: Sum of numbers within 100 that can be divisible by 3

#!/bin/bashdeclare i=1declare sum=0while ["$i"-lt];d o let i++ if [$[i%3]-ne 0];then continue fi let sum+= $idoneecho sum is $sum

A special use of the while (traversing each line of the file):

while read line; Do
Loop body
Done </path/from/somefile
Read each line in the/path/from/somefile file sequentially and assign the row to the variable line.

Example: Find all users with an even ID number and display their user name and ID number.

While read line;d o if [$[' echo $line |cut-d:-f3 '%2]-eq 0];then echo-ne "username: ' echo $line |cut -d:-f1 ' \ t ' echo ' UID: ' echo $line |cut-d:-f3 ' "fi done</etc/passwd

or pass the content over the pipe to the while loop:

Example: Notifies the user if the disk utilization is greater than 80%.

#!/bin/bashdf|grep "/DEV/SD" |while read disk;d o diskused=$ (echo $disk |sed-r ' s/.* ([0-9]+)%.*/\1/') diskname=$      (Echo $disk |cut-d ""-f1) [$diskused-ge] && Echo ' $diskname'll bi full: $diskused% ' done

5. Untill cycle

grammar until CONDITION; Do
Loop body

Done
Entry condition: CONDITION is False
Exit Condition: CONDITION is True

Example: printing a 99 multiplication table with until

#!/bin/bashj=1i=1until ["$j"-gt 9];d o until ["$i"-GT "$j"];d o echo-ne "$i * $j =$[i*j]\t" let i++ Do echo let I=1 let J++done

6. Circular Control Statement continue

Continue used in the loop body
Continue [n]: End of the nth layer of the current cycle, and directly into the next round of judgment; the inner layer is the 1th floor.
While CONDTIITON1; Do
CMD1
...
if CONDITION2; Then
Continue
Fi
Cmdn
...
Done

Example: Print a number other than 5 1-10.

#!/bin/bashfor i in {1..10};d o    if [  $i  -eq 5 ]; then        continue    else         echo  $i     fi                                                                                                                                                         done

7. Loop Control Statement Break

Grammar:

Break [n]: Early end of the nth layer cycle, the inner layer is the 1th layer while CONDTIITON1; Do CMD1 ... if CONDITION2; Then break fi cmdn ... done

The difference between break and continue is that continue skips a single loop that satisfies a condition in a loop, and break jumps out of its own loop body by default. In the continue example, if you change to break, only 1-4 is printed, because the 5 trigger breaks out of the loop after it is not printed.

8. Loop Control Shift Command

Shift [N]
n is used to move the list of parameters to the left for a specified number of times, with the default shift left once.
Parameter list once moved, the left-most argument is removed from the list. The while loop iterates through the position parameter list, often to shift.
Example

#!/bin/bashwhile (($#>0));d o echo $* shiftdone display results: [[email protected] app]#./haha.sh a b c d ea b c D EB c D EC D Ed EE

9. Select Cycle and Menu

Select variable in listdo loop body command done

• The Select loop is used primarily to create menus, and menu items in numerical order are displayed on standard errors, and a PS3 prompt is displayed waiting for user input.

• The user enters a number in the menu list and executes the corresponding command.

• User input is saved in the built-in variable REPLY

• Select is an infinite loop, so remember to exit the loop with the break command, or terminate the script with the Exit command. You can also press CTRL + C to exit the loop.

• Select is often used in conjunction with case

• Similar to a For loop, you can omit the in list and use positional parameters at this time

  Example

#!/bin/bashps3= "please choose your menu: " select menu in huimian  lamian hulatang yrt;do      case  $REPLY  in                                                                                                                                                  1)           echo  "The price  is \$10 "   ;;       2)           echo  " the price is \$15 "   ;;       3)           echo  " The price is \$5 "    ;;       4)           echo  " the price is \$20 "   ;;       *)           echo  " Get out! "           break      esacdone

10. Signal Capture Trap

Trap ' trigger command ' signal
When a custom process receives a specified signal from the system, it executes the triggering instruction without performing the original operation
Trap ' signal
Operations that ignore signals
Trap '-' signal
Operation to restore the original signal
Trap-p
column-defined signal operation

Example:

#!/bin/bash trap ' echo ' signal:sigint "' int trap-p for ((i=0;i<=10;i++)) does sleep 1 echo $i done trap ' int trap-p fo R ((i=11;i<=20;i++)) do sleep 1 echo $i doing trap '-' int trap-p for ((i=21;i<=30;i++)) does sleep 1 echo $i done

Process Control statements in shell programming