Shell Script Process Control

Shell scripts become used in the process usually requires process control, in general, sequential execution, in the actual use of different situations need to execute different commands, then use the choice to execute such as if, case, sometimes need to repeatedly execute, loop execution such as for, while, until

Conditional Select EXECUTE statement if

Single Branch	Dual Branch
if judgment condition; then Branch code with true condition Fi	if judgment condition; Then Branch code with true condition Else The condition is a false branch code Fi

[Email protected] ~]# vim score.sh 1 #!/bin/bash 2 read-p "Please input you score:" Score 3 if [$score-eq N 4 echo "excellent" 5 elif[$score-ge];then 6 echo "Keep Tring" 7 fi

Conditional Judgment Statement case

Case variable reference in PAT1) Branch 1 ;; PAT2) Branch 2 ;; *) Default Branch ;; Esac

Example: Scripting/root/bin/yesorno.sh, prompting the user to enter Yes or no, and to determine whether the user entered Yes or no, or other information

[Email protected] ~]# cat/bin/yesorno.sh #!/bin/bashread-p "Marry me? Please answer yes or no: "Anscase $ANS in[yy][ee][ss]|     [Yy]) echo "Me too";; [Nn] [oo]|     [Nn]) echo "My heart broken";; *) echo "Once again" Esac

Loop Execution Statement

For loop number of cycles known beforehand

Assigns the element in the list to the "variable name" in turn; The loop body is executed once each assignment; Until the elements in the list are exhausted, the loop ends

Method One	Method Two: Special format
For variable name in list; Loop body Done	for (initialize EXP1; conditional judgment EXP2; control variable correction expression EXP3)) Loop body Done

Example: Writing a script that prompts for the value of a positive integer n to calculate the sum of 1+2+...+n

Method One: #!/bin/bashread-p "Please input positive integer:" Nsum=0for i in ' seq 1 $n ' do sum=$[$sum + $i]done echo $sum method two:    [Email protected] ~]# vim sum.sh #!/bin/bashread-p "Please input positive integer:" Nsum=0for ((i=1;i<= $n; i++)) sum=$[$sum + $i]done echo $sum

While loop number is unknown in advance

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

Usage One:	Usage Two: A special usage of the while loop (traversing each line of the file)
While Cmd;d o Cmdn ... Done	while read line; Do Loop body Done </path/from/somefile Note: Read each line in the/path/from/somefile file sequentially and assign the row to the variable line

Example: Write a script that asks for the sum of all positive odd numbers within 100

[[email protected] ~]# vim score.sh 1 #!/bin/bash 2 Sum=0;i=1 3 while ((I<100)) 4 do 5 sum=$[$sum + $i] 6 let i=$[$i +2] 7 done 8 echo "sum= $sum"

Example: Scan each line of the/etc/passwd file, if the Gecos field is found to be empty, populate the user name and the unit phone, and prompt the user for Gecos information to be modified successfully.

#!/bin/bashwhile Read line;d o comment= ' echo $line |cut-d:-f5 ' username= ' echo $line |cut-d:-f1 ' If [-Z ' $c Omment "];then chfn-f" $username "$username &>/dev/null chfn-p" 0371-61703300 "$username & >/dev/null Fidone </APP/PASSWD

Until loops have entry conditions and exit conditions

Entry condition: CONDITION is False

Exit Condition: CONDITION is True

Until CONDITION; Do Loop body Done

Example: Write a script that asks for the sum of all positive odd numbers within 100

[Email protected] ~]# vim score.sh 1 #!/bin/bash 2 Sum=0;i=1 3 until ((I>100)) 4 do 5 sum=$[$sum + $i] 6 let i=$[$i +2] 7 done 8 echo "sum= $sum"

Select Loop statement

The Select loop is used primarily to create menus, and menu items in numerical order are displayed on standard errors, and the PS3 prompt is displayed.

Wait for user input

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

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 variable in list Do Circular Body Command Done

Loop Control statement Continue

Continue [n]: Early end of the nth layer of the current cycle, and directly into the next

The inner layer is the 1th floor.

While CONDTIITON1; Do CMD1 ... if CONDITION2; Then Continue Fi Cmdn ... Done

Loop control Statement Break

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 Cmd

Homework:

1, write a script, prompted to enter a network address, such as 192.168.0.0, determine the input network segment host online status, and statistics online and offline host how much

[[email protected] ~]# vim ping.sh#!/bin/bashread -p  "please input  network  (eg:192.168.0.0): " netecho  $net  |egrep  " ([0-9]|[ 1-9][0-9]|1[0-9]{2}|2[0-4][0-9]|25[0-5]). {3} [0-9]| [1-9] [0-9]|1[0-9]{2}|2[0-4][0-9]|25[0-4] " &> /dev/null   if [ $? -eq  0 ];then       netip= ' echo  $net  |egrep -o  ". *\." '        for  ( i=1;i<255;i++ );d o            {              ping -c 1 -w1  $netip $i &>/dev/null              if [ $? -eq 0 ];then              echo  "$netip $i is up  "             echo " $netip $ I " >> /app/${netip}txt;           fi            }&        done   else        echo  "Please input  correct format "   fi

2, scripting, printing chess board

#!/bin/bashyellow= "\033[1;43m  \033[0m" red= "\033[1;44m   \033[0m "for i in {1..8} ; do    if [ $[i%2]  -eq 0 ] ; then          for j  in {1..4} ; do                echo -en  "$yellow $red"                 done    else           for j in {1..4} ; do                echo -en  "$red $yellow"                 done   fi  echodone 

3, find a reject link that has more than 10 connections

Method One: Only a link greater than 10 times can be processed #!/bin/bashcheckinterval=30while true ;d o    link= ' netstat  -nt | sed -rn  '/^tcp/s/.*  (. *):. *$/\1/p '  |sort |uniq -c | Sort -nr |head -n 1 '     linknum= ' echo  $link  |cut -d   " "  -f1 '     ip= ' echo  $link  |cut -d  " "  -f2 '     [  $linknum  -ge 10 ]&& iptables -a innput  -s -$ip  -j REJECT    echo  "$ip  at  ' date + '%F  %t '  is reject  '  >> /app/checknet.log    sleep $ Checkintervaldone method Two: When there are more than 10 times the link can be processed #!/bin/bashwhile true ;d o    link= ' netstat  -nt | sed -rn  '/^tcp/s/.*  (. *):. *$/\1/p '  |sort |uniq -c | Sort -nr ' echo  ' $linK "|while read fileline ;d o    linknum= ' echo  $link  |cut -d   " "  -f1 '     ip= ' echo  $link  |cut -d  " "  -f2 '    if  [  $linknum  -ge 10 ];then         iptables -a input -s -$ip  -j REJECT        echo  "$ip  at  ' date + '%f %t '  is reject "  > > /app/checknet.log   fi    done

Shell Script Process Control