Shell process control

1 if else sentence

1.1 if grammar

# Multiple lines 
if   Conditions 
then
     command 
fi
# a line 
if  Conditions ; then  command ; fi

1.2 if else grammar

if   Conditions 
then
    command 
else
    command 
fi

1.3 if elif else grammar

if   Conditions 1
then
    command 1
elif  Conditions 2
then
     command 2
elif  Conditions 3
then
     command 3
……
else
    command N
fi

1.2 demo

 Tips " Please enter your test scores :"  Receive users to enter a score ,  Then use if else Conditional sentence judgement 
 requirement 1:  Less than  60  Output " fail, "
 requirement 2:  Greater than or equal to 60  also   Less than 70  Output " pass "
 requirement 2:  Greater than or equal to 70  also   Less than 80  Output " secondary "
 requirement 3:  Greater than or equal to 80  also   Less than 90  Output " good "
 requirement 4:  Greater than or equal to 90  also   Less than or equal to 100  Output " good "
 requirement 5:  The above does not conform to the output " The results are illegal "

#!/bin/bash
read -p " Please enter your examination result ："  score
if ((score<60))
then
   echo " fail, "
elif ((score>=60 && score<70))
then
   echo " pass "
elif ((score>=70 && score<80))
then
   echo " secondary "
elif ((score>=80 && score<90))
then
   echo " good "
elif ((score>=90 && score<=100))
then
   echo " good "
else
   echo " The results are illegal "

fi

2 Built in commands ：test

Shell Medium test The command is used to check if a condition holds , It can do numerical 、 Character and file testing .

2.1 grammar

if test  Numbers 1 options  Numbers 2 
then  
...
fi

options As follows ：

Parameters	explain
-eq	Equal is true
-ne	True if not equal
-gt	Greater than is true
-ge	True if greater than or equal to
-lt	Less than is true
-le	True if less than or equal to

2.2 demo

#/bin/bash
num1=1 num2=1 num3=2
echo "num1=${num1},num2=${num2},num3=${num3}"


if test $num1 -eq $num2
then
   echo "num1  and  num2  equal "
else
   echo "num1  and  num2  It's not equal "
fi


if test $num3 -gt $num2
then
   echo "num3  Greater than  num2"
else
   echo "num3  Less than  num2"
fi

3 Process control ：case sentence

3.1 grammar

case  value  in
 Matching mode 1)
     command 1
     command 2
    ...
    ;;
 Matching mode 2）
     command 1
     command 2
    ...
    ;;
*)
     command 1
     command 2
    ...
    ;;
esac

3.2 demo

#!/bin/bash
read -p " Please enter a 0-7 The number of ：" number
case $number in
1)
   echo " Monday "
   ;;
2)
   echo " Tuesday "
   ;;
3)
   echo " Wednesday "
   ;;
4)
   echo " Thursday "
   ;;
5)
   echo " Friday "
   ;;
6)
   echo " Saturday "
   ;;
7)
   echo " Sunday "
   ;;
*)
   echo " The number you entered is invalid "
   ;;
esac

4 Process control ：while sentence

4.1 grammar

while  Conditions 
do
	 command 1
	 command 2
	...
	continue; #  End the current cycle ,  Enter next cycle 
	break; #  End the current cycle 
done

4.2 demo

#!/bin/bash
read -p " Please enter a number :" number
let i=1
while (( $i <= $number))
do
   echo " The first ${i} Second print : hello world"
   ((i++))
done

5 Process control ：until sentence

5.1 grammar

until  Conditions 
do
     command 
done

5.2 demo

#!/bin/bash
read -p " Please enter a number :" number
let i=1
until (( $i >= $number  ))
do
   echo " The first ${i} Second print ：hello world"
   ((i++))
done

6 Process control ：for sentence

6.1 grammar

for var in item1 item2 ... itemN
do
     command 1
     command 2
    ...
done

#start:  The starting value of the cycle range , Must be an integer 
#end:  The end value of the loop range ,  Must be an integer 
for var in {
    start..end}
do
	 command 
done

for((i=start;i<=end;i++))
do
	 command 
done

6.2 demo

#!/bin/bash
echo "########## Circulation mode 1############"
for i in 1 2 3 4 5
do
  echo " The first  ${i} Second print : hello world"
done


echo "########## Circulation mode 2############"
for i in {
    1..5}
do
 echo " The first  ${i} Second print : hello world"
done


echo "########## Circulation mode 3############"
for((i=1;i<=5;i++))
do
 echo " The first  ${i} Second print : hello world"
done

6.3 Wireless loop

for((;;)); do  command ; done

#!/bin/bash
let i=1

for((;;))
do
   echo " The first ${i} Second print : hello world"
   if test $i -eq 10
   then
      break
   fi
   ((i++))
done

7 Process control ：select sentence

select in Loops are used to enhance interactivity , It can display numbered menus , Users can choose different menus by entering different numbers , And perform different functions . select in yes Shell A unique cycle , Very suitable for terminals （Terminal） Such an Interaction scenario , No other language ;

7.1 grammar

select var in menu1 menu2 ...
do
     command 
done

7.2 demo

#!/bin/bash
echo " What is your hobby ？"
select hobby in " Programming " " game " " Basketball " " swimming "
do
   echo " Your chosen hobby is :${hobby}"
   break
done

echo " Your hobby is : ${hobby}"