Shell learning record (III)

Shell Process control

if else-if else    

Be careful ： stay sh/bash You can't write that in , If else Branch has no statement execution , Don't write this else

if condition1
then
    command1
elif condition2 
then 
    command2
else
    commandN
fi

Write in a line （ For terminal command prompt ）：

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

for loop

for var in item1 item2 ... itemN
do
    command1
    command2
    ...
    commandN
done

Write in a line ：

for var in item1 item2 ... itemN; do command1; command2… done;

Output string ：

for str in 'This is a string'
do
    echo $str
done

Output ：

This is a string

while sentence

while condition
do
    command
done

#!/bin/bash
int=1
while(( $int<=5 ))  # Be careful ： Two brackets 
do
    echo $int
    let "int++"
done

Output ：

1
2
3
4
5

The above example uses Bash let command , It is used to execute one or more expressions , There is no need to add $ To represent a variable

Self adding operation ：let no++

Since the reduction of operating ：let no--

Abbreviation form  let no+=10,let no-=20, They are equal to  let no=no+10,let no=no-20

#!/bin/bash

let a=5+4
let b=9-3 
echo $a $b

until loop

until Loop through a series of commands until the condition is true Stop when .

until Circulation and while The cycle is the opposite of the way it's handled .

commonly while Circulation is better than until loop , But sometimes — Only in a few cases ,until Circulation is more useful .

until Grammar format :

until condition
do
    command
done

#!/bin/bash

a=0

until [ ! $a -lt 10 ]
do
   echo $a
   a=`expr $a + 1`
done

Output ：

0
1
2
3
4
5
6
7
8
9

case

Shell case Statements are multiple choice statements . It can be used case Statement matches a value with a pattern , If the match is successful , Execute the matching command .case The statement format is as follows ：

case  value  in
 Pattern 1)
    command1
    command2
    ...
    commandN
    ;;
 Pattern 2）
    command1
    command2
    ...
    commandN
    ;;
esac

case Work as shown above . Value must be followed by word in, The first mock exam must be closed with right brackets. . Value can be variable or constant . Match found that after the first mock exam meets a certain pattern, , During this period, all commands are executed until ;;.

Value will detect every matching pattern . Once the patterns match , After the corresponding command of matching mode is executed, other modes will not be continued . If there is no matching pattern , Use the asterisk * Capture the value , Then execute the following command .

The following script prompts for 1 To 4, Match with each pattern ：

echo ' Input  1  To  4  Number between :'
echo ' The number you enter is :'
read aNum
case $aNum in
    1)  echo ' You chose  1'
    ;;
    2)  echo ' You chose  2'
    ;;
    3)  echo ' You chose  3'
    ;;
    4)  echo ' You chose  4'
    ;;
    *)  echo ' You didn't enter  1  To  4  Number between '
    ;;
esac

Output ：

 Input  1  To  4  Number between :
 The number you enter is :
3
 You chose  3

 Input  1  To  4  Number between :
 The number you enter is :
5
 You didn't enter  1  To  4  Number between 

Out of the loop

During the cycle , Sometimes it's necessary to force the loop out before the loop end condition is reached ,Shell Use two commands to do this ：break and continue.

break command

break The command allows you to jump out of all loops （ Terminate all loops after execution ）.

In the following example , The script goes into a dead loop until the number entered by the user is greater than 5. To get out of this cycle , Back to shell At the prompt , Need to use break command .

#!/bin/bash
while :
do
    echo -n " Input  1  To  5  Number between :"
    read aNum
    case $aNum in
        1|2|3|4|5) echo " The number you enter is  $aNum!"
        ;;
        *) echo " The number you entered is not  1  To  5  Between !  Game over "
            break
        ;;
    esac
done

Output ：

 Input  1  To  5  Number between :3
 The number you enter is  3!
 Input  1  To  5  Number between :7
 The number you entered is not  1  To  5  Between !  Game over 

continue

continue Command and break Command similar , There's only one difference , It doesn't jump out of all loops , Just jump out of the current loop .

Modify the above example ：

#!/bin/bash
while :
do
    echo -n " Input  1  To  5  Number between : "
    read aNum
    case $aNum in
        1|2|3|4|5) echo " The number you enter is  $aNum!"
        ;;
        *) echo " The number you entered is not  1  To  5  Between !"
            continue
            echo " Game over "
        ;;
    esac
done

Run code discovery , When the input is greater than 5 Digital time , The loop in this case does not end , sentence  echo " Game over "  It will never be carried out .