Shell Flow Control

The flow control of shell cannot be empty which is different from other languages ​​such as Java and PHP.The following is the writing of PHP flow control:

<?php
if (isset($_GET["q"])) {
    search(q);
}
else {
    // do nothing
}

We cannot do like this in sh / bash. If there is no statement execution in the else branch, don’t write ‘else’.

if else

if

The if statement has the following syntax:

if condition
then
    command1 
    command2
    ...
    commandN 
fi

Write in a line (applicable to terminal command prompt):

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

The fi at the end is if spelled reverse, we will encounter similar situation later.

if else

The if else statement has the following syntax:

if condition
then
    command1 
    command2
    ...
    commandN
else
    command
fi

if else-if else

The if else-if else statement has the following syntax:

if condition1
then
    command1
elif condition2 
then 
    command2
else
    commandN
fi

The following example determines whether two variables are equal:

a=10
b=20
if [ $a == $b ]
then
   echo "a is equal to b"
elif [ $a -gt $b ]
then
   echo "a is greater than  b"
elif [ $a -lt $b ]
then
   echo "a is less than b"
else
   echo "no conditions meet"
fi

Output is

a is less than b

The if else statement is often used in conjunction with the test command, as follows:

num1=$[2*3]
num2=$[1+5]
if test $[num1] -eq $[num2]
then
    echo 'The two numbers are equal!'
else
    echo 'The two numbers are not equal!'
fi

Output is

The two numbers are not equal!

The for loop

Shell supports for loops which is similar to other programming languages.

The general format of the for loop is:

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;

The command can be any valid shell commands and statements.The in list can contain substitutions, strings, and file names.

The in is optional.

For example, output the numbers in the current list sequentially:

for loop in 1 2 3 4 5
do
    echo "The value is: $loop"
done

Output is

The value is: 1
The value is: 2
The value is: 3
The value is: 4
The value is: 5

For example, output the characters in the string sequentially:

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

Output is

This is a string

The while loop

The while loop enables you to execute a set of commands repeatedly, or read data from input files.The syntax is

while condition
do
    command
done

The following is a basic while loop. The while tests: if int is less than or equal to 5, then the condition returns true.int starts from 0, and int increases by 1 each time when the loop is processed.Run the above script, return the numbers 1 to 5, and then terminate.

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

Execute the script and output is

1
2
3
4
5

The above example uses the Bash command let, which is used to execute one or more expressions. There is no need to add $ to indicate the variable in the variable calculation.

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

echo 'press <CTRL-D> to exit'
echo -n 'input your favorite website name: '
while read FILM
do
    echo "Yes!$FILM is a good network"
done

Execute the script and output is

press <CTRL-D> to exit
input your favorite website name: linuxcommands
Yes!linuxcommands is a good network

The infinite Loop

The syntax is

while :
do
    command
done

Or

while true
do
    command
done

Or

for (( ; ; ))

The until loop

The while loop is perfect for a situation where you need to execute a set of commands while some condition is true.

The until loop is just the opposite of while loop.

In general, the while loop is better than the until loop, but in some cases—only in rare cases, until loop is more useful.

The syntax

until condition
do
    command
done

The condition is generally a conditional expression.If the resulting value is false, given command are executed, otherwise the program jumps to the next line after the done statement.

Here is a simple example that uses the until loop to display the numbers zero to nine.

#!/bin/bash

a=0

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

Upon execution, you will receive the following result

0
1
2
3
4
5
6
7
8
9

case

The case statement is multiple choice statement.You can use a case statement to match a value with a pattern. If the match is successful, execute the matching command.The case syntax is as follows:

case word in
pattern1)
    command1
    command2
    ...
    commandN
    ;;
pattern2)
    command1
    command2
    ...
    commandN
    ;;
esac
After the word is the keyword in, then various matching patterns follow by the in . Each pattern must end with a closing parenthesis.The word can be a variable or a constant.After the word to match the certain pattern, then all commands began to execute until ;;
The word will detect every pattern that matches.Once the pattern matches, the other patterns will not be continued after all the commands in the matching pattern are executed.If there is no matching pattern, use the * to capture the word, and then execute the following commands.
The following script need to input the number between 1 and 4 to match each pattern:
echo 'Input the number between 1 and 4:'
echo 'The number you input is:'
read aNum
case $aNum in
    1)  echo 'You choose 1'
    ;;
    2)  echo 'You choose 2'
    ;;
    3)  echo 'You choose 3'
    ;;
    4)  echo 'You choose 4'
    ;;
    *)  echo 'The number you input is not between 1 and 4'
    ;;
esac

Enter different number will have different results, for example:

Input the number between 1 and 4::
The number you input is:
3
You choose 3

Out of the loop

In the loop process, sometimes it is necessary to force out of the loop when the loop end condition is not reached. Shell uses two commands to achieve this: break and continue.

break

The break statement is used to terminate the execution of the entire loop.

The following example shows that the script enters an infinite loop as soon as a is greater than 5.To jump out of this loop and return to the shell prompt, we need to use the command break.

#!/bin/bash
while :
do
    echo -n "Enter a number between 1 and 5:"
    read aNum
    case $aNum in
        1|2|3|4|5) echo "The number you input is $aNum!"
        ;;
        *) echo "The number you input is not between 1 and 5! Game over"
            break
        ;;
    esac
done

Upon execute and you will receive the following result

Enter a number between 1 and 5:4
The number you input is 4
Enter a number between 1 and 5:14
The number you input is not between 1 and 5! Game over

continue

The continue statement is similar to the break command, except that it causes the current iteration of the loop to exit, rather than the entire loop.

Edit the above example

#!/bin/bash
while :
do
    echo -n "Enter a number between 1 and 5:"
    read aNum
    case $aNum in
        1|2|3|4|5) echo "The number you input is $aNum!"
        ;;
        *) echo "The number you input is not between 1 and 5!"
            continue
            echo "Game over"
        ;;
    esac
done

Execute the script we found that when a number greater than 5 is entered, the loop in the above example will not end, and the statement echo “game over” will never be executed.

case … esac

The case…esac statement is very similar to the switch…case statement we have in other programming languages.The case…esac statement is a multiway branch structure.Each case branch starts with a right parenthesis.Use two semicolons ;; to express break which is to end the entire case … esac statement. Esac (that is the case in turn) is the end marker.

The syntax

case word in
pattern1)
    command1
    command2
    command3
    ;;
pattern2)
    command1
    command2
    command3
    ;;
*)
    command1
    command2
    command3
    ;;
esac

The word is after the case, and the word can be a variable or a constant.

After the word is the keyword in, then various matching patterns follow by the in . Each pattern must end with a closing parenthesis. The pattern supports regular expressions.

For example

#!/bin/sh

site="linuxcommands"

case "$site" in
   "linuxcommands") echo "www.linuxcommands.site"
   ;;
   "google") echo "Google Search"
   ;;
   "taobao") echo "taobao site"
   ;;
esac

Upon execute and you will get the result

www.linuxcommands.site

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