Shell Scripting Tutorial [8] - Shell Process Control

Shell Scripting Tutorial [8] - Shell Process Control

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Directory : https://blog.csdn.net/shn111/article/details/131590488

Reference tutorial : https://www.runoob.com/linux/linux-shell.html

Online editor : https://www.runoob.com/try/runcode.php?filename=helloworld&type=bash

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The flow control of the shell cannot be empty

The following writing is wrong, if there is no statement execution in the else branch, do not write this else

if [ true ]
then
	echo "hello"
else
	
fi

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if else

ifStatement Syntax Format

if condition
then
    command1 
    command2
    ...
    commandN 
fi

write in one line (for terminal command prompt)

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

if else grammatical format

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

if else-if elsegrammatical format

if condition1
then
    command1
elif condition2 
then 
    command2
else
    commandN
fi

The […] judgment statement of if else is greater than using -gt, less than using -lt

if [ "$a" -gt "$b" ]; then
    ...
fi

If you use ((…)) as a judgment statement, you can directly use > and < for greater than and less than.

if (( a > b )); then
    ...
fi

Example:

a=10
b=20
if [ $a == $b ]
then
   echo "a 等于 b"
elif [ $a -gt $b ]
then
   echo "a 大于 b"
elif [ $a -lt $b ]
then
   echo "a 小于 b"
else
   echo "没有符合的条件"
fi
# a 小于 b

Use ((…)) as a judgment statement:

a=10
b=20
if (( $a == $b ))
then
   echo "a 等于 b"
elif (( $a > $b ))
then
   echo "a 大于 b"
elif (( $a < $b ))
then
   echo "a 小于 b"
else
   echo "没有符合的条件"
fi
# a 小于 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 '两个数字相等!'
else
    echo '两个数字不相等!'
fi
# 两个数字相等!

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for loop

Format:

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

Written in one line:

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

When the variable value is in the list, the for loop executes all the commands once, and uses the variable name to get the current value in the list. command can be any valid shell command and statement. The in list can contain substitutions, strings, and filenames.

The in list is optional, and if you don't use it, the for loop uses positional arguments from the command line.

For example, to print the numbers in the current list sequentially:

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

Print the characters in a string sequentially:

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

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while loop

A while loop is used to continuously execute a sequence of commands, and is also used to read data from an input file. Its syntax format is:

while condition
do
    command
done

example

int=1
while(( $int<=5 ))
do
    echo $int
    let "int++"
done
# 1
# 2
# 3
# 4
# 5

The above example uses the Bash let command, which is used to execute one or more expressions, and there is no need to add $ to represent variables in variable calculations. For details, please refer to: Bash let command

The while loop can be used to read keyboard information. In the example below, the input information is set to the variable FILM, press to end the loop.

echo '按下 <CTRL-D> 退出'
echo -n '输入你最喜欢的网站名: '
while read FILM
do
    echo "是的！$FILM 是一个好网站"
done

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Infinite loop

Grammar format:

while :
do
    command
done

or

while true
do
    command
done

or

for (( ; ; ))

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until loop

until Loop executes a sequence of commands until the condition is truetrue.

The until loop is the exact opposite of the while loop.

In general a while loop is better than an until loop, but sometimes—and only rarely—an until loop is more useful.

Grammar format:

until condition
do
    command
done

condition is generally a conditional expression, if the return value is false, continue to execute the statement in the loop body, otherwise jump out of the loop.

Example:

a=0

until [ ! $a -lt 10 ]
do
   echo $a
   a=`expr $a + 1`
done
# 0
# 1
# 2
# 3
# 4
# 5
# 6
# 7
# 8
# 9

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case…esac

case ... esac is a multi-choice statement, which is similar to switch ... case statements in other languages. It is a multi-branch selection structure. Each case branch starts with a right parenthesis, and two semicolons ;; indicate break, which means the end of execution , jump out of the entire case ... esac statement, and esac (that is, the reverse of case) is used as the end tag.

You can use a case statement to match a value and a pattern, and if the match is successful, execute the matching command.

grammatical format

case 值 in
模式1)
    command1
    command2
    ...
    commandN
    ;;
模式2)
    command1
    command2
    ...
    commandN
    ;;
esac

The working method of case is as shown above, the value must be followed by the word in, and each pattern must end with a closing bracket. The value can be a variable or a constant. After the matching finds that the value matches a certain pattern, all commands will be executed until ;;.

A value will detect every pattern that matches. Once the pattern is matched, no other patterns will be continued after the corresponding command matching the pattern is executed. If none of the patterns match, use an asterisk * to capture the value before executing the following command.

Example:

echo '输入 1 到 4 之间的数字:'
echo '你输入的数字为:'
read aNum
case $aNum in
    1)  echo '你选择了 1'
    ;;
    2)  echo '你选择了 2'
    ;;
    3)  echo '你选择了 3'
    ;;
    4)  echo '你选择了 4'
    ;;
    *)  echo '你没有输入 1 到 4 之间的数字'
    ;;
esac

Match string:

site="runoob"

case "$site" in
   "runoob") echo "菜鸟教程"
   ;;
   "google") echo "Google 搜索"
   ;;
   "taobao") echo "淘宝网"
   ;;
esac
# 菜鸟教程

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break out of the loop

breakCommand
The break command allows breaking out of all loops (terminating the execution of all subsequent loops).

Example: The script enters an infinite loop until the user enters a number greater than 5. To break out of this loop and return to the shell prompt, you need to use the break command.

while :
do
    echo -n "输入 1 到 5 之间的数字:"
    read aNum
    case $aNum in
        1|2|3|4|5) echo "你输入的数字为 $aNum!"
        ;;
        *) echo "你输入的数字不是 1 到 5 之间的! 游戏结束"
            break
        ;;
    esac
done
# 输入 1 到 5 之间的数字:3
# 你输入的数字为 3!
# 输入 1 到 5 之间的数字:7
# 你输入的数字不是 1 到 5 之间的! 游戏结束

continueCommand
The continue command is similar to the break command, the only difference is that it does not break out of all loops, only the current loop.

Example: When a number greater than 5 is entered, the loop in this example will not end, and the statement echo "game over" will never be executed.

while :
do
    echo -n "输入 1 到 5 之间的数字: "
    read aNum
    case $aNum in
        1|2|3|4|5) echo "你输入的数字为 $aNum!"
        ;;
        *) echo "你输入的数字不是 1 到 5 之间的!"
            continue
            echo "游戏结束"
        ;;
    esac
done