Shell script-conditional judgment | if else statement structure | case statement structure | super detailed

1. Condition test

test 条件表达式

Example format①:

• test 1 == 1

Testing 1is equal to 1the return value 0is thetrue

Example format②:

• [ 1 == 1 ]

2. File testing

[ 操作符 文件或目录 ]

Commonly used test operators

• -d test whether it is a directory or whether it exists
• -e test whether the file or directory exists
• -f test whether it is a file
• -r test whether the current user has permission to read
• -w test whether the current user has permission to write
• -x test whether the current user has permission to execute

[ 操作符 文件或目录 ] && echo "YES"

For example:

• [ -e a ] & echo “target found”

Three, integer value comparison

[ 整数1 操作符 整数2 ]

Commonly used test operators

• -eq is equal to
• -ne is not equal to
• -gt is greater than
• -lt is less than
• -le is less than or equal to
• -ge is greater than or equal to

For example:

[ `who | wc-l` -gt 5 ] && echo "没有超额"

Example: Simple regular inspection

a=`free -m | grep "Mem" | awk '{print $4}'`
[ $a -lt 100 ] && echo "剩余"${a}MB
if [ $a -lt 100 ]
then reboot fi;

Fourth, string comparison

Commonly used test operators

• = String content is the same
• != The string content is different,! Sign means the opposite
• -z string content is empty
• n string has content

[ str1 = str2 ]

= And == have the same function

[str1 != str2] Is the string in the variable different?
[-z str1] Check that the string content is empty

Spaces are also strings

[-n str1] Check whether there is a string

Variables without double quotes will be judged as simple strings

For example:

[ $PWD = `pwd` ] && echo "The current directory is `pwd`"

For example:

[ "en_US" != $LANG ] && echo "not en_US"

Five, logic test

[ 表达式1 ] 操作符 [ 表达式2 ]

命令1 操作符 命令2

Commonly used test operators

• -a or &&: logical and, and the meaning
• -o or ||: logical or, or meaning
• !: Logical no

[ $a -ne 1 ] && [ $a != 2 ] 等同于 [ $a -ne 1 -a $a != 2 ]

For example:

• echo [ “1” == “1” ] && echo “true” || echo “yes”

As long as one does not set up to perform the next

note: &&, ||operator can exist normal [[ ]]condition decision structure, but if it appears in a [ ]structure, the error will be

Example: Use PING to determine whether the website is in stock

• -c: the number of packets sent
• -i: time interval for sending packets
• -w: how many seconds to stop

ping -c 3 -i 0.5 -w 2 baidu.com > /dev/null && echo "$1 online" || echo "$1 off"
ping -c 3 -i 0.5 -w 2 example.com > /dev/null && echo "$1 online" || echo "$1 off"

Example: multiple judgments:

[ -d /boot ] && [ -r /boot ] && echo "you can read"

Six, judgment sentence

if statement

Condition is trueperformed when thenthe back part is executed
determination must be added last fito be closed

if  condition
then
    statement1
fi

Example: It
(()) is a mathematical calculation command. In addition to the most basic addition, subtraction, multiplication and division operations, it can also perform relational operations such as greater than, less than, equal to, and, or, and non-logical operations.

#!/bin/bash
if ((10 > 1))
then
    echo "true"
fi

if else statement

If conditionis true, will perform statement1, as is false, will be executedstatement2

if condition
then
   statement1
else
   statement2
fi

For example:

#!/bin/bash
read input_1
read input_2
if (( $input_1 == $input_2 ))
then
    echo "${input_1}=${inut_2}"
else
    echo "${input_1}!=${input_2}"
fi

if elif else statement

As condition1to truethe time, will be performed statement1, will next be determined condition2, if truethe execution statement2, from the beginning if condition1it is falseto be performed when elsethe statement after thestatementn3

if  condition1
then
   statement1
elif condition2
then
    statement2
else
   statementn3
fi

For example:

#!/bin/bash
if ((10 > 1))
then
      echo "1"
elif ((2 > 1))
then
      echo "2"
else
      echo "3"
fi

case in statement

►The beginning of the case line must start with the word "in", and each pattern must end with a single right bracket ")"►Double
semicolon";;" indicates the end of the command sequence
►In the pattern string, square brackets can be used to indicate a continuous Range, such as "[0-9]"; ► You can also use a vertical bar | to indicate or, such as a|b
►*) to indicate the default mode, where * is equivalent to a wildcard

case 变量值 in
判断1)
命令序列
;;
判断2)
命令序列
;;
……
*)
默认命令序列
esac

For example:

read num -p "请输入数字："
case $num in
    1)
        echo "1"
        ;;
    2)
        echo "2"
        ;;
    *)
        echo "error"
esac