shell中if-then的高级使用

if 条件表达式; then
    # 当条件表达式为真时执行的命令
    命令1
    命令2
    ...
else
    # 当条件表达式为假时执行的命令
    命令A
    命令B
    ...
fi

#!/bin/bash

# 获取用户输入
read -p "请输入一个数字: " num

# 判断数字是否大于10
if [ "$num" -gt 10 ]; then
    echo "数字大于10"
else
    echo "数字小于或等于10"
fi

#!/bin/bash

# 获取用户输入
read -p "请输入一个数字: " num

# 判断数字是否在1到10之间
if [ "$num" -ge 1 ] && [ "$num" -le 10 ]; then
    echo "数字在1到10之间"
else
    echo "数字不在1到10之间"
fi

#!/bin/bash

# 获取用户输入
read -p "请输入一个数字: " num

# 判断数字是否为正数、负数还是零
if [ "$num" -gt 0 ]; then
    echo "数字是正数"
elif [ "$num" -lt 0 ]; then
    echo "数字是负数"
else
    echo "数字是零"
fi

# 比较数值
[ "$num" -eq 10 ]       # 等于
[ "$num" -ne 10 ]       # 不等于
[ "$num" -gt 10 ]       # 大于
[ "$num" -lt 10 ]       # 小于
[ "$num" -ge 10 ]       # 大于或等于
[ "$num" -le 10 ]       # 小于或等于

# 比较字符串
[ "$str" = "hello" ]    # 等于
[ "$str" != "hello" ]   # 不等于
[ "$str" \< "hello" ]   # 字典序小于
[ "$str" \> "hello" ]   # 字典序大于

# 测试文件属性
[ -f "$file" ]          # 是普通文件
[ -d "$file" ]          # 是目录
[ -r "$file" ]          # 可读
[ -w "$file" ]          # 可写
[ -x "$file" ]          # 可执行
[ -s "$file" ]          # 文件非空

#!/bin/bash

# 获取用户输入的文件名
read -p "请输入一个文件名: " filename

# 测试文件是否存在且为普通文件
if [ -f "$filename" ]; then
    echo "文件存在且为普通文件"
else
    echo "文件不存在或不是普通文件"
fi

$ cat SingleParentheses.sh
#!/bin/bash
# Testing a single parentheses condition
#
echo $BASH_SUBSHELL
#
if (echo $BASH_SUBSHELL)
then
    echo "The subshell command operated successfully."
#
else
    echo "The subshell command was NOT successful."
#
fi
$
$ ./SinglePArentheses.sh
01
The subshell command operated successfully.

$ cat SingleParentheses.sh
#!/bin/bash
# Testing a single parentheses condition
#
echo $BASH_SUBSHELL
#
if (cat /etc/PASSWORD)
then
    echo "The subshell command operated successfully."
#
else
    echo "The subshell command was NOT successful."
#
fi
$
$ ./SinglePArentheses.sh
cat: /etc/PASSWORD: NO such file or directory
The subshell command was NOT successful.

$ cat DoubleParentheses.sh
#!/bin/bash
# Testing a double parentheses command
#
val1=10
#
if (( $val1 ** 2 > 90 ))
then
    (( val2 = $val1 ** 2 ))
    echo "The square of $val1 is $val2,"
    echo "which is greater than 90."
#
fi
$
$ ./DoubleParentheses.sh
The square of 10 is 100,
which is greater than 90.

$ cat DoubleBrackets.sh
#!/bin/bash
# Using double brackets for pattern matching
#
#
if [[ $BASH_VERSION == 5.* ]]
then
    echo "You are using the Bash Shell version 5 series."
fi
$
$ ./DoubleBrackets.sh
You are using the Bash Shell version 5 series.