Linux - 替换语句

当Shell遇到包含一个或多个特殊字符的表达式时，它将执行替换语句。

在此，变量的打印值由其值代替。同时，"\n" 换行-

#!/bin/sh

a=10
echo -e "Learnfk.com of age is $a\n"

您将收到以下输出。在这里， -e 选项可以解释反斜杠转义符。

Learnfk.com of age is 10

以下是没有 -e 选项的输出-

Learnfk.com of age is 10\n

以下可在echo命令中使用的转义序列-

命令替换

`command`

执行命令替换时，请确保使用反引号，而不是单引号字符。

命令替换通常用于将命令的输出分配给变量。以下每个示例演示命令替换-

#!/bin/sh

DATE=`date`
echo "Date is $DATE"

USERS=`who | wc -l`
echo "Logged in user are $USERS"

UP=`date ; uptime`
echo "Uptime is $UP"

执行后，您将收到以下输出-

无涯教程网

Date is Thu Jul  2 03:59:57 MST 2021
Logged in user are 1
Uptime is Thu Jul  2 03:59:57 MST 2021
03:59:57 up 20 days, 14:03,  1 user,  load avg: 0.13, 0.07, 0.15

可变替换

变量替换使Shell程序员可以根据变量的状态来操作变量的值。

下表是所有可能的替代方法-

以下是显示上述替换的各种状态的示例-

#!/bin/sh

echo ${var:-"Variable is not set"}
echo "1 - Value of var is ${var}"

echo ${var:="Variable is not set"}
echo "2 - Value of var is ${var}"

unset var
echo ${var:+"This is default value"}
echo "3 - Value of var is $var"

var="Prefix"
echo ${var:+"This is default value"}
echo "4 - Value of var is $var"

echo ${var:?"Print this message"}
echo "5 - Value of var is ${var}"

执行后，您将收到以下输出-

无涯教程网

Variable is not set
1 - Value of var is
Variable is not set
2 - Value of var is Variable is not set

3 - Value of var is
This is default value
4 - Value of var is Prefix
Prefix
5 - Value of var is Prefix