The command will return nothing as the value of thecountrywasn’t set in the first place. If the value of the variable is unset, using the following technique, we can assign a value. $echo${country=Greenland} Here, Bash will check if the variable country has any value stored. As the ...
nameref attribute)andvalueofany existing variablewiththe same name at a surrounding scope.Ifthereisno existing variable, the local variableisinitially unset. The following options can be usedtorestrict outputtovariableswiththe specified attributeortogive variables attributes: -aEachnameisan indexed array...
-bash: a: unbound variable 条件判断 虽然你可以通过上面的测试和&&、||控制操作符来完成一个大量的程序设计,不过bash环境中还包含了更亲切的“if,then,else”和case语法结构。当你学会了这两种结构之后,你还会学到循环结构,这时你的知识结构才会真正拓展开来。 if-then-else语句 bash环境下的if命令是一个复合的...
- Test if a given variable is [eq]ual/[n]ot [e]qual/[g]reater [t]han/[l]ess [t]han/[g]reater than or [e]qual/[l]ess than or [e]qual to the specified number: [[ $variable -eq|ne|gt|lt|ge|le integer ]] - Test if the specified variable has a [n]on-empty value: ...
Variable ConditionalsExpressionValueWhat does it do? -o opt If shell option is enabled. -v var If variable has a value assigned. -R var If variable is a name reference. -z var If the length of string is zero. -n var If the length of string is non-zero....
通常shell将空格堪称命令行中单词的分隔符→if 要表示某参数包含空格(eg:包含空格的文件名),then 需要使用以下三种界定符使空格作为整体中的一部分 一对单引号:shell以字面意思看待内部的字符串,不加以解释 一对双引号:shell会解读里面的特殊字符 $ echo 'The variable HOME has value $HOME' ...
if [[ ! -v var ]]; then echo "Variable is not set" elif [[ -z "$var" ]]; then echo "Variable is set to the empty string" else echo "Variable has the value: $var" fi OUTPUT 1 2 3 Variable has the value: /root Use the if-else statement with the -n option to check...
forvariable_nameinvalue1 value2 value3 .. ndocommand1 command2 commandndone 现在让我们检查 bash for 循环的几个示例用法。 循环示例 在最简单的形式中,for 循环采用以下基本格式。在此示例中,变量 n 遍历一组用花括号括起来的数值,并将它们的值打印到标准输出。
-i If the -i option is present, the shell is interactive. 1. 2. 译:-i的作用是,是shell变为交互式shell -l Make bash act as if it had been invoked as a login shell (see INVOCATION below). 1. 2. 译:-l的作用是使bash变成一个login shell ...
forvariable_nameinvalue1 value2 value3..ndocommand1 command2 commandn done 现在让我们检查 bash for 循环的几个示例用法。 循环示例 在最简单的形式中,for 循环采用以下基本格式。在此示例中,变量 n 遍历一组用花括号括起来的数值,并将它们的值打印到标准输出。