检查环境变量

2018-03-03 751 words 2 minutes

Contents

1. 概述

作为 Linux 用户，我们经常使用 shell 脚本。脚本中的常见要求之一是测试是否设置了特定的环境变量。这有助于处理各种错误情况。

在本教程中，我们将看到各种示例来检查是否设置了特定的环境变量。

2. 使用if条件表达式

我们可以使用if 条件表达式 来检查是否将值分配给变量：

$ VAR= 
$ if [ x"${VAR}" == "x" ]; then 
     echo "Value is not assigned to a variable"
  else
     echo "Value is assigned to a variable"
  fi
Value is not assigned to a variable

让我们为变量赋值并验证结果：

$ VAR="sample-value"
$ if [ x"${VAR}" == "x" ]; then
     echo "Value is not assigned to a variable"
  else 
     echo "Value is assigned to a variable"
  fi
Value is assigned to a variable

3. 使用双方括号

检查变量是否设置的另一种方法是使用双方括号：

$ VAR=
$ [[ x"${VAR}" == "x" ]] && echo "Value is not assigned to a variable" || echo "Value is assigned to a variable"
Value is not assigned to a variable

让我们通过为变量赋值来验证输出：

$ VAR="sample-value"
$ [[ x"${VAR}" == "x" ]] && echo "Value is not assigned to a variable" || echo "Value is assigned to a variable"
Value is assigned to a variable

请注意，此方法仅适用于 Bash、Z Shell (zsh) 和 Korn Shell (ksh)。

4. 使用参数表达式

我们可以使用bash 内置参数表达式 来检查是否设置了变量：

$ VAR=
$ [[ ${VAR:-"unset"} == "unset" ]] && echo "Value is not assigned to a variable" || echo "Value is assigned to a variable"
Value is not assigned to a variable

让我们设置一个变量的值并检查它是否有效：

$ VAR="sample-value"
$ [[ ${VAR:-"unset"} == "unset" ]] && echo "Value is not assigned to a variable" || echo "Value is assigned to a variable"
Value is assigned to a variable

5. 使用-z条件表达式

在bash中，有一个-z条件表达式，如果字符串的长度为零，则返回true 。**我们可以使用这个属性来检查一个变量是否被设置：

$ VAR=
$ [[ -z "${VAR}" ]] && echo "Value is not assigned to a variable" || echo "Value is assigned to a variable"
Value is not assigned to a variable

让我们试一试：

$ VAR="sample-value"
$ [[ -z "${VAR}" ]] && echo "Value is not assigned to a variable" || echo "Value is assigned to a variable"
Value is assigned to a variable

6. 使用-n条件表达式

类似地，如果字符串的长度不为零，则-n条件表达式返回true 。**我们可以使用这个属性来检查一个变量是否被设置：

$ VAR=
$ [[ ! -n "${VAR}" ]] && echo "Value is not assigned to a variable" || echo "Value is assigned to a variable"
Value is not assigned to a variable

让我们更新一个变量的值并确认结果：

$ VAR="sample-value"
$ [[ ! -n "${VAR}" ]] && echo "Value is not assigned to a variable" || echo "Value is assigned to a variable"
Value is assigned to a variable