bash 如何检查环境变量是否存在并获取其值？
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How to check if an environment variable exists and get its value?
提问by Nicolas El Khoury
I am writing a shell script. In this shell script, I am have a variable that either takes a default value, or the value of an environment variable. However, the environment variable doesn't have to be present.
我正在编写一个shell脚本。在这个 shell 脚本中，我有一个变量，它要么采用默认值，要么采用环境变量的值。但是，环境变量不必存在。
For instance, assume, before running the script, I perform the following operation:
例如，假设在运行脚本之前，我执行以下操作：
export DEPLOY_ENV=dev
How do I tell the script to search for this environment variable, and store its value in a variable inside the script. Moreover, how do I tell the script that if this environment variable does not exist, store a default variable?
我如何告诉脚本搜索此环境变量，并将其值存储在脚本内的变量中。此外，如何告诉脚本如果此环境变量不存在，则存储一个默认变量？
回答by endurogizer
[ -z "${DEPLOY_ENV}" ]checks whether DEPLOY_ENVhas length equal to zero. So you could run:
[ -z "${DEPLOY_ENV}" ]检查DEPLOY_ENV长度是否为零。所以你可以运行：
if [[ -z "${DEPLOY_ENV}" ]]; then
MY_SCRIPT_VARIABLE="Some default value because DEPLOY_ENV is undefined"
else
MY_SCRIPT_VARIABLE="${DEPLOY_ENV}"
fi
# or using a short-hand version
[[ -z "${DEPLOY_ENV}" ]] && MyVar='default' || MyVar="${DEPLOY_ENV}"
# or even shorter use
MyVar="${DEPLOY_ENV:-default_value}"
回答by Eugene Yarmash
You could just use parameter expansion:
你可以只使用参数扩展：
${parameter:-word}
If parameteris unset or null, the expansion of wordis substituted. Otherwise, the value of parameteris substituted.
${参数:-word}
如果参数未设置或为空，则替换单词的扩展。否则，替换参数的值。
So try this:
所以试试这个：
var=${DEPLOY_ENV:-default_value}
There's also the ${parameter-word}form, which substitutes the default value only when parameteris unset (but not when it's null).
还有${parameter-word}形式，它仅在参数未设置时（但不为空时）才替换默认值。
To demonstrate the difference between the two:
为了证明两者之间的区别：
$ unset DEPLOY_ENV
$ echo "'${DEPLOY_ENV:-default_value}' '${DEPLOY_ENV-default_value}'"
'default_value' 'default_value'
$ DEPLOY_ENV=
$ echo "'${DEPLOY_ENV:-default_value}' '${DEPLOY_ENV-default_value}'"
'default_value' ''
回答by chepner
If you don't care about the difference between an unset variable or a variable with an empty value, you can use the default-value parameter expansion:
如果您不关心未设置变量或具有空值的变量之间的区别，则可以使用默认值参数扩展：
foo=${DEPLOY_ENV:-default}
If you docare about the difference, drop the colon
如果您确实关心差异，请删除冒号
foo=${DEPLOY_ENV-default}
You can also use the -voperator to explicitly test if a parameter is set.
您还可以使用-v运算符显式测试是否设置了参数。
if [[ ! -v DEPLOY_ENV ]]; then
echo "DEPLOY_ENV is not set"
elif [[ -z "$DEPLOY_ENV" ]]; then
echo "DEPLOY_ENV is set to the empty string"
else
echo "DEPLOY_ENV has the value: $DEPLOY_ENV"
fi
回答by Robert
There is no difference between environment variables and variables in a script. Environment variables are just defined earlier, outside the script, before the script is called. From the script's point of view, a variable is a variable.
环境变量和脚本中的变量没有区别。环境变量只是在调用脚本之前在脚本之外定义的。从脚本的角度来看，变量就是变量。
You can check if a variable is defined:
您可以检查是否定义了变量：
if [ -z "$a" ]
then
echo "not defined"
else
echo "defined"
fi
and then set a default value for undefined variables or do something else.
然后为未定义的变量设置默认值或做其他事情。
The -zchecks for a zero-length (i.e. empty) string. See man bashand look for the CONDITIONAL EXPRESSIONS section.
在-z一个零长度的检查（即空的）的字符串。查看man bash并查找 CONDITIONAL EXPRESSIONS 部分。
You can also use set -uat the beginning of your script to make it fail once it encounters an undefined variable, if you want to avoid having an undefined variable breaking things in creative ways.
set -u如果您想避免未定义的变量以创造性的方式破坏事物，您还可以在脚本的开头使用，使其在遇到未定义的变量时失败。
回答by Preethi Srinivasan
NEW_VAR=""
if [[ ${ENV_VAR} && ${ENV_VAR-x} ]]; then
NEW_VAR=${ENV_VAR}
else
NEW_VAR="new value"
fi
回答by Nicolas El Khoury
All the answers worked. However, I had to add the variables that I needed to get to the sudoers files as follows:
所有的答案都有效。但是，我必须添加获取 sudoers 文件所需的变量，如下所示：
sudo visudo
Defaults env_keep += "<var1>, <var2>, ..., <varn>"
