blocks|key|1864552|text|环境变量和脚本中的变量没有区别。在脚本被调用之前，环境变量只是在脚本之外定义的。从脚本的角度来看，变量就是变量。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1864553|您可以检查是否定义了变量：|1864554|if+[+-z+"$a"+]
then
++++echo+"not+defined"
else+
++++echo+"defined"
fi|code-block|syntax|javascript|1864555|然后为未定义的变量设置默认值或执行其他操作。|1864556|-z检查长度为零(即空)的字符串。请参见man+bash并查找条件表达式部分。|offset|length|style|CODE|1864557|如果你不想让一个未定义的变量以创造性的方式破坏事情，你也可以在你的脚本的开头使用set+-u，使它在遇到一个未定义的变量时失败。|1864558|entityMap^0|0|0|0|0|0|2|K|8|0|14|6|0^^$0|@$1|2|3|4|5|6|7|U|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|V|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|W|8|@]|9|@]|A|$G|H]]|$1|I|3|J|5|6|7|X|8|@]|9|@]|A|$]]|$1|K|3|L|5|6|7|Y|8|@$M|Z|N|10|O|P]|$M|11|N|12|O|P]]|9|@]|A|$]]|$1|Q|3|R|5|6|7|13|8|@$M|14|N|15|O|P]]|9|@]|A|$]]|$1|S|3|-4|5|6|7|16|8|@]|9|@]|A|$]]]|T|$]]

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.

You can check if a variable is defined:

<pre><code>if [ -z "$a" ]
then
 echo "not defined"
else 
 echo "defined"
fi
</code></pre>

and then set a default value for undefined variables or do something else.

The <code>-z</code> checks for a zero-length (i.e. empty) string. See <code>man bash</code> and look for the CONDITIONAL EXPRESSIONS section.

You can also use <code>set -u</code> at 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.

blocks|key|1864618|text|[+-z+"${DEPLOY_ENV}"+]检查DEPLOY_ENV的长度是否等于零。这样你就可以运行：|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1864619|if+[[+-z+"${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+"${DEPLOY_ENV}"+]]+&&+MyVar='default'+%7C%7C+MyVar="${DEPLOY_ENV}"

#+or+even+shorter+use

MyVar="${DEPLOY_ENV:-default_value}"|code-block|syntax|javascript|1864620|entityMap^0|0|M|O|A|0|0^^$0|@$1|2|3|4|5|6|7|M|8|@$9|N|A|O|B|C]|$9|P|A|Q|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|R|8|@]|D|@]|E|$I|J]]|$1|K|3|-4|5|6|7|S|8|@]|D|@]|E|$]]]|L|$]]

<code>[ -z "${DEPLOY_ENV}" ]</code> checks whether <code>DEPLOY_ENV</code> has length equal to zero. So you could run:

<pre><code>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 "${DEPLOY_ENV}" ]] &amp;&amp; MyVar='default' || MyVar="${DEPLOY_ENV}"

# or even shorter use

MyVar="${DEPLOY_ENV:-default_value}"
</code></pre>

blocks|key|1864889|text|如果您不关心未设置的变量和具有空值的变量之间的区别，则可以使用默认值参数扩展：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1864890|foo=${DEPLOY_ENV:-default}|code-block|syntax|javascript|1864891|如果您确实关心差异，请去掉冒号|1864892|foo=${DEPLOY_ENV-default}|1864893|您还可以使用-v运算符来显式测试是否设置了参数。|offset|length|style|CODE|1864894|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|1864895|entityMap^0|0|0|0|0|6|2|0|0^^$0|@$1|2|3|4|5|6|7|U|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|V|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|W|8|@]|9|@]|A|$]]|$1|I|3|J|5|D|7|X|8|@]|9|@]|A|$E|F]]|$1|K|3|L|5|6|7|Y|8|@$M|Z|N|10|O|P]]|9|@]|A|$]]|$1|Q|3|R|5|D|7|11|8|@]|9|@]|A|$E|F]]|$1|S|3|-4|5|6|7|12|8|@]|9|@]|A|$]]]|T|$]]

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:

<pre><code>foo=${DEPLOY_ENV:-default}
</code></pre>

If you do care about the difference, drop the colon

<pre><code>foo=${DEPLOY_ENV-default}
</code></pre>

You can also use the <code>-v</code> operator to explicitly test if a parameter is set.

<pre><code>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
</code></pre>

blocks|key|1650672|text|NEW_VAR=""
if+[[+${ENV_VAR}+&&+${ENV_VAR-x}+]];+then
++NEW_VAR=${ENV_VAR}
else
++NEW_VAR="new+value"
fi|type|code-block|depth|inlineStyleRanges|entityRanges|data|syntax|javascript|1650673|unstyled|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|G|8|@]|9|@]|A|$B|C]]|$1|D|3|-4|5|E|7|H|8|@]|9|@]|A|$]]]|F|$]]

<pre><code>NEW_VAR=""
if [[ ${ENV_VAR} &amp;&amp; ${ENV_VAR-x} ]]; then
 NEW_VAR=${ENV_VAR}
else
 NEW_VAR="new value"
fi
</code></pre>

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.

For instance, assume, before running the script, I perform the following operation:

<pre><code>export DEPLOY_ENV=dev
</code></pre>

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?

How to check if an environment variable exists and get its value?

我正在写一个shell脚本。在这个shell脚本中，我有一个变量，它要么接受默认值，要么接受环境变量值。但是，环境变量不一定要存在。例如，假设在运行脚本之前，我执行了以下操作：export DEPLOY_ENV=dev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，我如何告诉脚本，如果此环境变量不存在，...

问如何检查环境变量是否存在并获取其值？
EN

社区

活动

资源

关于

腾讯云开发者

热门产品

热门推荐

更多推荐

问如何检查环境变量是否存在并获取其值？EN

社区

活动

资源

关于

腾讯云开发者

热门产品

热门推荐

更多推荐

问如何检查环境变量是否存在并获取其值？
EN