我有一些Unix shell脚本，需要在开始执行操作之前检查某些环境变量是否设置好，因此我会做类似这样的事情：
if [ -z "$STATE" ]; then
echo "Need to set STATE"
exit 1
fi
if [ -z "$DEST" ]; then
echo "Need to set DEST"
exit 1
fi
这需要输入很多内容。有没有更优雅的习语来检查一个环境变量集是否已设置？
编辑：我应该提到这些变量没有有意义的默认值 - 如果任何一个未设置，脚本应该出错。
显然的答案是使用参数扩展的特殊形式之一：
: ${STATE?"Need to set STATE"}
: ${DEST:?"Need to set DEST non-empty"}
或者更好（请参阅下文关于双引号位置的部分）：
: "${STATE?Need to set STATE}"
: "${DEST:?Need to set DEST non-empty}"
第一种变体（仅使用?）需要设置STATE，但STATE =“”（空字符串）是可以的-不完全是您想要的，但是是替代和较旧的表示法。
第二个变体（使用：？）要求设置并且非空的DEST。
如果您没有提供消息，系统会提供默认的消息。
${var?}结构可移植回到 Version 7 UNIX 和 Bourne Shell（大约在1978年）。${var:?}结构稍微新一些：我认为它是在 System III UNIX（大约在1981年）中出现的，但它可能先出现在 PWB UNIX 中。因此它存在于 Korn Shell 中，并在 POSIX shells 中，包括特别是 Bash。
它通常在 shell 的 man 页面中记录在名为 "Parameter Expansion" 的部分中。例如，bash 手册说：
${parameter:?word}如果为Null或未设置，则显示错误。 如果参数为null或未设置，则会将单词扩展（或一条关于此单词的消息）写入标准错误流，并退出非交互式shell。否则，将替换参数的值。
我应该补充一下，冒号命令只是对其参数进行求值，然后成功执行。这是原始的shell注释符号（在＃到行末之前）。很长一段时间，Bourne shell脚本的第一个字符都是冒号。C Shell会读取脚本，并使用第一个字符来确定它是为C Shell（＃哈希）还是Bourne shell（：冒号）而编写的。然后内核加入了支持#!/path/to/program，Bourne shell增加了＃注释，冒号传统就被淘汰了。但是，如果您遇到以冒号开头的脚本，现在您将知道为什么。
对于这个讨论有什么想法？https://github.com/koalaman/shellcheck/issues/380#issuecomment-145872749
讨论的要点是：
但是，当我使用0.4.1版本的
shellcheck进行检查时，我收到了这条信息：In script.sh line 13: : ${FOO:?"The environment variable FOO must be set and non-empty"} ^-- SC2086: Double quote to prevent globbing and word splitting.关于这件事，你有什么建议？
简短的回答是“按照shellcheck所建议的去做”。
: "${STATE?Need to set STATE}"
: "${DEST:?Need to set DEST non-empty}"
To illustrate why, study the following. Note that the : command doesn t echo its arguments (but the shell does evaluate the arguments). We want to see the arguments, so the code below uses printf "%s
" in place of :.
$ mkdir junk
$ cd junk
$ > abc
$ > def
$ > ghi
$
$ x="*"
$ printf "%s
" ${x:?You must set x} # Careless; not recommended
abc
def
ghi
$ unset x
$ printf "%s
" ${x:?You must set x} # Careless; not recommended
bash: x: You must set x
$ printf "%s
" "${x:?You must set x}" # Careful: should be used
bash: x: You must set x
$ x="*"
$ printf "%s
" "${x:?You must set x}" # Careful: should be used
*
$ printf "%s
" ${x:?"You must set x"} # Not quite careful enough
abc
def
ghi
$ x=
$ printf "%s
" ${x:?"You must set x"} # Not quite careful enough
bash: x: You must set x
$ unset x
$ printf "%s
" ${x:?"You must set x"} # Not quite careful enough
bash: x: You must set x
$
请注意，在整个表达式没有双引号的情况下，$x 中的值会扩展为首先是*，然后是文件名列表。这就是shellcheck建议修复的内容。我没有验证它是否不接受将表达式放在双引号中的形式，但是合理的假设是可以的。
尝试一下：
[ -z "$STATE" ] && echo "Need to set STATE" && exit 1;
你的问题取决于你所使用的 shell。
Bourne shell 对你追求的东西几乎没有什么帮助。
但是...
它确实在几乎所有地方都可以工作。
尽量远离csh。与Bourne shell相比，它的铃铛和口哨增加了很多优点，但现在它已经开始出问题了。如果你不相信我，就尝试在csh中分离STDERR! (-:
There are two possibilities here. The example above, namely using:
${MyVariable:=SomeDefault}
第一次使用，您需要引用$MyVariable。 这将获取环境变量MyVariable，如果当前未设置，则为变量分配SomeDefault的值以供以后使用。
你也有可能：
${MyVariable:-SomeDefault}
这只是将SomeDefault替换为您使用此结构的变量。它不会将SomeDefault的值分配给变量，且在遇到此语句后，MyVariable的值仍将为null。
Surely the simplest approach is to add the -u switch to the shebang (the line at the top of your script), assuming you’re using bash:
#!/bin/sh -u
This will cause the script to exit if any unbound variables lurk within.
${MyVariable:=SomeDefault}
If MyVariable is set and not null, it will reset the variable value (= nothing happens).
Else, MyVariable is set to SomeDefault.
The above will attempt to execute ${MyVariable}, so if you just want to set the variable do:
MyVariable=${MyVariable:=SomeDefault}
In my opinion the simplest and most compatible check for #!/bin/sh is:
if [ "$MYVAR" = "" ]
then
echo "Does not exist"
else
echo "Exists"
fi
Again, this is for /bin/sh and is compatible also on old Solaris systems.
bash 4.2 introduced the -v operator which tests if a name is set to any value, even the empty string.
$ unset a
$ b=
$ c=
$ [[ -v a ]] && echo "a is set"
$ [[ -v b ]] && echo "b is set"
b is set
$ [[ -v c ]] && echo "c is set"
c is set
I always used:
if [ "x$STATE" == "x" ]; then echo "Need to set State"; exit 1; fi
Not that much more concise, I m afraid.
Under CSH you have $?STATE.
For future people like me, I wanted to go a step forward and parameterize the var name, so I can loop over a variable sized list of variable names:
#!/bin/bash
declare -a vars=(NAME GITLAB_URL GITLAB_TOKEN)
for var_name in "${vars[@]}"
do
if [ -z "$(eval "echo $$var_name")" ]; then
echo "Missing environment variable $var_name"
exit 1
fi
done
We can write a nice assertion to check a bunch of variables all at once:
#
# assert if variables are set (to a non-empty string)
# if any variable is not set, exit 1 (when -f option is set) or return 1 otherwise
#
# Usage: assert_var_not_null [-f] variable ...
#
function assert_var_not_null() {
local fatal var num_null=0
[[ "$1" = "-f" ]] && { shift; fatal=1; }
for var in "$@"; do
[[ -z "${!var}" ]] &&
printf %s
"Variable $var not set" >&2 &&
((num_null++))
done
if ((num_null > 0)); then
[[ "$fatal" ]] && exit 1
return 1
fi
return 0
}
Sample invocation:
one=1 two=2
assert_var_not_null one two
echo test 1: return_code=$?
assert_var_not_null one two three
echo test 2: return_code=$?
assert_var_not_null -f one two three
echo test 3: return_code=$? # this code shouldn t execute
Output:
test 1: return_code=0
Variable three not set
test 2: return_code=1
Variable three not set
More such assertions here: https://github.com/codeforester/base/blob/master/lib/assertions.sh
This can be a way too:
if (set -u; : $HOME) 2> /dev/null
...
...
http://unstableme.blogspot.com/2007/02/checks-whether-envvar-is-set-or-not.html
None of the above solutions worked for my purposes, in part because I checking the environment for an open-ended list of variables that need to be set before starting a lengthy process. I ended up with this:
mapfile -t arr < variables.txt
EXITCODE=0
for i in "${arr[@]}"
do
ISSET=$(env | grep ^${i}= | wc -l)
if [ "${ISSET}" = "0" ];
then
EXITCODE=-1
echo "ENV variable $i is required."
fi
done
exit ${EXITCODE}
Rather than using external shell scripts I tend to load in functions in my login shell. I use something like this as a helper function to check for environment variables rather than any set variable:
is_this_an_env_variable ()
local var="$1"
if env |grep -q "^$var"; then
return 0
else
return 1
fi
}