我如何知道是否在Bash中设置了变量?
例如,如何检查用户是否向函数提供了第一个参数?
function a {
# if $1 is set ?
}
我如何知道是否在Bash中设置了变量?
例如,如何检查用户是否向函数提供了第一个参数?
function a {
# if $1 is set ?
}
当前回答
if [[ ${1:+isset} ]]
then echo "It was set and not null." >&2
else echo "It was not set or it was null." >&2
fi
if [[ ${1+isset} ]]
then echo "It was set but might be null." >&2
else echo "It was was not set." >&2
fi
其他回答
我喜欢辅助功能来隐藏Bash的粗糙细节。在这种情况下,这样做会增加更多(隐藏的)粗糙度:
# The first ! negates the result (can't use -n to achieve this)
# the second ! expands the content of varname (can't do ${$varname})
function IsDeclared_Tricky
{
local varname="$1"
! [ -z ${!varname+x} ]
}
因为我在这个实现中首先遇到了bug(灵感来自Jens和Lionel的回答),所以我想出了一个不同的解决方案:
# Ask for the properties of the variable - fails if not declared
function IsDeclared()
{
declare -p $1 &>/dev/null
}
我发现它更直接,更害羞,更容易理解/记住。测试用例表明它是等效的:
function main()
{
declare -i xyz
local foo
local bar=
local baz=''
IsDeclared_Tricky xyz; echo "IsDeclared_Tricky xyz: $?"
IsDeclared_Tricky foo; echo "IsDeclared_Tricky foo: $?"
IsDeclared_Tricky bar; echo "IsDeclared_Tricky bar: $?"
IsDeclared_Tricky baz; echo "IsDeclared_Tricky baz: $?"
IsDeclared xyz; echo "IsDeclared xyz: $?"
IsDeclared foo; echo "IsDeclared foo: $?"
IsDeclared bar; echo "IsDeclared bar: $?"
IsDeclared baz; echo "IsDeclared baz: $?"
}
main
测试用例还显示,局部var不声明var(除非后面跟着“=”)。很长一段时间以来,我以为我是这样声明变量的,只是为了发现我只是表达了我的意图。。。我想这是不可能的。
IsDeclared_Tricky xyz:1IsDeclared_Tricky foo:1IsDeclared_Tricky条:0我声明的Tricky baz:0IsDeclared xyz:1IsDeclared foo:1IsDeclared栏:0IsDeclared baz:0
奖金:用例
我主要使用此测试以某种“优雅”和安全的方式(几乎类似于接口…)向函数提供(和返回)参数:
# Auxiliary functions
function die()
{
echo "Error: $1"; exit 1
}
function assertVariableDeclared()
{
IsDeclared "$1" || die "variable not declared: $1"
}
function expectVariables()
{
while (( $# > 0 )); do
assertVariableDeclared $1; shift
done
}
# Actual example
function exampleFunction()
{
expectVariables inputStr outputStr
outputStr="$inputStr, World!"
}
function bonus()
{
local inputStr='Hello'
local outputStr= # Remove this to trigger the error
exampleFunction
echo $outputStr
}
bonus
如果调用时声明了所有必需的变量:
你好,世界!
其他:
错误:未声明变量:outputStr
[[ $foo ]]
Or
(( ${#foo} ))
Or
let ${#foo}
Or
declare -p foo
对于那些希望在使用set-u的脚本中检查未设置或空的脚本:
if [ -z "${var-}" ]; then
echo "Must provide var environment variable. Exiting...."
exit 1
fi
常规[-z“$var”]检查将因var而失败;未绑定变量如果设置-u但[-z“${var-}”]如果var未设置而不失败,则扩展为空字符串。
为了明确回答OP关于如何确定变量是否已设置的问题,Lionel的回答是正确的:
if test "${name+x}"; then
echo 'name is set'
else
echo 'name is not set'
fi
这个问题已经有很多答案,但没有一个提供真正的布尔表达式来明确区分变量值。
以下是我得出的一些明确表达:
+-----------------------+-------------+---------+------------+
| Expression in script | name='fish' | name='' | unset name |
+-----------------------+-------------+---------+------------+
| test "$name" | TRUE | f | f |
| test -n "$name" | TRUE | f | f |
| test ! -z "$name" | TRUE | f | f |
| test ! "${name-x}" | f | TRUE | f |
| test ! "${name+x}" | f | f | TRUE |
+-----------------------+-------------+---------+------------+
顺便说一下,这些表达式是等价的:测试<表达式><=>〔<表达式>〕
其他需谨慎使用的歧义表达:
+----------------------+-------------+---------+------------+
| Expression in script | name='fish' | name='' | unset name |
+----------------------+-------------+---------+------------+
| test "${name+x}" | TRUE | TRUE | f |
| test "${name-x}" | TRUE | f | TRUE |
| test -z "$name" | f | TRUE | TRUE |
| test ! "$name" | f | TRUE | TRUE |
| test ! -n "$name" | f | TRUE | TRUE |
| test "$name" = '' | f | TRUE | TRUE |
+----------------------+-------------+---------+------------+
要检查是否设置了变量,请执行以下操作:
var=""; [[ $var ]] && echo "set" || echo "not set"