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

我总是使用这个,因为任何第一次看到代码的人都很容易理解:

if [ "$variable" = "" ]
    then
    echo "Variable X is empty"
fi

如果要检查是否为空;

if [ ! "$variable" = "" ]
    then
    echo "Variable X is not empty"
fi

就是这样。

以下是如何测试参数是否未设置、是否为空(“Null”)或是否设置了值:

+--------------------+----------------------+-----------------+-----------------+
|   Expression       |       parameter      |     parameter   |    parameter    |
|   in script:       |   Set and Not Null   |   Set But Null  |      Unset      |
+--------------------+----------------------+-----------------+-----------------+
| ${parameter:-word} | substitute parameter | substitute word | substitute word |
| ${parameter-word}  | substitute parameter | substitute null | substitute word |
| ${parameter:=word} | substitute parameter | assign word     | assign word     |
| ${parameter=word}  | substitute parameter | substitute null | assign word     |
| ${parameter:?word} | substitute parameter | error, exit     | error, exit     |
| ${parameter?word}  | substitute parameter | substitute null | error, exit     |
| ${parameter:+word} | substitute word      | substitute null | substitute null |
| ${parameter+word}  | substitute word      | substitute word | substitute null |
+--------------------+----------------------+-----------------+-----------------+

来源:POSIX:参数扩展:

在所有显示为“替换”的情况下,表达式将替换为显示的值。在所有显示为“assign”的情况下,参数都被指定该值,该值也会替换表达式。

要在操作中显示此内容,请执行以下操作:

+--------------------+----------------------+-----------------+-----------------+
|   Expression       |  When FOO="world"    |  When FOO=""    |    unset FOO    |
|   in script:       |  (Set and Not Null)  |  (Set But Null) |     (Unset)     |
+--------------------+----------------------+-----------------+-----------------+
| ${FOO:-hello}      | world                | hello           | hello           |
| ${FOO-hello}       | world                | ""              | hello           |
| ${FOO:=hello}      | world                | FOO=hello       | FOO=hello       |
| ${FOO=hello}       | world                | ""              | FOO=hello       |
| ${FOO:?hello}      | world                | error, exit     | error, exit     |
| ${FOO?hello}       | world                | ""              | error, exit     |
| ${FOO:+hello}      | hello                | ""              | ""              |
| ${FOO+hello}       | hello                | hello           | ""              |
+--------------------+----------------------+-----------------+-----------------+
[[ $foo ]]

Or

(( ${#foo} ))

Or

let ${#foo}

Or

declare -p foo

如果未设置,则要退出

这对我很有用。如果没有设置参数,我希望脚本退出并显示错误消息。

#!/usr/bin/env bash

set -o errexit

# Get the value and empty validation check all in one
VER="${1:?You must pass a version of the format 0.0.0 as the only argument}"

运行时返回错误

peek@peek:~$ ./setver.sh
./setver.sh: line 13: 1: You must pass a version of the format 0.0.0 as the only argument

仅检查,不退出-空和未设置无效

如果您只想检查值set=VALID或unset/empty=INVALID,请尝试此选项。

TSET="good val"
TEMPTY=""
unset TUNSET

if [ "${TSET:-}" ]; then echo "VALID"; else echo "INVALID";fi
# VALID
if [ "${TEMPTY:-}" ]; then echo "VALID"; else echo "INVALID";fi
# INVALID
if [ "${TUNSET:-}" ]; then echo "VALID"; else echo "INVALID";fi
# INVALID

或者,即使是短期测试;-)

[ "${TSET:-}"   ] && echo "VALID" || echo "INVALID"
[ "${TEMPTY:-}" ] && echo "VALID" || echo "INVALID"
[ "${TUNSET:-}" ] && echo "VALID" || echo "INVALID"

仅检查,不退出-仅空为无效

这就是问题的答案。如果您只想检查值set/empty=VALID或unset=INVALID,请使用此选项。

注意,“..-1}”中的“1”无关紧要,它可以是任何东西(比如x)

TSET="good val"
TEMPTY=""
unset TUNSET

if [ "${TSET+1}" ]; then echo "VALID"; else echo "INVALID";fi
# VALID
if [ "${TEMPTY+1}" ]; then echo "VALID"; else echo "INVALID";fi
# VALID
if [ "${TUNSET+1}" ]; then echo "VALID"; else echo "INVALID";fi
# INVALID

短期测试

[ "${TSET+1}"   ] && echo "VALID" || echo "INVALID"
[ "${TEMPTY+1}" ] && echo "VALID" || echo "INVALID"
[ "${TUNSET+1}" ] && echo "VALID" || echo "INVALID"

我把这个答案献给了@mklement0(comments),他要求我准确回答这个问题。

参考:2.6.2参数扩展