Bash function to check if a given variable is set

Question

As explained in this answer, the "right" way to check if a variable is set in bash looks like this:

if [ -z ${var+x} ]; then
    echo "var is unset"
else
    echo "var is set to '$var'"
fi

What I'm interested in is how to extract this into a function that can be reused for different variables.

The best I've been able to do so far is:

is_set() {
  local test_start='[ ! -z ${'
  local test_end='+x} ]'
  local tester=$test_start$1$test_end

  eval $tester
}

It seems to work, but is there a better way that doesn't resort to calling eval?

Answer 1

在 Bash 中，您可以使用[[ -v var ]]. 不需要函数或复杂的方案。

从联机帮助页：

   -v varname
          True if the shell variable varname is set (has been assigned a value).

前 2 个命令序列打印ok：

[[ -v PATH ]] && echo ok

var="" ; [[ -v var ]] && echo ok

unset var ; [[ -v var ]] && echo ok