Bash 中的“${1+/$1}”是什么意思？

Question

I was reading a Bash script with this line:

FOO=${1+/$1}

What does this line do?

Answer 1

The / makes it look slightly more confusing than it is, but this is just an example of ${foo+bar} , which expands to bar if $foo is set.

In this case, the variable is $1 , the first positional parameter passed to the script or function.

example () {
  echo "${1+/$1}"
}

example     # outputs nothing
example ''  # outputs "/"
example foo # outputs "/foo"

There is a table that summarises these parameter expansions in the spec. The rules for ${parameter+word} are:

• Set and Not Null: substitute word
• Set But Null: substitute word
• Unset: substitute null

So to answer your question directly, FOO=${1+/$1} assigns /$1 to FOO is $1 is set, otherwise FOO is set to null (an empty string).