What does this shell test achieve

问题

I have a very simple question that I can't answer. In shell, what would this command do:

test -d $VIRTUAL_ENV || virtualenv $VIRTUAL_ENV

It seems like it tests if the virtualenv directory exists, but I dont understand what if does with that information. Will it always create the virtualenv afterwards, or would it only do it if it doesn't already exist?

回答1:

The || is the OR condition. Hence, this will test if $VIRTUAL_ENV directory exists. If not, it will run virtualenv $VIRTUAL_ENV.

Other examples:

$ test -d /tmp || echo "yes"
$
$ test -d /tmpblabla || echo "this dir does not exist"
this dir does not exist
$ test -d /tmp && echo "/tmp exists" || echo "yes"
/tmp exists

回答2:

It tests if the directory $VIRTUAL_ENV exists and otherwise creates it using virtualenv

来源：https://stackoverflow.com/questions/19768796/what-does-this-shell-test-achieve