how to get Logical Not in shell?

问题:

I am testing whether command exist like this:

if hash pm2 2>/dev/null; then
    echo "already exist"
else
    npm install --global pm2
fi

but in fact I just want to do this

if not exist
   install it
fi

I tried this

if [ ! hash pm2 2>/dev/null ]; then
    npm install --global pm2
fi

it is not ok

回答1:

Just negate the condition in your if:

if ! hash pm2 2>/dev/null; then
#  ^
    npm install --global pm2
fi

If you want to use the test command [ you have to enclose the command within a $() to get it evaluated:

if [ ! $(hash pm2 2>/dev/null) ]; then

Example

Let's create an empty file:

$ touch a

And check if it does not contain some text, eg, 5:

$ if ! grep -sq 5 a; then echo "no 5 here"; fi
no 5 here
$ if [ ! $(grep -sq 5 a) ]; then echo "no 5 here"; fi
no 5 here