CODEWITHSUNDEEP
bash
serv-inc
serv-inc

Reputation: 38207

Where to negate logical expression in bash

How do you negate a test in bash if you want to combine multiple tests?

The code is

if ! [ $(pgrep Xvfb) ] || [ ! -v DISPLAY ]; then
    echo starting xvfb
    mkdir -p /tmp/xvfb
    Xvfb :1 -fbdir /tmp/xvfb > /tmp/xvfb_output 2>&1 &
    export DISPLAY=:1
fi

It is supposed to be sourced and to start Xvfb if not already running. Previously, it lacked the || [ ! -v DISPLAY ] part to check the existence of said variable.

To negate a test, you could do either ! [ ... ] or [ ! ... ], which both seem to work.

Is the reasoning correct that you should use [ ! ...] because it is inside the test and thus clearer (and a bit more efficient)?

Upvotes: 3

Views: 5267

Answers (1)

Julien Lopez
Julien Lopez

Reputation: 1844

I'm pretty sure it doesn't matter where you put the negation in terms of efficiency. As for readability, you could write your if statement like this:

if ! (test "$(pgrep Xvfb)" -a -n "${DISPLAY:+1}"); then

This way you have only one negation on one test.

I agree with you that

if ! [ $(pgrep Xvfb) ] || [ ! -v DISPLAY ]; then

is ambiguous and

if [ ! $(pgrep Xvfb) ] || [ ! -v DISPLAY ]; then

is not.

Upvotes: 6

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