Negate if condition in bash script

问题

I'm new to bash and I'm stuck at trying to negate the following command:

wget -q --tries=10 --timeout=20 --spider http://google.com
if [[ $? -eq 0 ]]; then
        echo "Sorry you are Offline"
        exit 1

This if condition returns true if I'm connected to the internet. I want it to happen the other way around but putting ! anywhere doesn't seem to work.

回答1:

You can choose:

if [[ $? -ne 0 ]]; then       # -ne: not equal

if ! [[ $? -eq 0 ]]; then     # -eq: equal

if [[ ! $? -eq 0 ]]; then

! inverts the return of the following expression, respectively.

回答2:

Better

if ! wget -q --spider --tries=10 --timeout=20 google.com
then
  echo 'Sorry you are Offline'
  exit 1
fi

回答3:

If you're feeling lazy, here's a terse method of handling conditions using || (or) and && (and) after the operation:

wget -q --tries=10 --timeout=20 --spider http://google.com || \
{ echo "Sorry you are Offline" && exit 1; }

回答4:

Since you're comparing numbers, you can use an arithmetic expression, which allows for simpler handling of parameters and comparison:

wget -q --tries=10 --timeout=20 --spider http://google.com
if (( $? != 0 )); then
    echo "Sorry you are Offline"
    exit 1
fi

Notice how instead of -ne, you can just use !=. In an arithmetic context, we don't even have to prepend $ to parameters, i.e.,

var_a=1
var_b=2
(( var_a < var_b )) && echo "a is smaller"

works perfectly fine. This doesn't appply to the $? special parameter, though.

Further, since (( ... )) evaluates non-zero values to true, i.e., has a return status of 0 for non-zero values and a return status of 1 otherwise, we could shorten to

if (( $? )); then

but this might confuse more people than the keystrokes saved are worth.

The (( ... )) construct is available in Bash, but not required by the POSIX shell specification (mentioned as possible extension, though).

This all being said, it's better to avoid $? altogether in my opinion, as in Cole's answer and Steven's answer.

回答5:

You can use unequal comparison -ne instead of -eq:

wget -q --tries=10 --timeout=20 --spider http://google.com
if [[ $? -ne 0 ]]; then
    echo "Sorry you are Offline"
    exit 1
fi

来源：https://stackoverflow.com/questions/26475358/negate-if-condition-in-bash-script

标签

Linux

bash

if-statement

negate