Linux – Evaluation of backticks in bash

bashlinux

I'm confused about the evaluation of backticks in bash. I've seen code like this before which makes sense:

RESULT=`whoami`

This stores the output of the whoami command in the RESULT variable and confirms that backticks evaluate to the output of the command within them. But I've also seen code like the following:

if `wget google.com -T 2 -t 1 -o /dev/null`; then 
   echo "Internet ok"
fi

This is testing for internet connectivity by trying to get the google homepage (with a -t 1 for a single try, a -T 2 for a 2 second timeout, and a redirection to /dev/null since it doesn't actually want the output rather it just wants to see if it succeeds). The code reads as if the author was expecting that the backticks would evaluate to the exit code of the wget command rather than the output.

But it actually works. It prints Internet ok if it can connect to google.com and if I change the url to some nonsense url that doesn't exist, it doesn't satisfy the if statement and prints nothing. I don't understand why this works.

Independently running the wget command with valid and invalid urls prints nothing in both cases and only differs by exit code.

My conclusion is that there is a special construct for if followed by backticks that returns the exit code rather than the output. Am I wrong?

Best Answer

No, the backticks do not have a special meaning and you even could run the wget command without them in the if statement.
if always evaluates the exit code of the command followed.

A more comprehensive overview can be found here.

EDIT

Backticks initiate a command substitution which is executed in a subshell and returns an exit code. if just checks return codes, it does not make use of the output of the command.

To compare the output of a command you could use the [ , which must be closed with ], which basically is a test. The test could be anything

  • from man test

  • a string comparison 

    [ "hello" == "test" ]

  • an integer test 

    [ 2 -eq 3 ]

If the test succeeds you get an exit code 0 ( true ), otherwise not 0 ( false ) which is again evaluated by if.

So you have something in mind like follows.

if [ "`wget google.com -T 2 -t 1 -o /dev/null`" == "" ]
then 
    echo "emptY"
fi

But this wouldn't make much sense, due to redirecting the output to /dev/null you will always get true from [ "wget google.com -T 2 -t 1 -o /dev/null" == "" ].
On the other hand it also would not be useful to check if the output of the wget command does contain output, leaving out the -o /dev/null, because even in an error situation you will get output, but different return codes.

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