Meaning of $? (dollar question mark) in shell scripts

$? returns the exit value of the last executed command. echo $? prints that value on console. zero implies a successful execution while non-zero values are mapped to various reason for failure.

Hence when scripting; I tend to use the following syntax

if [ $? -eq 0 ]; then
 # do something
else
 # do something else
fi

The comparison is to be done on equals to 0 or not equals 0.

** Update Based on the comment: Ideally, you should not use the above code block for comparison, refer to @tripleee comments and explanation.

This is the exit status of the last executed command.

For example the command true always returns a status of 0 and false always returns a status of 1:

true
echo $? # echoes 0
false
echo $? # echoes 1

From the manual: (acessible by calling man bash in your shell)

?       Expands to the exit status of the most recently executed foreground pipeline.

By convention an exit status of 0 means success, and non-zero return status means failure. Learn more about exit statuses on wikipedia.

There are other special variables like this, as you can see on this online manual: https://www.gnu.org/s/bash/manual/bash.html#Special-Parameters

echo $? - Gives the EXIT STATUS of the most recently executed command . This EXIT STATUS would most probably be a number with ZERO implying Success and any NON-ZERO value indicating Failure

? - This is one special parameter/variable in bash.

$? - It gives the value stored in the variable "?".

Some similar special parameters in BASH are 1,2,*,# ( Normally seen in echo command as $1 ,$2 , $* , $# , etc., ) .

It has the last status code (exit value) of a command.