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There is this line in a shell script i have seen:
grep -e ERROR ${LOG_DIR_PATH}/${LOG_NAME} > /dev/null if [ $? -eq 0 ] 
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$? is the exit status of the most recently-executed command; by convention, 0 means success and anything else indicates failure. That line is testing whether the grep command succeeded. The grep manpage states: The exit status is 0 if selected lines are found, and 1 if not found.
If you execute ./script.sh , $0 will give output ./script.sh but if you execute it with bash script.sh it will give output script.sh . Show activity on this post. They are called the Positional Parameters.
Show activity on this post. 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.
As explained in this comment on that answer you link to, echo $0 simply shows you the name of the currently running process: $0 is the name of the running process. If you use it inside of a shell then it will return the name of the shell. If you use it inside of a script, it will be the name of the script.
$? is the exit status of the most recently-executed command; by convention, 0 means success and anything else indicates failure. That line is testing whether the grep command succeeded.
The grep manpage states:
The exit status is 0 if selected lines are found, and 1 if not found. If an error occurred the exit status is 2. (Note: POSIX error handling code should check for '2' or greater.)
So in this case it's checking whether any ERROR lines were found.
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