How to assign a variable inside if condition in shell script - bash

Question

I would like to know how to assign a variable inside if block in shell script..
Below is my code..
if [[ -z "$MMBOX_PATH" || -z "$BACKUP_PATH" || -z "$REMOTE_SERVER" || -z "$LOG_PATH" ]]
then
echo -e "Must Provide All Required Paths [$FLAG is Empty].."
exit 1
fi
The above code will run whenever it found empty variable, but I also wants to know which variable is empty (E.g., In above code suppose if LOG_PATH variable is empty then it should display in echo output in place of $FLAG )
I tried following codes..
if [[ `FLAG='MMBOX_PATH'` && -z "$MMBOX_PATH" || `FLAG='BACKUP_PATH'` && -z "$BACKUP_PATH" || `FLAG='REMOTE_SERVER'` && -z "$REMOTE_SERVER" || `FLAG='LOG_PATH'` && -z "$LOG_PATH" ]]
then
echo -e "Must Provide All Required Paths [$FLAG is Empty].."
exit 1
fi
But above code returns false hence it is not printing the content inside echo.
I also tried to keep FLAG variable before condition execution, but every time it returns 'Nothing'
if FLAG='MMBOX_PATH' && [[ -z "$MMBOX_PATH" ]]
then
echo -e "Must Provide All Required Paths [$FLAG is Empty].."
exit 1
fi
In above case I'm getting FLAG='MMBOX_PATH' in output but if I add one more condition to that if nothing is printing (Means if I check same thing for BACKUP_PATH,REMOTE_SERVER..)
if FLAG='MMBOX_PATH' && [[ -z "$MMBOX_PATH" ]] && FLAG='LOG_PATH' && [[ -z "$LOG_PATH" ]]
then
echo -e "Must Provide All Required Paths [$FLAG is Empty].."
exit 1
fi
In this case nothing is printing even though MMBOX_PATH present and LOG_PATH empty.
Note: Using if condition each and every variable it is possible to know which variable is empty,but I don't want to extend my lines with if-else conditions I just want to know in that if block itself how to assign a variable and prints once condition is true.
Can anybody help me how to get empty variable..? (/bin/bash)

Answer 1

If all you are doing is checking existence with the if you could use a function.
check() {
for i in "$#";do
if [[ -z "${!i}" ]]
then
echo -e "Must Provide All Required Paths [\$$i is Empty].."
exit 1
fi
done
}
check MMBOX_PATH BACKUP_PATH REMOTE_SERVER LOG_PATH

Answer 2

Shell already provides a syntax for verifying that a variable has a value and exits if it does not:
: ${MMBOX_PATH:?Must provide MMBOX_PATH}
: ${BACKUP_PATH:?Must provide BACKUP_PATH}
: ${REMOTE_SERVER:?Must provide REMOVE_SERVER}
: ${LOG_PATH:?Must provide LOG_PATH}
There's no need to define a check function that does the same thing.
The initial colon is the do-nothing command; the shell evaluates its arguments, and : exits with status 0 immediately. The parameter expansion is what verifies that the named parameter has a value. If it does not, the given error message is printed. If the shell is not interactive, it also exits with status 1.