Expression used with if condtition

Bash expression is the combination of operators, features, or values used to form a bash conditional statement. Conditional expression could be binary or unary expression which involves numeric, string or any commands whose return status is zero when success. Following are few conditional expressions that are helpful.

[-a FILE ] True if FILE exists.
[ -d FILE ] True if FILE exists and is a directory.
[ -e FILE ] True if FILE exists.
[ -f FILE ] True if FILE exists and is a regular file.
[ -h FILE ] True if FILE exists and is a symbolic link.
[ -s FILE ] True if FILE exists and has a size greater than zero.
[ -x FILE ] True if FILE exists and is executable.
[ -O FILE ] True if FILE exists and is owned by the effective user ID.
[ -G FILE ] True if FILE exists and is owned by the effective group ID.
[ -L FILE ] True if FILE exists and is a symbolic link.
[ -N FILE ] True if FILE exists and has been modified since it was last read.
[ -S FILE ] True if FILE exists and is a socket.
[ FILE1 -nt FILE2 ] True if FILE1 has been changed more recently than FILE2, or if FILE1 exists and FILE2 does not.
[ FILE1 -ot FILE2 ] True if FILE1 is older than FILE2, or if FILE2 exists and FILE1 does not.
[ FILE1 -ef FILE2 ] True if FILE1 and FILE2 refer to the same device and inode numbers.
[ -o OPTIONNAME ] True if shell option “OPTIONNAME” is enabled.
[ -z STRING ] True if the length of “STRING” is zero.
[ -n STRING ] or [ STRING ] True if the length of “STRING” is non-zero.
[ STRING1 == STRING2 ] True if the strings are equal. “=” may be used instead of “==” for strict POSIX compliance.
[ STRING1 != STRING2 ] True if the strings are not equal.
[ STRING1 STRING2 ] True if “STRING1” sorts after “STRING2” lexicographically in the current locale.
[ ARG1 OP ARG2 ] “OP” is one of -eq, -ne, -lt, -le, -gt or -ge. These arithmetic binary operators return true if “ARG1” is equal to, not equal to, less than, less than or equal to, greater than, or greater than or equal to “ARG2”, respectively. “ARG1” and “ARG2” are integers.

[ expr1 -a expr2 ] Returns true if both the expression is true.
[ expr1 -o expr2 ] Returns true if either of the expression1 or 2 is true.

if [ $first -eq 0 ] && [ $second -eq 0 ]
then
echo “Num1 and Num2 are zero”

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