The shell uses a command called test to evaluate conditional expressions. Full details of this command can be found in the test manual page. For example:
if test ! -f $FILE then if test "$WARN" = "yes" then echo "$FILE does not exist" fi fi
First, we test to see if the filename specified by the variable $FILE exists and is a regular file. If it does not then we test to see if the variable $WARN is assigned the value yes, and if it is a message that the filename does not exist is displayed.