bash how to sum variables

Answer

Let's we define two variables:

FILES_ETC=1416
FILES_VAR=7928

If you want to get total value, you can do:

TOTAL_FILES=$((FILES_ETC+FILES_VAR))
echo $TOTAL_FILES

For 100% compatibility with old shells, you can use external command expr. In modern systems (ec. Centos 7 64bit), in fact, expr is not an external command. It is a internal shell function. So there is not significal difference in speed between above and this command:

TOTAL_FILES=`expr $FILES_ETC + $FILES_VAR`
echo $TOTAL_FILES

Funny thing is that there must be spaces between arguments. In fact, expr is command, $FILES_ETC is the first argument, + is the second argument and $FILE_VAR is the third argument.

If you want to add numbers with decimal points, you can use bc. bc is very useful external command for making arithmetical operations:

TOTAL_FILES=$(echo $FILES_ETC + $FILES_VAR  | bc)
echo $TOTAL_FILES

Common mistakes are spaces before or after '=' sign. However bc and expr command expect spaces between arguments and operators, so in these case don't forget to spaces.