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How to perform multiple operations on a number before storing to a variable?

 
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Old 10-23-2015
How to perform multiple operations on a number before storing to a variable?
(I am using bash)
I have a command that will find the line number in a file by searching for a string where it exists.

Code:
linenumber=$(grep -n "string" $FILENAME | cut -d : -fi)

This returns the line number and removes the string. Now that I have the line number I want to subtract 4 from it and give me a new line number.

Code:
newlinenumber=$((linenumber - 4))

What I would like to do is find the line number and subtract by 4 BEFORE storing to the variable "linenumber" so I can avoid creating a new variable "newlinenumber" altogether. Essentially I want to combine the two with the idea pictured as follows:

Code:
linenumber=$(grep -n "string" $FILENAME | cut -d : -fi) | $((linenumber - 4))

I want to know the correct syntax to make this possible.
 
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