Referencing a text line in Linux

Posted on 2011-09-19
Last Modified: 2012-08-14

I'm new to Linux and I’m trying to write a simple script to reference lines in a text file and perform math. I have the basics of what I from doing some research online but I cant find how to reference text within a text file.

var2= "needs to referance line 4 of file calc2"
var3= "needs to referance line 5 of fil calc3"
var4=$[$var2 / $var3]
echo calculation is $var4
Question by:JacobC
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LVL 68

Accepted Solution

woolmilkporc earned 500 total points
ID: 36563622

this can be done with sed:

var2=$(sed -n '4p' calc2)
var3=$(sed -n '5p' calc3)

var4=$(($var2 / $var3))    # Integer math only!
echo calculation is $var4

Or with awk:

var2=$(awk 'NR==4 {print}' calc2)
var3=$(awk 'NR==5 {print}' calc3)

var4=$(awk -v v2=$var2 -v v3=$var4 'BEGIN {printf "%f\n", v2/v3}')  # Floating point math!
echo calculation is $var4


Author Comment

ID: 36563729
awk says
awk: fatal: division by zero attempted

sed says
awk: fatal: division by zero attempted
LVL 68

Expert Comment

ID: 36563748

so line 5 of calc3 contains zero. I'd claim not being responsible for this.


Author Comment

ID: 36563762
Oops I’m sorry I messed up my file names, it works great!

Do you know any good reference sites I can read up on to get a better understanding? I know how to do basic math in Linux but I want to learn more about exporting/importing information out of files into equations.
LVL 68

Expert Comment

ID: 36563839
This is the bash reference manual:

Selecting from files (to assign variables for later use e.g. in calculations) can be done with sed, awk, perl, and also with combinations of "head", "tail" and "cut". There are many ways, "read" and "set" can also be used in particular situations.

See the manpages of sed, awk, head, tail, cut etc.

This is for GNU sed:

and GNU awk:

Perl is a very powerful programming language. A tutorial:

Have fun!


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