Storing part of a filename as a variable

VTGuitarMan · 03-11-2008, 10:02 PM

Hi All,

I'm writing a simple bash shell script in which I need to store part of a filename as a variable. I have seen a number of posts about how to append an environment variable to a filename, but not the other way around. What I'd like to do is the following:

Using a for-loop structure, I want to extract numbers from filenames, i.e.:

file_1.txt
file_2.txt
file_3.txt
...
file_1000.txt

Each time I extract the number, I want to use it in a simple calculation, then discard the variable value in favor of the next file in the series. So, run a calculation using 1 as the value, then run calculation with 2, 3, etc., for the entire series of file_*.txt. I hope that makes sense.

Can someone point me in the right direction? I am figuring it might take some crafty use of sed or grep, or some such tool, but I haven't been able to suss it out for myself.

Thanks in advance!

jschiwal · 03-11-2008, 10:54 PM

You can simply use variable expansion:

Code:

file=file_1.txt
temp=${file#file_}
num=${temp%.txt}

Or use sed:

Code:

file=file_1.txt
num=$(echo $file | sed 's/[^0-9]*\([0-9]*\)\.txt/\1/')

syg00 · 03-11-2008, 11:15 PM

Think I'd prefer

Code:

egrep -o '[[:digit:]]+'

instead of sed for something like this.

ghostdog74 · 03-11-2008, 11:24 PM

Code:

#!/bin/sh
ls -1 file_* | sort -n -t"_" -k2 | while read -r fi; 
                                   do  
                                       echo ${fi##*_}
                                       num=${fi##*_}
                                       ## do calculation with $num
                                   done

VTGuitarMan · 03-12-2008, 06:15 AM

Thanks everyone for your quick replies and helpful input! I've been able to grab the variable from the files and do my calculations.

ta0kira · 03-12-2008, 06:47 AM

Quote:

Originally Posted by syg00

Think I'd prefer

Code:

egrep -o '[[:digit:]]+'

instead of sed for something like this.

I'd have to agree with this since a failed pattern match in sed will give you the entire file name, whereas this will give you nothing if a match isn't made.
ta0kira