Why bc and args doesn't work together in one line?

Question

I need help using xargs(1) and bc(1) in the same line. I can do it multiple lines, but I really want to find a solution in one line.

Here is the problem: The following line will print the size of a file.txt

ls -l file.txt | cut -d" " -f5

And, the following line will print 1450 (which is obviously 1500 - 50)

echo '1500-50' | bc

Trying to add those two together, I do this:

ls -l file.txt | cut -d" " -f5 | xargs -0 -I {} echo '{}-50' | bc

The problem is, it's not working! :)

I know that xargs is probably not the right command to use, but it's the only command I can find who can let me decide where to put the argument I get from the pipe.

This is not the first time I'm having issues with this kind of problem. It will be much of a help..

Thanks

score 2 · Answer 1 · answered Jul 28 '13 at 08:23

2

This might work for you:

ls -l file.txt | cut -d" " -f5 | sed 's/.*/&-50/' | bc

Infact you could remove the cut:

ls -l file.txt | sed -r 's/^(\S+\s+){4}(\S+).*/\2-50/' | bc

Or use awk:

ls -l file.txt | awk '{print $5-50}'

answered Jul 28 '13 at 08:23

potong

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Thanks man, Didn't know about the '&' replacement for a pattern match. – winter Jul 28 '13 at 08:49

score 2 · Accepted Answer · answered Jul 28 '13 at 08:33

If you do

ls -l file.txt | cut -d" " -f5 | xargs -0 -I {} echo '{}-50'

you will see this output:

23
-50

This means, that bc does not see a complete expression.

Just use -n 1 instead of -0:

ls -l file.txt | cut -d" " -f5 | xargs -n 1 -I {} echo '{}-50'

and you get

23-50

which bc will process happily:

ls -l file.txt | cut -d" " -f5 | xargs -n 1 -I {} echo '{}-50' | bc
-27

So your basic problem is, that -0 expects not lines but \0 terminated strings. And hence the newline(s) of the previous commands in the pipe garble the expression of bc.

Better to know my mistakes then to get an alternative :) – winter Jul 28 '13 at 09:43 — winter, Jul 28 '13 at 09:43

score 1 · Answer 3 · answered Jul 28 '13 at 10:36

Parsing output from the ls command is not the best idea. (really).

you can use many other solutions, like:

find . -name file.txt -printf "%s\n"

or

stat -c %s file.txt

or

wc -c <file.txt

and can use bash arithmetics, for avoid unnecessary slow process forks, like:

find . -type f -print0 | while IFS= read -r -d '' name
do
        size=$(wc -c <$name)
        s50=$(( $size - 50 ))
        echo "the file=$name= size:$size minus 50 is: $s50"
done

score 1 · Answer 4 · answered Jul 28 '13 at 15:16

Here is another solution, which only use one external command: stat:

file_size=$(stat -c "%s" file.txt) # Get the file size
let file_size=file_size-50         # Subtract 50

If you really want to combine them into one line:

let file_size=$(stat -c "%s" file.txt)-50

The stat command gets you the file size in bytes. The syntax above is for Linux (I tested against Ubuntu). On the Mac the syntax is a little different:

let file_size=$(stat -f "%z" mini.csv)-50

Why bc and args doesn't work together in one line?

4 Answers4