Weird behaviour with bash script

Question

i wrote a bash script that suppose to create some files and write to them. anyway it doesnt really matter what the script does, the thing is - there is a comment in the code (at the end) which suppose only to print something to screen. and if I try to run it like that- the program doesnt write output in the files, but if put this line not as a comment (i.e remove the '##' ) then it works- the program writes the ouput to files. I reallly tried but i dont understand whats going on... If u need the rest of the script, or some more explanation about what it does just say.

files=( `ls $artists` )
echo artists=%${files[*]}%
for file in ${files[*]}; do
    echo file= $file:
    lines=`wc -l $artists/$file | cut -d" " -f1`
    echo lines=$lines
    counter=0
    while read -a line; do
        if (( $counter==$lines ));then
            break;
        fi

        if [[ ! $line =~ [^[:space:]] ]] ; then
            continue
        fi
        rank=$(( ${line[3]}+$(( 5*${line[4]} )) ))
        echo  ${line[*]}
        echo rank = $rank
        echo  "${line[*]} $rank" >> $artists/$file
        let counter++
    done < $artists/$file

    ##cat  $artists/$file | tail -$lines
    cat $artists/$file | tail -$lines > $artists/$file
done

A description of what the goal of the script is would be very helpful. — chepner
– chepner, Commented May 30, 2013 at 12:48

Ignacio Vazquez-Abrams · Accepted Answer · 2013-05-30 11:41:02Z

2

Redirecting into the source file will damage it. Redirect to a temporary file instead, and rename it after.

answered May 30, 2013 at 11:41

Ignacio Vazquez-Abrams

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2 Comments

kojiro Over a year ago

Here's why

rooster Over a year ago

Ho so thats the problem.... anyway, it is HW and im not allowed to use tmp files... any suggestions? thnks.

cdarke · Accepted Answer · 2013-05-30 12:20:06Z

0

cat $artists/$file | tail -$lines > $artists/$file

Let's consider what that is doing. First you are running the cat program, which is unnecessary. It might affect the symptoms, but I doubt it.

The shell has spotted the > $artists/$file. It will TRUNCATE the file to zero bytes before running the tail program. Depending on the exact order of events, cat will be reading an empty file.

So, don't do that, try this instead:

if tail -$lines "$artists/$file" > "$artists/$file.$$"
then
    mv "$artists/$file.$$" "$artists/$file"
else
    echo "Unable to tail $artists/$file" >&2
fi

No need for cat. We redirect to a temporary filename which has the PID appended ($$). If the tail worked then we rename the temporary file to the filename required, otherwise we write an error message to stderr (fd 2). You might also consider if you want to break out of the loop or exit the program if this fails.

Defensive proramming also demands that we place any filename variables inside double quotes, in case some numpty has put whitespace inside a file or directory name (Program Files).

edited May 30, 2013 at 12:20

answered May 30, 2013 at 12:14

cdarke

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2 Comments

rooster Over a year ago

thnks but as i said, not allowed to use tmp files

cdarke Over a year ago

@rooster: why the hell not?

jaypal singh · Accepted Answer · 2013-05-30 13:12:39Z

0

Redirecting to the source file will truncate the file before the next command reads it. Since you mentioned, you cannot create temporary files, how about creating named pipes.

You can re-direct and read from pipes just like they are files. Plus you can do that in parallel.

answered May 30, 2013 at 13:12

jaypal singh

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