Shell – String to integer in Shell

arithmeticnumeric datashell-script

I'm currently working on a project called 'dmCLI' which means 'download manager command line interface'. I'm using curl to multi-part download a file. And I'm having trouble with my code. I can't convert string to int.

Here's my full code. I also uploaded my code into Github Repo. Here it is:
dmCLI GitHub Repository

#!/bin/bash
RED='\033[0;31m'
NC='\033[0m'

help() {
    echo "dmcli [ FILENAME:path ] [ URL:uri ]"
}

echo -e "author: ${RED}@atahabaki${NC}"
echo -e "     __      _______   ____"
echo -e " ___/ /_ _  / ___/ /  /  _/"
echo -e "/ _  /  ' \/ /__/ /___/ /  "
echo -e "\_,_/_/_/_/\___/____/___/  "
echo -e "                           "
echo -e "${RED}Downloading${NC} has never been ${RED}easier${NC}.\n"

if [ $# == 2 ]
then
    filename=$1;
    url=$2
    headers=`curl -I ${url} > headers`
    contentlength=`cat headers | grep -E '[Cc]ontent-[Ll]ength:' | sed 's/[Cc]ontent-[Ll]ength:[ ]*//g'`
    acceptranges=`cat headers | grep -E '[Aa]ccept-[Rr]anges:' | sed 's/[Aa]ccept-[Rr]anges:[ ]*//g'`
    echo -e '\n'
    if [ "$acceptranges" = "bytes" ]
    then 
        echo File does not allow multi-part download.   
    else
        echo File allows multi-part download.
    fi
    echo "$(($contentlength + 9))"
    # if [acceptranges == 'bytes']
    # then
    # divisionresult = $((contentlength/9))
    # use for to create ranges...
else 
    help
fi

# First get Content-Length via regex or request,
# Then Create Sequences,
# After Start Downloading,
# Finally, re-assemble to 1 file.

I want to divide contentlength's value by 9. I tried this:

echo "$(($contentlength/9))"

It's getting below error:

/9")syntax error: invalid arithmetic operator (error token is "

I'm using localhost written in node.js. I added head responses. It returns Content-Length of the requested file, and the above dmCLI.sh gets correctly the Content-Length value. 

./dmcli.sh home.html http://127.0.0.1:404/

A HEAD request to http://127.0.0.1:404/ returns: Content-Length: 283, Accept-Range: bytes

The dmCLI works for getting the value, but when I want to access its value, it won't work.

Simple actions work like:

echo "$contentlength"

But I can't access it by using this:

echo "$contentlength bytes"

and here:

echo "$(($contentlength + 9))"

returns 9 but I'm expecting 292. Where's the problem; why is it not working?

Best Answer

The regex below will extract the number of bytes, only the number:

contentlength=$(
  LC_ALL=C sed '
    /^[Cc]ontent-[Ll]ength:[[:blank:]]*0*\([0-9]\{1,\}\).*$/!d
    s//\1/; q' headers
)

After the above change, the contentlength variable will be only made of decimal digits (with leading 0s removes so the shell doesn't consider the number as octal), thus the 2 lines below will display the same result:

echo "$(($contentlength/9))"

echo "$((contentlength/9))"
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