Require explaination for some variables used in bash script

Question

Simple bash script to generate subnet from ip and netmask which i got after some googling.It as below

#!/bin/bash
sub_net()
{
    A=$1
    B=$2
    OFS=$IFS
    IFS="."
    set ${A:-0.0.0.0}
    A1=${1:-0}
    A2=${2:-0}
    A3=${3:-0}
    A4=${4:-0}

    IFS="."
    set ${B:-0.0.0.0}
    B1=${1:-0}
    B2=${2:-0}
    B3=${3:-0}
    B4=${4:-0}

    IFS=$OFS

    C4=$(( B4 & A4 ))
    C3=$(( B3 & A3 ))
    C2=$(( B2 & A2 ))
    C1=$(( B1 & A1 ))

    echo ${C1}.${C2}.${C3}.${C4}
}

sub_net 181.172.0.111 255.255.255.0

i know the IFS usage it's for field separator but at around line:8 the codeset ${A:-0.0.0.0} written so what is use of it. For debugging i just comment that line but got following error

./script.bash: line 33: 255.255.255.0: syntax error: invalid arithmetic operator (error token is ".255.255.0")

Can anyone explain usage of it?

devnull · Accepted Answer · 2014-01-22 07:02:34Z

1

The line:

set ${A:-0.0.0.0}

would set positional parameters. Since you've said:

IFS="."

before this, it'd imply that if the variable A was 123.42.10.42 then positional parameters 123, 42, 10, 42 would be set which can be accessed using $1, $2, ... (And, indeed, you could see those being utilized in setting other variables: A1, A2, ...)

Regarding the ${A1:-0} notation, you could read about it in Shell Parameter Expansion:

${parameter:-word}

If parameter is unset or null, the expansion of word is substituted. Otherwise, the value of parameter is substituted.

answered Jan 22, 2014 at 7:02

devnull

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