Bash shell script not working

Question

I have a script I am trying to work out to scan my LAN and send me notification if there is a new MAC address that does not appear in my master list. I believe my variables may be messed up. This is what I have:

#!/bin/bash

LIST=$HOME/maclist.log
MASTERFILE=$HOME/master
FILEDIFF="$(diff $LIST $MASTERFILE)"

# backup the maclist first
if [ -f $LIST ]; then
       cp $LIST maclist_`date +%Y%m%H%M`.log.bk
else
        touch $LIST
fi 

# this will scan the network and extract the IP and MAC address
nmap -n -sP 192.168.122.0/24 | awk '/^Nmap scan/{IP=$5};/^MAC/{print IP,$3};{next}' > $LIST

# this will use a diff command to compare the maclist created above and master list of known good devices on the LAN
if [ $FILEDIFF ] 2> /dev/null; then
        echo
        echo "---- All is well on `date` ----" >> macscan.log
        echo
else
       # echo -e "\nWARNING!!" | `mutt -e 'my_hdr From:[email protected]' -s "WARNIG!! NEW DEVICE ON THE LAN" -i maclist.log [email protected]`
        echo "emailing you"
fi

When I execute this when the maclist.log does not exist I get this response:

diff: /root/maclist.log: No such file or directory

If I execute it again with the maclist.log file existing the file gets renamed from the cp line without any issue.

Why don't you just execute FILEDIFF="$(diff $LIST $MASTERFILE)" after your first conditional block (if [ -f $LIST ] ...) ? — Thomas W.
– Thomas W., Commented Sep 23, 2016 at 17:45
@ThomasWilmotte So I tried that and I get no error but the FILEDIFF variable still does not work because I get that email. Or, there could be something wrong with my if [ $FILEDIFF ] statement. — t3kg33k
– t3kg33k, Commented Sep 23, 2016 at 17:52
@t3kg33k FILEDIFF=diff ... will be an empty string if diff didn't find any difference between the files. You can then check if FILEDIFF is an empty string or not with if [[ "$diff" == "" ]];then ... — Thomas W.
– Thomas W., Commented Sep 23, 2016 at 18:06
@ThomasWilmotte Ah! Okay. That makes sense now. And that worked. Thanks! Now, how do I promote your answer as the correct answer? :) — t3kg33k
– t3kg33k, Commented Sep 23, 2016 at 18:18

psmears · Accepted Answer · 2016-09-23 18:00:27Z

1

The line

FILEDIFF="$(diff $LIST $MASTERFILE)"

executes the diff when it is run (not when you use $FILELIST later). At that time the list file hasn't been created.

The easiest fix is just to put the diff command in full where $FILELIST is currently used.

answered Sep 23, 2016 at 18:00

psmears

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