[SOLVED] bash scripts

hopeless_n00b · 05-08-2017, 03:27 PM

I must be looking in the wrong places, because I have not found documentation for bash scripts which explain in clear-cut language exactly when it is that a bash script executes all the commands, and when it is that a bash script stops before executing all the commands.

My best guess from experimenting is that a bash script stops early at the first command whose exit code is different from zero.

Can anyone confirm, or are there exceptions? How does this work, and can anyone point me to a short introduction which does not skip over such fundamentally crucial issues?

NevemTeve · 05-08-2017, 03:59 PM

That depends on 'errexit' option (see command 'set -e')

hopeless_n00b · 05-08-2017, 04:22 PM

At this point I am so confused I can't form a reasonable conjecture or ask a reasonable question...

Let me give a semi-concrete example; sorry I can't make a fully concrete example -- unless it is not sanitized -- but perhaps you understand enough that my shortcomings don't get in the way...

Consider

Code:

while read host id
do
echo "host = $host"
mac="$(ssh $host "ifconfig | sed -n 's/://g;s/.*HWaddr \([^ ]*\).*/\1/p'")"
echo "$mac"
done < /tmp/hostIDs

The result is that the "do loop" seems only to execute one time -- I get a host and mac printed out -- even though the file /tmp/hostIDs has many lines. This is very confusing because if I simply comment out the line containing the ssh, like so

Code:

while read host id
do
echo "host = $host"
#mac="$(ssh $host "ifconfig | sed -n 's/://g;s/.*HWaddr \([^ ]*\).*/\1/p'")"
echo "$mac"
done < /tmp/hostIDs

then the loop executes many times and the host is printed out for every line in /tmp/hostIDs.

At first I thought it was because the ssh caused early termination, but if that is true, how can I force the loop to complete?
I have no clue!

NevemTeve · 05-08-2017, 04:44 PM

The first ssh might have read all your input. Try sg like this:

Code:

...
mac="$(ssh $host "..." </dev/null)"
...

hopeless_n00b · 05-08-2017, 04:59 PM

Partial progress (although I have no clue as to what is going on)...
Consider

Code:

while read host id                                                                       
do                                                                                       
command="ssh $host \"ifconfig | sed -n 's/://g;s/.*HWaddr \([^ ]*\).*/\1/p'\""           
echo "$command"                                                                          
done < /tmp/hostIDs

This loop completes with no early termination. Moreover it prints out a bunch of lines like

Code:

ssh machine0 "ifconfig | sed -n 's/://g;s/.*HWaddr \([^ ]*\).*/\1/p'"
ssh machine1 "ifconfig | sed -n 's/://g;s/.*HWaddr \([^ ]*\).*/\1/p'"
.
.
.

Moreover if I cut and paste one of those lines into an xterm, it executes and prints out the mac address of the machine.

How do I preserve the fact that the loop does not terminate early, and also assign the (printed out) result of the ssh command -- i.e., the mac address -- to a variable in my script?

Neither

Code:

mac=`$command`

nor

Code:

mac=$(eval $command)

work, and both cause the loop to terminate early.

I tried

Code:

mac="$($command </dev/null)"

and that seems pointed in the right direction because the loop does not terminate early, but I still can't make it work.

michaelk · 05-08-2017, 05:01 PM

Are there any error messages displayed when you run the script?

What is the format of your hostIDs file. Your read statement expects two variables with the default separator being a space i.e.

host1 id1
host2 id2

The id variable is unused. What is the id variable?

If you are using a regular username not all distributions include /sbin in a regular users path and since ifconfig is depreciated it may not even be installed if running the latest version.

Mind like a steal trap... Nothing gets in or out.

NevemTeve is correct that ssh reads from standard input therefore it is reading the rest of the values in the loop. An alternative solution is to add a -n i.e.

Code:

mac=$(ssh -n $host ...)

Although the command should still work you do not need the quotes.

Code:

mac=$(ssh $host "ifconfig | sed -n 's/://g;s/.*HWaddr \([^ ]*\).*/\1/p'" </dev/null)

BW-userx · 05-08-2017, 06:07 PM

Code:

#!/bin/bash

count=0
bcount=0

while read host id
do
((++count))

   while [[ $bcount -lt $count ]] ;
   do

     HoldMacArray[$bcount]=input
     ((++bcount))

   done
done

# to look inside of array.length

for (( i=0 ; i < ${#HoldMacArray[@]} ; i++ ))
{
  echo "${HoldMacArray[i]}"
 }

or just one loop

Code:

count=0
while read file
do

arrayMac=[$count]=input
((++count))

done

NevemTeve · 05-08-2017, 11:33 PM

@OP: here a complete example

Code:

#!/bin/sh

remote_cmd="ifconfig | sed -n 's/://g;s/.*HWaddr \([^ ]*\).*/\1/p'"

while read host id
do
    echo "host = $host"
    mac="$(ssh -n $host "$remote_cmd")"
    RC="$?"
    echo "RC=$RC mac=$mac"
done </tmp/hostIDs

pan64 · 05-09-2017, 12:56 AM

I do not understand why do we need to run sed on the remote side (and also why sed?):

Code:

ssh host ifconfig | awk '/HWaddr/ { gsub(":", ""); print $NF }'

or even better:

Code:

while read host
do
    echo "host = $host"
    ssh $host ifconfig </dev/null
done </tmp/hostIDs | awk '/host =/; /HWaddr/ { gsub(":", ""); print $NF }'

by the way what is that id good for? while read host id

hopeless_n00b · 05-09-2017, 08:27 AM

The post from michaelk sorted me out (though if I were more preceptive, the first answer might have).

My problem was indeed that I needed a -n flag for ssh. I'll peruse the remaining answers to solidify my understanding.

This community is great! I appreciate all your help.

pan64 · 05-09-2017, 01:49 PM

-n is equal to </dev/null
see man ssh