Question

I have a set of piped commands that work on the command line, but do not produce output when run within a script.

The command is:

STRNG=$( ip mroute show | tr -d "()," | awk ' {print "/usr/sbin/smcroute -a eth3", $1, $2, "vtun0 vtun1"}' ); echo "$STRNG"`

And the output is:

/usr/sbin/smcroute -a eth3 192.0.1.19 224.1.1.1 vtun0 vtun1
/usr/sbin/smcroute -a eth3 192.0.1.18 224.1.1.1 vtun0 vtun1

However, if I put the very same command line into a script, I get no output from the echo "$STRNG" command.

What I’m trying to do is execute every line in $STRNG as a command, but for whatever reason it appears $STRNG doesn’t contain any text in the script, whereas $STRNG does contain text when run from the command line. I’m sure this is due to limited bash understanding.

Could someone help me with this?

Answer 1

I want to thank everyone for their help – these will be good things to look for in the future.

The first line of my script was different from the command line:

/usr/sbin/smcroute -k; /usr/sbin/smcroute -d

It turns out that I was getting different results from ip mroute show each time, possibly because multicast packets had not yet arrived on the interface. Adding a
sleep 1 after the first line and before the ip mroute show chain fixed it.

I wouldn’t have found it if not for Fosco’s help, and I also didn’t know how to debug or expand aliases before.