Bash commands work on cmd line but not in script

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?

try to run it with -x : /bin/bash -x your_script.sh It will dump debug informations. — Scharron
– Scharron, Commented Jul 21, 2010 at 19:03

Jim Lewis · Accepted Answer · 2010-07-21 19:08:03Z

1

Is one of the commands in your pipeline an alias? If so, you'll need to do

shopt -s expand_aliases

in order for bash to expand it in your script....generally this is only enabled by default in interactive shells.

answered Jul 21, 2010 at 19:08

Jim Lewis

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ninjalj · Accepted Answer · 2010-07-21 19:16:20Z

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As Scharron said, run it with sh -x. Other than that:

Is ip in the script's $PATH?
Do you use a different locale ($LANG or $LC_???) in your script, so the output of commands gets translated to another language?

answered Jul 21, 2010 at 19:16

ninjalj

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Mark Wagner · Accepted Answer · 2010-07-21 22:29:24Z

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I would break it into smaller pieces to debug it.

I.e., first have a script that does this:

ip mroute show

Run the script. If that produces output then tack on more.

ip mroute show | tr -d "(),"

ip mroute show | tr -d "()," | awk ' {print $0 } '

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

answered Jul 21, 2010 at 22:29

Mark Wagner

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cuttcards · Accepted Answer · 2010-07-22 12:11:15Z

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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.

answered Jul 22, 2010 at 12:11

cuttcards

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