How can I substitute a name of a variable into a file name within a command?

Question

I am trying to make a script that runs through all the machines available and saves some data into my home folder.

machines=`cat $OAR_FILE_NODES | uniq`
for machine in ${machines}
do
    echo "connecting to:" ${machine}
    oarsh ${machine} 'tar -zcvf data_${machine}.tar.gz /tmp/data'
done

The problem is that all data gets saved to data_.tar.gz archive, overwriting it several times.

How can I make the shell substitute the variable machine into the command passed to oarsh?

What if you use double quotes in the oarsh line? This way, $machine will get expanded. — fedorqui
– fedorqui, Commented Feb 28, 2014 at 14:09
Like this oarsh ${machine} tar -zcvf "data_${machine}.tar.gz" /tmp/data ? — Martin Drozdik
– Martin Drozdik, Commented Feb 28, 2014 at 14:13
Like oarsh ${machine} "tar -zcvf data_${machine}.tar.gz /tmp/data". — fedorqui
– fedorqui, Commented Feb 28, 2014 at 14:13
Nice to read that! I posted as an answer to make it clear for future people entering in your question. Mark as accepted if you want. — fedorqui
– fedorqui, Commented Feb 28, 2014 at 14:28

fedorqui · Accepted Answer · 2014-02-28 14:23:51Z

1

As seen on the comments, it was about the variable not expanding because of the single quote '. By changing to double quotes " the variable is expanded properly:

oarsh ${machine} "tar -zcvf data_${machine}.tar.gz /tmp/data"
                 ^                                          ^

Note that when using a single quote, what oarsh would receive is the literal:

tar -zcvf data_${machine}.tar.gz /tmp/data

while using double quotes would provide the $machine value replaced:

tar -zcvf data_THE_MACHINE.tar.gz /tmp/data

answered Feb 28, 2014 at 14:23

fedorqui

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