Why does ssh in a while loop exit early?

Bit of a gotcha this one.

If you have a ssh call within a while loop, ssh will utilise the contents of the loop variable in one go and thus the loop will exit on the first iteration.

For example

/tmp/file has two filenames in it, file1.txt and file2.txt. If you write the following: –

infile=/tmp/file
remotehost=192.168.1.1
cat ${infile} | while read file
do
   echo "processing $file"
   ssh $remotehost "ls $file"
done

The script will only process the first line of /tmp/file. If you comment out the ssh and re-run, you will see both lines are processed.

The solution is to use a for loop instead of the while loop. i.e.

for transfer in $(cat $infile)

This works correctly.