While loop stops reading after the first line in Bash

Question

I have the following shell script. The purpose is to loop thru each line of the target file (whose path is the input parameter to the script) and do work against each line. Now, it seems only work with the very first line in the target file and stops after that line got processed. Is there anything wrong with my script?

#!/bin/bash # SCRIPT: do.sh # PURPOSE: loop thru the targets   FILENAME=$1 count=0  echo "proceed with $FILENAME"  while read LINE; do    let count++    echo "$count $LINE"    sh ./do_work.sh $LINE done < $FILENAME  echo "\ntotal $count targets" 

In do_work.sh, I run a couple of ssh commands.

Answer 1

The problem is that do_work.sh runs ssh commands and by default ssh reads from stdin which is your input file. As a result, you only see the first line processed, because the command consumes the rest of the file and your while loop terminates.

This happens not just for ssh, but for any command that reads stdin, including mplayer, ffmpeg, HandBrakeCLI, and more.

To prevent this, pass the -n option to your ssh command to make it read from /dev/null instead of stdin. Other commands have similar flags, or you can universally use < /dev/null.

Answer 2

More generally, a workaround which isn't specific to ssh is to redirect standard input for any command which might otherwise consume the while loop's input.

while read -r LINE; do    let count++    echo "$count $LINE"    sh ./do_work.sh "$LINE" </dev/null done < "$FILENAME" 

The addition of </dev/null is the crucial point here (though the corrected quoting is also somewhat important; see also When to wrap quotes around a shell variable?). You will want to use read -r unless you specifically require the legacy slightly odd behavior you get without -r.

Another workaround of sorts which is somewhat specific to ssh is to make sure any ssh command has its standard input tied up, e.g. by changing

ssh otherhost some commands here 

to instead read the commands from a here document, which conveniently (for this particular scenario) ties up the standard input of ssh for the commands:

ssh otherhost <<'____HERE'     some commands here ____HERE