Shell script can read file line by line but not perform actions for each line

Question

I'm trying to run this command over multiple machines

sshpass -p 'nico' ssh -o 'StrictHostKeyChecking=no' [email protected] "mkdir test"

The IPs are stored in the following .txt file

$ cat ips.txt
10.0.2.15
10.0.2.5

I created a bash script that reads this file line by line. If I run it with an echo:

#!/bin/bash
input="ips.txt"
while IFS= read -r line
do
  echo "$line"
  #sshpass -p 'nico' ssh -o 'StrictHostKeyChecking=no' nico@$line "mkdir test"
done < "$input"

It prints every line:

$ ./variables.sh
10.0.2.15
10.0.2.5

This makes me understand that the script is working as intended. However, when I replace the echo line with the command I want to run for each line:

#!/bin/bash
    input="ips.txt"
    while IFS= read -r line
    do
      #echo "$line"
      sshpass -p 'nico' ssh -o 'StrictHostKeyChecking=no' nico@$line "mkdir test"
    done < "$input"

It only performs the action for the first IP on the file, then stops. Why?

Answer 1

While your example is a solution that works, it's not the explanation.

Your could find the explanation here : ssh breaks out of while-loop in bash

In two words :

1. "while" loop continue reading from the same file-descriptor that defined in the loop header ( $input in your case )
2. ssh (or sshpass) read data from stdin (but in your case from file descriptor $input). And here is the point that hide the things as we didn't exect "ssh" to read the data.

Just to understand the problem you could have same strange experience for example using commands like "ffmpeg" or "mplayer" in while loop. Mplayer and ffmpeg use the keyboards while they are running, so they will consume all the the file-descriptor.

Another good and funny example :

#!/bin/bash

{
  echo first
  for ((i=0; i < 16384; i++)); do echo "testing"; done
  echo "second"
} > test_file

while IFS= read -r line
do
  echo "Read $line"
  cat | uptime > /dev/null
done < test_file

At first part we write 1st line : first 16384 lines : testing then last line : second

16384 lines "testing" are equal to 128Kb buffer

At the second part, the command "cat | uptime" will consume exactly 128Kb buffer, so our script will give

Read first
Read second

As solution, as you did, we could use "for" loop. Or use "ssh -n" Or playing with some file descriptor - you could find the example in the link that I gave.

Answer 2

Managed to solve this by using a for instead of a while. Script ended up looking like this:

for file in $(cat ips.txt)
do
  sshpass -p 'nico' ssh -o 'StrictHostKeyChecking=no' nico@$file "mkdir test"
done