while loop to read file ends prematurely

Question

The eventual goal is to have my bash script execute a command on multiple servers. I almost have it set up. My SSH authentication is working, but this simple while loop is

Answer 1

Assign to an array before the loop, so that you are not using stdin for your loop variables. The ssh inside the loop can then use stdin without interfering with your loop.

readarray a <  hosts.list
for HOST in "${a[@]}"; do 
       ssh $HOST "uname -a"
       #...other stuff in loop
done

Answer 2

maybe with python XD

#!/usr/bin/python

import sys
import Queue
from subprocess import call

logfile = sys.argv[1]
q = Queue.Queue()

with open(logfile) as data:
    datalines = (line.rstrip('\r\n') for line in data)
    for line in datalines:
        q.put(line)


while not q.empty() :
    host = q.get()
    print "++++++ " + host + " ++++++"
    call(["ssh", host, "uname -a"])
    call(["ssh", host, "oslevel -s"])
    print "++++++++++++++++++++++++++"

Answer 3

If you run commands which read from stdin (such as ssh) inside a loop, you need to ensure that either:

• Your loop isn't iterating over stdin
• Your command has had its stdin redirected:

...otherwise, the command can consume input intended for the loop, causing it to end.

The former:

while read -u 5 -r hostname; do
  ssh "$hostname" ...
done 5<file

...which, using bash 4.1 or newer, can be rewritten with automatic file descriptor assignment as so:

while read -u "$file_fd" -r hostname; do
  ssh "$hostname" ...
done {file_fd}<file

The latter:

while read -r hostname; do
  ssh "$hostname" ... </dev/null
done <file

...can also, for ssh alone, can also be approximated with the -n parameter (which also redirects stdin from /dev/null):

while read -r hostname; do
  ssh -n "$hostname"
done <file

Answer 4

As the solution specified here use -n option for ssh or open file with a different handle:

while read -u 4 HOST
  do
    echo $HOST
    ssh $HOST "uname -a"
    ssh $HOST "oslevel -s"
    echo ""
  done 4< hosts.list`