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

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

...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}

The latter:

while read -r hostname; do
  ssh "$hostname" ... 

...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