While loops over ssh: solving problems

Introduction

A simple task that we may have performed infinite times perhaps is to put a list of servers in a file and then pass that list to a for loop, ssh to that list of servers in the loop and run some commands on them. When you try to do the same thing with a while loop, you think it'll work but it doesn't unless you are an ssh sensei and know some of its tricks.

The problem

When you try to feed input to a while loop from a file and try to run ssh commands on the listed servers in the while loop, the loop either hangs or it works on the first server i.e. runs one iteration and then stops. The problem is that ssh reads from standard input, therefore it eats all the remaining lines. We can just connect its standard input to nowhere.

ssh $USER@$SERVER "cd ${REMOTE_PATH}; mkdir -p $i" < /dev/null

In some case re-directing stdin from /dev/null is known not to work. A simple solution to that scenario is to use ssh with -n option to redirect its stdin from /dev/null like below.

cat dev_list | tr ',' ' ' | while read INS SERVER ETC ; do echo $SERVER -- $INS; sudo ssh -o StrictHostKeyChecking=no -o "BatchMode yes"  -n -q  $SERVER "ls /tmp | grep dbstart | grep log";  [[ $? -eq 0 ]] && echo  $SERVER -- $INS >> log_found.txt; done

Here is the description of the -n option straight from the man page for ssh:

     -n      Redirects stdin from /dev/null (actually, prevents reading from stdin).  This must be used when ssh is run in the background.  A common trick is to use this to run X11 programs on a remote machine. For example, ssh -n shadows.cs.hut.fi emacs & will start an emacs on shadows.cs.hut.fi, and the X11 connection will be automatically forwarded over an encrypted channel.  The ssh program will be put in the background.  (This does not work if ssh needs to ask for a password or passphrase; see also the -f option.)

Conclusion

We hope that you found this article to be useful and this ssh trick helps in future when you run scripts remotely via ssh over a while loop.