[SOLVED] shell script - loop stops when ping fails

shell script - loop stops when ping fails

I'm trying to write a shell script to ping every server listed in a specified file and get its uptime. The intended behavior is that if a server is not reached after ${TIMEOUT} seconds, the script stops trying to ping that server and moves on to the next one in the list. However, when a ping for a server times out, the program stops completely instead of just moving on to the next server in the list. How can I fix this?

Here is the relevant code snippet (${ALS_HOSTS} is the list of servers to be pinged):

while read hostid
do
        ping -c ${NUM_PING_PACKETS} ${hostid}
        if ping -w ${TIMEOUT} -c ${NUM_PING_PACKETS} ${hostid} | grep ' from' 2>/dev/null 1>&2 # check for output
        then
                ssh ${hostid} uptime > ${WORKDIR}/${hostid} &
        fi
done  < ${ALS_HOSTS}

(this script will be running on a CentOS 5 machine that cannot be upgraded or reconfigured at this time)

Solution

This could be because ssh shares stdin with the while read loop, and drains all the data to send it to the remote host.

Use ssh -n to prevent this:

ssh -n ${hostid} uptime > ${WORKDIR}/${hostid} &