how do i reduce timeout on unix telnet on connection

Question

I have a unix shell script which test ftp ports of multiple hosts listed in a file.

for i in `cat ftp-hosts.txt`         do         echo "QUIT" | telnet $i 21 done 

In general this scripts works, however if i encounter a host which does not connect, i.e telnet is "Trying...", how can I reduce this wait time so it can test the next host ?

Answer 1

Have you tried using netcat (nc) instead of telnet? It has more flexibility, including being able to set the timeout:

echo 'QUIT' | nc -w SECONDS YOUR_HOST PORT # e.g. echo "QUIT" | nc -w 5       localhost 21 

The -w 5 option will timeout the connection after 5 seconds.

Answer 2

Try using timeout3 script is very robust and I used a lot without problems on different situations. Example to wait just 3 seconds trying to check if ssh port is open.

> echo QUIT > quit.txt > ./timeout3 -t 3 telnet HOST 22 < quit.txt  

outputs: you can grep for "Connected" or "Terminated"

timeout3 file contents:

#

#!/bin/bash # # The Bash shell script executes a command with a time-out. # Upon time-out expiration SIGTERM (15) is sent to the process. If the signal # is blocked, then the subsequent SIGKILL (9) terminates it. # # Based on the Bash documentation example. # If you find it suitable, feel free to include # anywhere: the very same logic as in the original examples/scripts, a # little more transparent implementation to my taste. # # Dmitry V Golovashkin <[email protected]>  scriptName="${0##*/}" declare -i DEFAULT_TIMEOUT=9 declare -i DEFAULT_INTERVAL=1 declare -i DEFAULT_DELAY=1 # Timeout. declare -i timeout=DEFAULT_TIMEOUT  # Interval between checks if the process is still alive. declare -i interval=DEFAULT_INTERVAL  # Delay between posting the SIGTERM signal and destroying the process by SIGKILL. declare -i delay=DEFAULT_DELAY  function printUsage() {     cat <<EOF  Synopsis     $scriptName [-t timeout] [-i interval] [-d delay] command     Execute a command with a time-out.     Upon time-out expiration SIGTERM (15) is sent to the process. If SIGTERM     signal is blocked, then the subsequent SIGKILL (9) terminates it.      -t timeout         Number of seconds to wait for command completion.         Default value: $DEFAULT_TIMEOUT seconds.      -i interval         Interval between checks if the process is still alive.         Positive integer, default value: $DEFAULT_INTERVAL seconds.      -d delay         Delay between posting the SIGTERM signal and destroying the         process by SIGKILL. Default value: $DEFAULT_DELAY seconds.  As of today, Bash does not support floating point arithmetic (sleep does), therefore all delay/time values must be integers. EOF }  # Options. while getopts ":t:i:d:" option; do       case "$option" in         t) timeout=$OPTARG ;;         i) interval=$OPTARG ;;         d) delay=$OPTARG ;;         *) printUsage; exit 1 ;;     esac done shift $((OPTIND - 1))  # $# should be at least 1 (the command to execute), however it may be strictly # greater than 1 if the command itself has options.  if (($# == 0 || interval <= 0)); then      printUsage     exit 1 fi  # kill -0 pid   Exit code indicates if a signal may be sent to $pid process. (     ((t = timeout))      while ((t > 0)); do         sleep $interval         kill -0 $$ || exit 0         ((t -= interval))     done     # Be nice, post SIGTERM first.     # The 'exit 0' below will be executed if any preceeding command fails.     kill -s SIGTERM $$ && kill -0 $$ || exit 0     sleep $delay     kill -s SIGKILL $$ ) 2> /dev/null &  exec "$@" 

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