Test if remote TCP port is open from a shell script

Question

I m looking for a quick and simple method for properly testing if a given TCP port is open on a remote server  from inside a Shell script   I ve managed to do it with the telnet command  and it works fine when the port is opened  but it doesn t seem to timeout when it s not and just hangs there     Here s a sample   l TELNET  echo  quit    telnet  SERVER  PORT   grep  Escape character is   if        -ne 0    then   echo  Connection to  SERVER on port  PORT failed    exit 1 else   echo  Connection to  SERVER on port  PORT succeeded    exit 0 fi   I either need a better way  or a way to force telnet to timeout if it doesn t connect in under 8 seconds for example  and return something I can catch in Shell  return code  or string in stdout    I know of the Perl method  which uses the IO  Socket  INET module and wrote a successful script that tests a port  but would rather like to avoid using Perl if possible   Note  This is what my server is running  where I need to run this from   SunOS 5 10 Generic 139556-08 i86pc i386 i86pc

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It s easy enough to do with the -z and -w TIMEOUT options to nc  but not all systems have nc installed  If you have a recent enough version of bash  this will work     Connection successful    timeout 1 bash -c  cat  lt   dev null  gt   dev tcp google com 80    echo    0    Connection failure prior to the timeout   timeout 1 bash -c  cat  lt   dev null  gt   dev tcp sfsfdfdff com 80  bash  sfsfdfdff com  Name or service not known bash   dev tcp sfsfdfdff com 80  Invalid argument   echo    1    Connection not established by the timeout   timeout 1 bash -c  cat  lt   dev null  gt   dev tcp google com 81    echo    124   What s happening here is that timeout will run the subcommand and kill it if it doesn t exit within the specified timeout  1 second in the above example   In this case bash is the subcommand and uses its special  dev tcp handling to try and open a connection to the server and port specified  If bash can open the connection within the timeout  cat will just close it immediately  since it s reading from  dev null  and exit with a status code of 0 which will propagate through bash and then timeout  If bash gets a connection failure prior to the specified timeout  then bash will exit with an exit code of 1 which timeout will also return  And if bash isn t able to establish a connection and the specified timeout expires  then timeout will kill bash and exit with a status of 124

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With netcat you can check whether a port is open like this   nc my example com 80  lt   dev null   The return value of nc will be success if the TCP port was opened  and failure  typically the return code 1  if it could not make the TCP connection   Some versions of nc will hang when you try this  because they do not close the sending half of their socket even after receiving the end-of-file from  dev null  On my own Ubuntu laptop  18 04   the netcat-openbsd version of netcat that I have installed offers a workaround  the -N option is necessary to get an immediate result   nc -N my example com 80  lt   dev null

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As pointed by B  Rhodes  nc  netcat  will do the job  A more compact way to use it  nc -z  lt host gt   lt port gt   That way nc will only check if the port is open  exiting with 0 on success  1 on failure  For a quick interactive check  with a 5 seconds timeout   nc -z -v -w5  lt host gt   lt port gt

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check ports using bash  Example      test port bash sh 192 168 7 7 22   the port 22 is open  Code  HOST  1 PORT  2 exec 3 gt   dev tcp   HOST    PORT  if      -eq 0   then echo  the port  2 is open  else echo  the port  2 is closed  fi

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TOC   Using bash and timeout  Command Examples   Using nc  Command RHEL 6  nc-1 84   Installation Examples   RHEL 7  nmap-ncat-6 40   Installation Examples     Remarks  Using bash and timeout  Note that timeout should be present with RHEL 6   or is alternatively found in GNU coreutils 8 22  On MacOS  install it using brew install coreutils and use it as gtimeout  Command    timeout  TIMEOUT SECONDS bash -c  quot  lt  dev tcp   HOST    PORT  quot   echo     If parametrizing the host and port  be sure to specify them as   HOST  and   PORT  as is above  Do not specify them merely as  HOST and  PORT  i e  without the braces  it won t work in this case  Example  Success    timeout 2 bash -c  quot  lt  dev tcp canyouseeme org 80 quot   echo    0  Failure    timeout 2 bash -c  quot  lt  dev tcp canyouseeme org 81 quot   echo    124  If you must preserve the exit status of bash    timeout --preserve-status 2 bash -c  quot  lt  dev tcp canyouseeme org 81 quot   echo    143  Using nc  Note that a backward incompatible version of nc gets installed on RHEL 7  Command  Note that the command below is unique in that it is identical for both RHEL 6 and 7  It s just the installation and output that are different    nc -w  TIMEOUT SECONDS -v  HOST  PORT  lt  dev null  echo     RHEL 6  nc-1 84   Installation    sudo yum install nc  Examples  Success    nc -w 2 -v canyouseeme org 80  lt  dev null  echo    Connection to canyouseeme org 80 port  tcp http  succeeded  0  Failure    nc -w 2 -v canyouseeme org 81  lt  dev null  echo    nc  connect to canyouseeme org port 81  tcp  timed out  Operation now in progress 1  If the hostname maps to multiple IPs  the above failing command will cycle through many or all of them  For example    nc -w 2 -v microsoft com 81  lt  dev null  echo    nc  connect to microsoft com port 81  tcp  timed out  Operation now in progress nc  connect to microsoft com port 81  tcp  timed out  Operation now in progress nc  connect to microsoft com port 81  tcp  timed out  Operation now in progress nc  connect to microsoft com port 81  tcp  timed out  Operation now in progress nc  connect to microsoft com port 81  tcp  timed out  Operation now in progress 1  RHEL 7  nmap-ncat-6 40   Installation    sudo yum install nmap-ncat  Examples  Success    nc -w 2 -v canyouseeme org 80  lt  dev null  echo    Ncat  Version 6 40   http   nmap org ncat   Ncat  Connected to 52 202 215 126 80  Ncat  0 bytes sent  0 bytes received in 0 22 seconds  0  Failure    nc -w 2 -v canyouseeme org 81  lt  dev null  echo    Ncat  Version 6 40   http   nmap org ncat   Ncat  Connection timed out  1  If the hostname maps to multiple IPs  the above failing command will cycle through many or all of them  For example    nc -w 2 -v microsoft com 81  lt  dev null  echo    Ncat  Version 6 40   http   nmap org ncat   Ncat  Connection to 104 43 195 251 failed  Connection timed out  Ncat  Trying next address    Ncat  Connection to 23 100 122 175 failed  Connection timed out  Ncat  Trying next address    Ncat  Connection to 23 96 52 53 failed  Connection timed out  Ncat  Trying next address    Ncat  Connection to 191 239 213 197 failed  Connection timed out  Ncat  Trying next address    Ncat  Connection timed out  1  Remarks  The -v  --verbose  argument and the echo    command are of course for illustration only

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I m guessing that it s too late for an answer  and this might not be a good one  but here you go     What about putting it inside of a while loop with a timer on it of some sort   I m more of a Perl guy than Solaris  but depending on the shell you re using  you should be able to do something like   TIME    date   s    15 while TIME     date   s  do whatever   And then just add a flag in the while loop  so that if it times out before completing  you can cite the timeout as reason for failure   I suspect that the telnet has a timeout switch as well  but just off the top of my head  I think the above will work

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nmap-ncat to test for local port that is not already in use    availabletobindon       port   1    nc -w 2 -i 1 localhost   port  2 gt  amp 1   grep -v -q  Idle timeout expired    return

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This uses telnet behind the scenes  and seems to work fine on mac linux  It doesn t use netcat because of the differences between the versions on linux mac  and this works with a default mac install   Example     is port open sh 80 google com OPEN    is port open sh 8080 google com CLOSED   is port open sh  PORT  1 HOST  2 TIMEOUT IN SEC   3 -1  VALUE IF OPEN   4 - OPEN   VALUE IF CLOSED   5 - CLOSED    function eztern       if     1       2      then     echo  3   else     echo  4   fi      cross platform timeout util to support mac mostly   https   gist github com jaytaylor 6527607 function eztimeout     perl -e  alarm shift  exec  ARGV           function testPort       OPTS         find out if port is open using telnet     by saving telnet output to temporary file     and looking for  Escape character  response     from telnet   FILENAME   tmp   port check   uuidgen     RESULT   eztimeout  TIMEOUT IN SEC telnet  HOST  PORT  amp  gt   FILENAME  cat  FILENAME   tail -n1    rm -f  FILENAME    SUCCESS   eztern   RESULT   Escape character is          VALUE IF OPEN    VALUE IF CLOSED      echo   SUCCESS     testPort

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If you re using ksh or bash they both support IO redirection to from a socket using the  dev tcp IP PORT construct  In this Korn shell example I am redirecting no-op s     std-in from a socket   W  python -m SimpleHTTPServer  amp   1      16833 Serving HTTP on 0 0 0 0 port 8000     W     lt  dev tcp 127 0 0 1 8000   The shell prints an error if the socket is not open   W     lt  dev tcp 127 0 0 1 8001 ksh   dev tcp 127 0 0 1 8001  cannot open  Connection refused    You can therefore use this as the test in an if condition   SERVER 127 0 0 1 PORT 8000 if     lt   dev tcp  SERVER  PORT  2 gt  dev null then     print succeeded else     print failed fi   The no-op is in a subshell so I can throw std-err away if the std-in redirection fails   I often use  dev tcp for checking the availability of a resource over HTTP   W  print arghhh  gt  grr html W  python -m SimpleHTTPServer  amp   1      16863 Serving HTTP on 0 0 0 0 port 8000     W   print -u9  GET  grr html HTTP 1 0 n  cat  lt  amp 9  9 lt  gt  dev tcp 127 0 0 1 8000 HTTP 1 0 200 OK Server  SimpleHTTP 0 6 Python 2 6 1 Date  Thu  14 Feb 2013 12 56 29 GMT Content-type  text html Content-Length  7 Last-Modified  Thu  14 Feb 2013 12 55 44 GMT  arghhh W     This one-liner opens file descriptor 9 for reading from and writing to the socket  prints the HTTP GET to the socket and uses cat to read from the socket

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I needed short script which was run in cron and hasn t output  I solve my trouble using nmap   open  nmap -p  PORT  SERVER   grep   PORT    grep open  if   -z   open     then   echo  Connection to  SERVER on port  PORT failed    exit 1 else   echo  Connection to  SERVER on port  PORT succeeded    exit 0 fi   To run it You should install nmap because it is not default installed package

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While an old question  I ve just dealt with a variant of it  but none of the solutions here were applicable  so I found another  and am adding it for posterity   Yes  I know the OP said they were aware of this option and it didn t suit them  but for anyone following afterwards it might prove useful   In my case  I want to test for the availability of a local apt-cacher-ng service from a docker build   That means absolutely nothing can be installed prior to the test   No nc  nmap  expect  telnet or python   perl however is present  along with the core libraries  so I used this   perl -MIO  Socket  INET -e  exit   defined  IO  Socket  INET- gt new  172 17 42 1 3142

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In Bash using pseudo-device files for TCP UDP connections is straight forward  Here is the script      usr bin env bash SERVER example com PORT 80  lt  dev tcp  SERVER  PORT if        -ne 0    then   echo  Connection to  SERVER on port  PORT failed    exit 1 else   echo  Connection to  SERVER on port  PORT succeeded    exit 0 fi   Testing       test sh  Connection to example com on port 80 succeeded     Here is one-liner  Bash syntax     lt  dev tcp localhost 11211  amp  amp  echo Port open     echo Port closed      Note that some servers can be firewall protected from SYN flood attacks  so you may experience a TCP connection timeout   75secs   To workaround the timeout issue  try   timeout 1 bash -c   lt  dev tcp stackoverflow com 81   amp  amp  echo Port open     echo Port closed    See  How to decrease TCP connect   system call timeout

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In some cases where tools like curl  telnet  nc o nmap are unavailable you still have a chance with wget  if      wget -q -t 1 --spider --dns-timeout 3 --connect-timeout 10  host port  echo     -eq 0     then echo  OK   else echo  FAIL   fi

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Building on the most highly voted answer  here is a function to wait for two ports to be open  with a timeout as well  Note the two ports that mus be open  8890 and 1111  as well as the max attempts  1 per second    function wait for server to boot         echo  Waiting for server to boot up         attempts 0     max attempts 30     while   nc 127 0 0 1 8890  lt   dev null    nc 127 0 0 1 1111  lt   dev null     amp  amp      attempts  lt   max attempts      do         attempts    attempts 1           sleep 1          echo  waiting       attempts    max attempts        done

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I needed a more flexible solution for working on multiple git repositories so I wrote the following sh code based on 1 and 2  You can use your server address instead of gitlab com and your port in replace of 22   SERVER gitlab com PORT 22 nc -z -v -w5  SERVER  PORT result1      Do whatever you want  if      result1     0    then   echo   port 22 is closed  else   echo  port 22 is open  fi

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If you want to use nc but don t have a version that support -z  try using --send-only   nc --send-only  lt IP gt   lt PORT gt   lt  dev null   and with timeout   nc -w 1 --send-only  lt IP gt   lt PORT gt   lt  dev null   and without DNS lookup if it s an IP   nc -n -w 1 --send-only  lt IP gt   lt PORT gt   lt  dev null   It returns the codes as the -z based on if it can connect or not

[shell] Test if remote TCP port is open from a shell script

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