21

I'm writing a script that will backup data from my laptop to an Ubuntu server. To do so, I'm looking for a (ba)sh command to test if the server is available before starting the backup. something like ping on port 22 that returns a boolean.

How can I do this?

6 Answers 6

30

Like this:

nc -z hostname 22 > /dev/null
echo $?

If it's 0 then it's available. If it's 1 then it's not.

5
  • 1
    Note that this option works only with the netcat-openbsd package, otherwise the option is not present: nc: invalid option -- 'z' Commented Jan 18, 2017 at 7:26
  • 2
    without -w option nc will be stuck forever on Ubuntu
    – Alex Jones
    Commented Jan 21, 2019 at 10:09
  • can we have a similar command from windows Commented Mar 26, 2020 at 19:31
  • This tests if port $host:22 is open, not if a sshd is taking requests, which means a false positive on any/all proxied requests. Just timeout ssh if you want a valid/reliable check timeout1 ssh blah or ssh -o ConnectTimeout=1 blah Commented Nov 24, 2020 at 16:03
  • In my case the nc cmd was still outputting, have to use &> /dev/null to silence it.
    – robbash
    Commented Mar 21, 2021 at 9:22
16

Use netcat:

nc -z localhost 22

From the manpage

 -z      Specifies that nc should just scan for listening daemons,
         without sending any data to them.
4
  • Damn! You beat me to it.
    – bahamat
    Commented Feb 2, 2011 at 20:23
  • 2
    thanks, didn't know nc , i was wondering why nc -z -w 2 192.168.0.123 ( -w is the timeout option, in seconds ) does not timeout :|
    – Amo__
    Commented Feb 2, 2011 at 23:12
  • You need to specify a port too. nc -z -w 2 192.168.0.123 22 works as expected.
    – chris
    Commented Feb 3, 2011 at 7:36
  • +1 for the z into. whoever you scan still can get info of who did the scan only without providing the identification string
    – amrx
    Commented Sep 11, 2016 at 20:37
7

Alternate:

nc -vzw 1 hostname 22
3
  • this should be selected answer, without -w option nc will be stuck forever on Ubuntu
    – Alex Jones
    Commented Jan 21, 2019 at 10:10
  • Bool can be used like => nc -zw 2 examplehost.com 22 && { echo "You can call your backup function" ; } || { echo "SSH Unavailable" ; }
    – Mr. Pundir
    Commented Feb 23, 2019 at 11:38
  • It's a very fine use of netcat .. however beware that not all ssh servers listen on port 22, and this may be reflected in the local /etc/ssh/ssh_config file.
    – stevea
    Commented Feb 12, 2023 at 8:31
6

What about

MACHINE=192.168.0.8
exec 3>/dev/tcp/${MACHINE}/22
if [ $? -eq 0 ]
then
    echo "SSH up"
else
    echo "SSH down"
fi
5
  • Don't always assume that it's Linux. His laptop may be a Mac. And Debian disables /dev/tcp. Ubuntu being Debian derived may as well (but I have no specific knowledge). But if /dev/tcp is available, then yes this is perfectly valid.
    – bahamat
    Commented Feb 2, 2011 at 20:35
  • thx for your feedback, i've wrote it on a Mac.
    – user4418
    Commented Feb 2, 2011 at 20:49
  • thanks, even if exec 3>/dev/tcp/${HOST}/22 looks really "bizarre"
    – Amo__
    Commented Feb 2, 2011 at 23:17
  • 2
    This is the only answer that doesn't require to install additional packages.
    – Chen A.
    Commented Oct 23, 2017 at 13:14
  • Works great for my use case in Mac and Manjaro (Arch Linux) Commented Aug 25, 2020 at 18:10
0
ssh hostname exit 2>/dev/null ; echo $?

This actually creates a connection and performs authentication. Also it does not rely on the port 22 assumption which is sometimes false.

0

I improved Steveas answer by adding a timeout (here 2 secs):

ssh -o ConnectTimeout=2 -p <port> <hostname> exit 2>/dev/null ; echo $?

If port is the default 22 then you can run just:

ssh -o ConnectTimeout=2 <hostname> exit 2>/dev/null ; echo $?

Without timeout ssh may wait a long time.

Using ssh instead of nc has a big advantage: It checks that answering daemon is ssh and not just any daemon. Successful "ssh ping" with ssh most likely indicates that the real ssh connection will succeed.

NOTE also that there is available app called ssh-ping. It is part of ssh-tools

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