How can I check that a remote computer is online for ssh / script access?

Question

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?

Solution 1

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.

Solution 2

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.

Solution 3

Alternate:

nc -vzw 1 hostname 22

Comments

[bahamat]

Damn! You beat me to it.

[user441706]

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 :|

[chris]

You need to specify a port too. nc -z -w 2 192.168.0.123 22 works as expected.

[amrx]

+1 for the z into. whoever you scan still can get info of who did the scan only without providing the identification string

[valentin_nasta]

Note that this option works only with the netcat-openbsd package, otherwise the option is not present: nc: invalid option -- 'z'

[Edward Torvalds]

without -w option nc will be stuck forever on Ubuntu

[Edward Torvalds]

this should be selected answer, without -w option nc will be stuck forever on Ubuntu

[Mr. Pundir]

Bool can be used like => nc -zw 2 examplehost.com 22 && { echo "You can call your backup function" ; } || { echo "SSH Unavailable" ; }

[Harshal Zope]

can we have a similar command from windows

[christianlc]

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

[robbash]

In my case the nc cmd was still outputting, have to use &> /dev/null to silence it.