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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?
Like this:
nc -z hostname 22 > /dev/null
echo $?
If it's 0 then it's available. If it's 1 then it's not.
nc: invalid option -- 'z'
Jan 18, 2017 at 7:26
timeout1 ssh blah or ssh -o ConnectTimeout=1 blah
Nov 24, 2020 at 16:03
nc cmd was still outputting, have to use &> /dev/null to silence it.
Use netcat:
nc -z localhost 22
From the manpage
-z Specifies that nc should just scan for listening daemons,
without sending any data to them.
nc -z -w 2 192.168.0.123 ( -w is the timeout option, in seconds ) does not timeout :|
z into. whoever you scan still can get info of who did the scan only without providing the identification string
Alternate:
nc -vzw 1 hostname 22
-w option nc will be stuck forever on Ubuntu
Jan 21, 2019 at 10:10
nc -zw 2 examplehost.com 22 && { echo "You can call your backup function" ; } || { echo "SSH Unavailable" ; }
Feb 23, 2019 at 11:38
What about
MACHINE=192.168.0.8
exec 3>/dev/tcp/${MACHINE}/22
if [ $? -eq 0 ]
then
echo "SSH up"
else
echo "SSH down"
fi
/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.
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.
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
