Sometimes I restart a device and need to ssh back in when it's ready.
I want to run the ssh command every 5 seconds until the command succeeds.
My first attempt:
watch -n5 ssh [email protected] && exit 1
How can I do this?
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Do you want an interactive SSH session?Nobody– Nobody2017-11-16 20:48:12 +00:00Commented Nov 16, 2017 at 20:48
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@Nobody yes I doPhilip Kirkbride– Philip Kirkbride2017-11-16 20:49:57 +00:00Commented Nov 16, 2017 at 20:49
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Ok, I first thought my idea wouldn't work interactively, but a short test showed that it does so I wrote an answer anyway. :)Nobody– Nobody2017-11-16 21:11:27 +00:00Commented Nov 16, 2017 at 21:11
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7 Answers
Another option would be to use until.
until ssh [email protected]; do
sleep 5
done
If you do this repeatedly for a number of hosts, put it in a function in your ~/.bashrc.
repeat()
{
read -p "Enter the hostname or IP of your server :" servername
until ssh $servername; do
sleep 5
done
}
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1thanks, I'm going to use the repeat script but with
$instead of the prompt :-)Philip Kirkbride– Philip Kirkbride2017-11-15 18:00:37 +00:00Commented Nov 15, 2017 at 18:00 -
17It is worth noting that
sshcan exit with a non-zero status after having connected successfully if something external to the connection terminates the session, in which case that script will attempt to reconnect. This may or may not be a desirable side effect.Austin Hemmelgarn– Austin Hemmelgarn2017-11-15 20:22:32 +00:00Commented Nov 15, 2017 at 20:22
ssh [email protected]
until !!; do sleep 5 ; done
The !! to repeat the last command.
OpenSSH has a ConnectionAttempts setting that does almost what you want. The default is 1 but you can change it in ssh_config or on the command-line:
ssh -o 'ConnectionAttempts 10' ...
Unfortunately you can't adjust the attempt interval which is fixed at 1 second, but you can adjust the connection attempt time-out ConnectTimeout (in seconds) in the same fashion.
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This doesn't seem to be working for me :/Freedo– Freedo2019-08-17 10:57:47 +00:00Commented Aug 17, 2019 at 10:57
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@Freedo: Could you please explain what exactly you tried and what exactly didn't work? Also, it would likely be better to write a new question about your issue. You're welcome to draw my attention to it through a a link in the comments.David Foerster– David Foerster2019-08-18 11:36:28 +00:00Commented Aug 18, 2019 at 11:36
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If it wasn't for the spurious (in my opinion) 5 second interval, this would be the correct answer. It certainly gets my upvote. A suggested addition to make the thing even more convenient, put this ... alias YH="ssh -o 'ConnectionAttempts 20' you@yourhost" ... in your ${HOME}/.profile. Then, after reboot, you'd just have to type YH on the command line, pour another coffee, take a sip ... and you're in.Martin Bramwell– Martin Bramwell2020-07-21 16:23:32 +00:00Commented Jul 21, 2020 at 16:23
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created bash fn
sshr() { ssh $1 -o "ConnectionAttempts 10" }so I can do thissshr destinationJohn Lee– John Lee2022-02-16 22:44:49 +00:00Commented Feb 16, 2022 at 22:44 -
2@JohnLee Better would be
sshr() { ssh -o "ConnectionAttempts 10" $@ }so you can pass multiple arguments.AJMansfield– AJMansfield2024-03-13 17:40:55 +00:00Commented Mar 13, 2024 at 17:40
while ! ssh [email protected] true; do
sleep 5
done; echo "Host is back up at $(date)!"
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2Or, if you don't want to actually log in until the service is up,
while ! nc -w5 -z host.example.com 22; do [...].DopeGhoti– DopeGhoti2017-11-15 17:46:02 +00:00Commented Nov 15, 2017 at 17:46
autossh keeps ssh sessions alive. With the right parameters you can also launch it when it's currently impossible to connect and it will try until it succeeds. This works fine, but if you want the reconnect part to work nicely with interactive sessions you'll need to combine it with screen or equivalent.
I recommend this:
AUTOSSH_POLL=5 AUTOSSH_GATETIME=0 autossh -M 0 -o ServerAliveInterval=5 -o ServerAliveCountMax=1 user@host
But see its man page for details. With the above parameters, autossh will try launching ssh every AUTOSSH_POLL seconds, AUTOSSH_GATETIME=0 makes it try again if the first try fails (so definitely what you want), -M 0 disables connection checking by autossh, all later options are passed to ssh. The two -o options make ssh exit if the connections drops for more than 5s.
At the risk of just throwing code at an answer, this script works for me on a daily basis. I call it ressh and it can be used as simply as ressh {hostname}, or with care, ressh -S {session_name} {hostname}
#!/bin/bash
#
# Repeatedly attempt to reconnect to a remote ssh server
########################################################################
#
prefix="${0/*\/}"
port=
session=
action=ssh # How to connect remotely
while getopts 'l:Lp:P:S:' OPT
do
case "$OPT" in
l) [[ s == "$OPTARG" ]] && action=list ;;
L) action=list ;;
[Pp]) port="$OPTARG" ;;
S) session="$OPTARG";; ## ; [[ $session =~ ^[0-9]+$ ]] && session="${prefix}-$session" ;;
esac
done
shift $(($OPTIND - 1))
host="$1"
shift
if [[ -z "$host" ]]
then
echo "ERROR: Missing hostname" >&2
exit 1
fi
if [[ list == "$action" ]]
then
ssh ${port:+-p $port} "$host" "screen -ls"
exit 0
fi
# Connect repeatedly to the target host system
#
[[ -z "$session" ]] && session="${prefix}-$$.$host"
while :
do
ssh -tt ${port:+-p $port} "$host" "screen -dr $session || screen -S $session $*"
ss=$?
[[ 0 -eq $ss ]] && break
[[ 255 -eq $ss ]] && sleep 4
sleep 1
echo "Reconnecting to session $session on $host..."
done
# All done
#
exit 0
This is how I do it within a command for Shuttle App for Mac. Similar to David's answer. This command connects to and reboots the server and then tries to reconnect to it again until it succeeds.
{
"name": "Server - REBOOT",
"cmd": "ssh -t [email protected] 'reboot -p' ; sleep 1 ; echo 'Reconnecting. Standby...' ; sleep 2 ; ssh -o 'ConnectionAttempts 10' [email protected]"
},
