I run into an issue last week that drives me crazy. I wrote a BASH script which does a remote ssh connection to acamai and than performs a simple 'ls'. I want to redirect the 'ls' sdtout output to a given file.
While the script itself works like a charm when run manually, it does not while it runs via cron. The cronjob runs as root and each command works as expected expect the ssh command. My System is Gentoo Linux and cron is the old but gold vixie-cron.
To reduce the 200 LOC I put the basics herein which alone (as a single script) are enough to demonstrate the problem.
#!/bin/bash
PATH='/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/opt/bin'
#set -x
shopt -s lastpipe
exec 2>log.out
(ssh -i <path to key> -o HostKeyAlgorithms=+ssh-dss -o StrictHostKeyChecking=no <account#example.com> 'ls -r <path>') > '/root/listing.txt'
Even in -vvv debug mode of ssh I can see, that everything works...just except that I get no stdout output.
Than I tried something else that I found in another posting on the internet:
#!/bin/bash
PATH='/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/opt/bin'
#set -x
shopt -s lastpipe
exec 2>log.out
(ssh -T -i <path to key> -o HostKeyAlgorithms=+ssh-dss -o StrictHostKeyChecking=no <account#example.com> 'ls -r <path>' </dev/zero) > '/root/listing.txt'
Drawback here, I start a ssh session that I can't close and I guess its due to /dev/zero.
Another approach was to TEE Pipe the sub-shell of the ssh command...this worked for a short time ( and why not yet anymore ?!)
Now I'm clueless and need help. Cron has its PATH, uses BASH etc. Curious my boss did that with success with java (and he hates BASH...).
Any explanation and helpful tips are greatly welcome.
I have same issue, I make script for CRON and it gets output from remote SSH host.
If i run script manually - it works as should. But when CRON runs it - i get just a part of remote output.
I cant realise why its happening.
#!/bin/sh
pass=123
filelist=$(sshpass -p "$pass" ssh -q -tt -o StrictHostKeyChecking=no user#"10.10.10.10" "list")
filestring=$(echo "$filelist" | grep -Po "(\S+\s\S+\s+\d+\s\d{2}:\d{2}:\d{2}\s\d{4})\slist0\.lst")
filedate=${filestring% list0.lst}
echo $filedate
filestamp=$(date -d "$filedate" +"%s")
echo $filestamp
When i get echos in file via CRON - there are date from 0:00:00 - field with date (echo $filedate) is empty. But when i run manually - i get normal date with time...
It really bother me.
Help?
I found solution - add "-tt" to ssh command and all input goes to variable.
filelist=$(sshpass -p "$pass" ssh -q -tt -o StrictHostKeyChecking=no user#"10.10.10.10" "list")
I send a tee command from host 1 to host 2:
ssh user#host2 '/path/run |& tee myFile.txt'
I use tee so that I get the output of the binary to be added to myFile.txt
The problem I have then is after a bit of time, I want to regain control of my local host without having a lot of printout. So I do CTRL+C. This lets the process on host2 continue to run, which is what I want, but it stops the tee process itself, so the file is not populated.
I tried to replace |& tee myFile.txt' by 2>&1 myFile.txt' & but it did not help.
How can I ensure that the file continues to be populated on host2, while regaining control to my session on host1?
If you want to record the results in some file (work with IO redirection inside of the nohup), you need to enclose all the pipeline in the nohup. It does not use shell expansions, since argument is only COMMAND ARGS, so using a sh is a good way:
ssh user#host2 'nohup sh -c "/path/run |& tee myFile.txt" &'
but note that nohup will disconnect the terminal from the command ant it might fail. Useful would be to redirect it directly to the file:
ssh user#host2 'nohup sh -c "/path/run &> myFile.txt" &'
Inspiration from the SO answer.
use nohup, screen or tmux for backgrounding a process.
I'm using 'ssh myremoteserver -f -X myprogram 1>/dev/null 2>&1' to launch a remote gui app. But when I close the terminal, it closes the application. Is there a way to be able to close the terminal without closing the gui application ? I tried using nohup, but it didn't work, maybe I used it wrong.
Thanks
I normally ssh to launch applications with a "&" at the end of the whole string ocmando
try this
ssh myremoteserver -f -X myprogram & 1>/dev/null 2>&1
hope this help you
This is a follow-on question to the How do you use ssh in a shell script? question. If I want to execute a command on the remote machine that runs in the background on that machine, how do I get the ssh command to return? When I try to just include the ampersand (&) at the end of the command it just hangs. The exact form of the command looks like this:
ssh user#target "cd /some/directory; program-to-execute &"
Any ideas? One thing to note is that logins to the target machine always produce a text banner and I have SSH keys set up so no password is required.
I had this problem in a program I wrote a year ago -- turns out the answer is rather complicated. You'll need to use nohup as well as output redirection, as explained in the wikipedia artcle on nohup, copied here for your convenience.
Nohuping backgrounded jobs is for
example useful when logged in via SSH,
since backgrounded jobs can cause the
shell to hang on logout due to a race
condition [2]. This problem can also
be overcome by redirecting all three
I/O streams:
nohup myprogram > foo.out 2> foo.err < /dev/null &
This has been the cleanest way to do it for me:-
ssh -n -f user#host "sh -c 'cd /whereever; nohup ./whatever > /dev/null 2>&1 &'"
The only thing running after this is the actual command on the remote machine
Redirect fd's
Output needs to be redirected with &>/dev/null which redirects both stderr and stdout to /dev/null and is a synonym of >/dev/null 2>/dev/null or >/dev/null 2>&1.
Parantheses
The best way is to use sh -c '( ( command ) & )' where command is anything.
ssh askapache 'sh -c "( ( nohup chown -R ask:ask /www/askapache.com &>/dev/null ) & )"'
Nohup Shell
You can also use nohup directly to launch the shell:
ssh askapache 'nohup sh -c "( ( chown -R ask:ask /www/askapache.com &>/dev/null ) & )"'
Nice Launch
Another trick is to use nice to launch the command/shell:
ssh askapache 'nice -n 19 sh -c "( ( nohup chown -R ask:ask /www/askapache.com &>/dev/null ) & )"'
If you don't/can't keep the connection open you could use screen, if you have the rights to install it.
user#localhost $ screen -t remote-command
user#localhost $ ssh user#target # now inside of a screen session
user#remotehost $ cd /some/directory; program-to-execute &
To detach the screen session: ctrl-a d
To list screen sessions:
screen -ls
To reattach a session:
screen -d -r remote-command
Note that screen can also create multiple shells within each session. A similar effect can be achieved with tmux.
user#localhost $ tmux
user#localhost $ ssh user#target # now inside of a tmux session
user#remotehost $ cd /some/directory; program-to-execute &
To detach the tmux session: ctrl-b d
To list screen sessions:
tmux list-sessions
To reattach a session:
tmux attach <session number>
The default tmux control key, 'ctrl-b', is somewhat difficult to use but there are several example tmux configs that ship with tmux that you can try.
I just wanted to show a working example that you can cut and paste:
ssh REMOTE "sh -c \"(nohup sleep 30; touch nohup-exit) > /dev/null &\""
You can do this without nohup:
ssh user#host 'myprogram >out.log 2>err.log &'
Quickest and easiest way is to use the 'at' command:
ssh user#target "at now -f /home/foo.sh"
I think you'll have to combine a couple of these answers to get what you want. If you use nohup in conjunction with the semicolon, and wrap the whole thing in quotes, then you get:
ssh user#target "cd /some/directory; nohup myprogram > foo.out 2> foo.err < /dev/null"
which seems to work for me. With nohup, you don't need to append the & to the command to be run. Also, if you don't need to read any of the output of the command, you can use
ssh user#target "cd /some/directory; nohup myprogram > /dev/null 2>&1"
to redirect all output to /dev/null.
This worked for me may times:
ssh -x remoteServer "cd yourRemoteDir; ./yourRemoteScript.sh </dev/null >/dev/null 2>&1 & "
You can do it like this...
sudo /home/script.sh -opt1 > /tmp/script.out &
It appeared quite convenient for me to have a remote tmux session using the tmux new -d <shell cmd> syntax like this:
ssh someone#elsewhere 'tmux new -d sleep 600'
This will launch new session on elsewhere host and ssh command on local machine will return to shell almost instantly. You can then ssh to the remote host and tmux attach to that session. Note that there's nothing about local tmux running, only remote!
Also, if you want your session to persist after the job is done, simply add a shell launcher after your command, but don't forget to enclose in quotes:
ssh someone#elsewhere 'tmux new -d "~/myscript.sh; bash"'
Actually, whenever I need to run a command on a remote machine that's complicated, I like to put the command in a script on the destination machine, and just run that script using ssh.
For example:
# simple_script.sh (located on remote server)
#!/bin/bash
cat /var/log/messages | grep <some value> | awk -F " " '{print $8}'
And then I just run this command on the source machine:
ssh user#ip "/path/to/simple_script.sh"
If you run remote command without allocating tty, redirect stdout/stderr works, nohup is not necessary.
ssh user#host 'background command &>/dev/null &'
If you use -t to allocate tty to run interactive command along with background command, and background command is the last command, like this:
ssh -t user#host 'bash -c "interactive command; nohup backgroud command &>/dev/null &"'
It's possible that background command doesn't actually start. There's race here:
bash exits after nohup starts. As a session leader, bash exit results in HUP signal sent to nohup process.
nohup ignores HUP signal.
If 1 completes before 2, the nohup process will exit and won't start the background command at all. We need to wait nohup start the background command. A simple workaroung is to just add a sleep:
ssh -t user#host 'bash -c "interactive command; nohup backgroud command &>/dev/null & sleep 1"'
The question was asked and answered years ago, I don't know if openssh behavior changed since then. I was testing on:
OpenSSH_8.6p1, OpenSSL 1.1.1g FIPS 21 Apr 2020
I was trying to do the same thing, but with the added complexity that I was trying to do it from Java. So on one machine running java, I was trying to run a script on another machine, in the background (with nohup).
From the command line, here is what worked: (you may not need the "-i keyFile" if you don't need it to ssh to the host)
ssh -i keyFile user#host bash -c "\"nohup ./script arg1 arg2 > output.txt 2>&1 &\""
Note that to my command line, there is one argument after the "-c", which is all in quotes. But for it to work on the other end, it still needs the quotes, so I had to put escaped quotes within it.
From java, here is what worked:
ProcessBuilder b = new ProcessBuilder("ssh", "-i", "keyFile", "bash", "-c",
"\"nohup ./script arg1 arg2 > output.txt 2>&1 &\"");
Process process = b.start();
// then read from process.getInputStream() and close it.
It took a bit of trial & error to get this working, but it seems to work well now.
YOUR-COMMAND &> YOUR-LOG.log &
This should run the command and assign a process id you can simply tail -f YOUR-LOG.log to see results written to it as they happen. you can log out anytime and the process will carry on
If you are using zsh then use program-to-execute &! is a zsh-specific shortcut to both background and disown the process, such that exiting the shell will leave it running.
A follow-on to #cmcginty's concise working example which also shows how to alternatively wrap the outer command in double quotes. This is how the template would look if invoked from within a PowerShell script (which can only interpolate variables from within double-quotes and ignores any variable expansion when wrapped in single quotes):
ssh user#server "sh -c `"($cmd) &>/dev/null </dev/null &`""
Inner double-quotes are escaped with back-tick instead of backslash. This allows $cmd to be composed by the PowerShell script, e.g. for deployment scripts and automation and the like. $cmd can even contain a multi-line heredoc if composed with unix LF.
First follow this procedure:
Log in on A as user a and generate a pair of authentication keys. Do not enter a passphrase:
a#A:~> ssh-keygen -t rsa
Generating public/private rsa key pair.
Enter file in which to save the key (/home/a/.ssh/id_rsa):
Created directory '/home/a/.ssh'.
Enter passphrase (empty for no passphrase):
Enter same passphrase again:
Your identification has been saved in /home/a/.ssh/id_rsa.
Your public key has been saved in /home/a/.ssh/id_rsa.pub.
The key fingerprint is:
3e:4f:05:79:3a:9f:96:7c:3b:ad:e9:58:37:bc:37:e4 a#A
Now use ssh to create a directory ~/.ssh as user b on B. (The directory may already exist, which is fine):
a#A:~> ssh b#B mkdir -p .ssh
b#B's password:
Finally append a's new public key to b#B:.ssh/authorized_keys and enter b's password one last time:
a#A:~> cat .ssh/id_rsa.pub | ssh b#B 'cat >> .ssh/authorized_keys'
b#B's password:
From now on you can log into B as b from A as a without password:
a#A:~> ssh b#B
then this will work without entering a password
ssh b#B "cd /some/directory; program-to-execute &"
I think this is what you need:
At first you need to install sshpass on your machine.
then you can write your own script:
while read pass port user ip; do
sshpass -p$pass ssh -p $port $user#$ip <<ENDSSH1
COMMAND 1
.
.
.
COMMAND n
ENDSSH1
done <<____HERE
PASS PORT USER IP
. . . .
. . . .
. . . .
PASS PORT USER IP
____HERE
My server deployment script triggers a long-running process through SSH, like so:
ssh host 'install.sh'
Since my internet connection at home is not the best, I can sometimes be disconnected while the install.sh is running. (This is easily simulated by closing the terminal window.) I would really like for the install.sh script to keep running in those cases, so that I don't end up with interrupted apt-get processes and similar nuisances.
The reason why install.sh gets killed seems to be that stdout and stderr are closed when the SSH session is yanked, so writing to them fails. (It's not an issue of SIGHUP, by the way -- using nohup makes no difference.) If I put touch ~/1 && echo this fails && touch ~/2 into install.sh, only ~/1 is created.
So running ssh host 'install.sh &> install.out' solves the problem, but then I lose any "live" progress and error output.
So my question is: What's an easy/idiomatic way to run a process through SSH so that it doesn't crash if SSH dies, but so that I can still see the output as it runs?
Solutions I have tried:
When I run things manually, I use screen for cases like this, but I don't think it will be of much help here because I need to run install.sh automatically from a shell script. Screen seems to be made for interactive use (it complains "Must be connected to a terminal.").
Using install.sh 2>&1 | tee install.out didn't help either (silly of me to think it might).
You can redirect stdout/stderr into install.out and then tail -f it. The following snippet actually works:
touch install.out && # so tail does not bark (race condition)
(install.sh < /dev/null &> install.out &
tail --pid "$!" -F install.out)
But surely there must a less awkward way to do the same thing?
Try using screen:
screen ./install.sh
If your ssh session gets interrupted, you can simply reattach to the session via another ssh connection:
screen -x
You can provide a terminal to your ssh session using the -t switch:
ssh -t server screen ./install.sh
install.sh 2>&1 | tee install.out
if the only issue is not getting stderr. You didn't say exactly why the tee wasn't acceptable. You may need the other nohup/stdin tweaks.