Bash – How to view the output of a running process in another bash session

bashio-redirectionprocess

I have left a script running on a remote machine from when I was locally working at it. I can connect over SSH to the machine as the same user and see the script running in ps.

$ ps aux | grep ipcheck
myuser  18386  0.0  0.0  18460  3476 pts/0    S+   Dec14   1:11 /bin/bash ./ipchecker.sh

It is simply outputting to stdout on a local session (I ran ./ipchecker.sh form a local terminal window, no redirection, no use of screen etc).

Is there anyway from an SSH session I can view the output of this running command (without stopping it)?

So far the best I have found is to use strace -p 18386 but I get hordes of text flying up the screen, its far too detailed. I can stop strace and then sift through the output and find the text bring printed to stdout but its very long and confusing, and obviously whilst it's stopped I might miss something. I would like to find a way to see the script output live as if I was working locally.

Can anyone improve on this? The obvious answer is to restart the script with redirection or in a screen session etc, this isn't a mission critical script so I could do that. Rather though, I see this as a fun learning exercise.

Best Answer

If all you want to do is spy on the existing process, you can use strace -p1234 -s9999 -e write where 1234 is the process ID. (-s9999 avoids having strings truncated to 32 characters, and write the system call that produces output.) If you want to view only data written on a particular file descriptor, you can use something like strace -p1234 -e trace= -e write=3 to see only data written to file descriptor 3 (-e trace= prevents the system calls from being loged). That won't give you output that's already been produced.

If the output is scrolling by too fast, you can pipe it into a pager such as less, or send it to a file with strace -o trace.log ….

With many programs, you can divert subsequent output with a ptrace hack, either to your current terminal or to a new screen session. See How can I disown a running process and associate it to a new screen shell? and other linked threads.

Note that depending on how your system is set up, you may need to run all these strace commands as root even if the process is running under your user with no extra privileges. (If the process is running as a different user or is setuid or setgid, you will need to run strace as root.) Most distributions only allow a process to trace its children (this provides a moderate security benefit — it prevents some direct malware injection, but doesn't prevent indirect injection by modifying files). This is controlled by the kernel.yama.ptrace_scome sysctl.

Related Solutions

Bash – How to output to screen overriding redirection

Try a script like this:

#!/bin/bash
echo "to fd1" >&1
echo "to fd2" >&2
echo "to screen" >$(tty)

When you call it, it looks like this:

user@host:~# ./script
to fd1
to fd2
to screen
user@host:~# ./script 1>/dev/null
to fd2
to screen
user@host:~# ./script 2>/dev/null
to fd1
to screen
user@host:~# ./script > /dev/null 2>&1
to screen

SSH – How to Send a HEREDOC Over SSH to a Remote Host from a Local Script?

If you wrap the entire command that you want executed inside backticks it's likely you'll get the correct solution. There are issues regarding the evaluation of variables and other interpolated elements, but I'll put those to one side for a moment.

However, I'm not entirely sure why you want anything in backticks. Your Question says you want to run a local script remotely. Your first example does this.

ssh -q user@host << ENDSSH
hostname
ls -l
id
ENDSSH

Perhaps you want to get the output of the remote execution into a variable? It doesn't say that anywhere in the Question, but you'd do it like this (bash)

RESULT=$(ssh user@host <<ENDSSH
hostname
id
ENDSSH
)
...
echo "$RESULT"

Or, since you mentioned csh, like this

set result = `ssh -q user@host` <<ENDSSH
hostname
id
ENDSSH
...
echo "$result"

Note that for the csh variant all output lines are concatenated together.

I see that you have reported an error, stty: standard input: Invalid argument. This is probably occurring because you have an stty command in your login script on the remote server. This should be wrapped in a test for an interactive shell. In bash you would do something like this:

if test -n "$PS1"
then
    # Interactive setup here
    stty ...
fi

Best Answer

Related Solutions

Bash – How to output to screen overriding redirection

SSH – How to Send a HEREDOC Over SSH to a Remote Host from a Local Script?

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