Linux process stop resume

Is there a way to pause a running process on Linux systems and resume later?

I have to copy files on a machine. And the data is immensely large. Now servers need to serve normally, and there are usually a particular range of busy hours on those. So is there a way to run such commands in a way that if server hits busy hours, it pauses process, and when it gets out of that range, it resumes it? Intended-Result

cp src dst if time between 9:00-14:00 pause process After 14:00 resume cp command. 

Do you need the actual data to be copied as a backup? If not, could you use cp -al to make a hardlink farm? Or use a filesystem that supports block-level reflinks with copy-on-write, using cp -a —reflink=auto ? BTRFS and ZFS support that for copies within the same physical device.

Do any of the files in src change between 9:00 and 14:00? If so, simply pausing and resuming the cp process may result in corrupted files. It may be better to run rsync in combination with the timeout command.

From and to where are the files being copied? Is this a virtual system? What is the source filesystem? What’s the purpose of the copy?

@Braiam Im using rsync, and copying files from remote unto local machine. I just used cp command as example here btw

6 Answers 6

You can pause execution of a process by sending it a SIGSTOP signal and then later resume it by sending it a SIGCONT .

Assuming your workload is a single process (doesn’t fork helpers running in background), you can use something like this:

# start copy in background, store pid cp src dst & echo "$!" >/var/run/bigcopy.pid 

Then when busy time starts, send it a SIGSTOP :

# pause execution of bigcopy kill -STOP "$(cat /var/run/bigcopy.pid)" 

Later on, when the server is idle again, resume it.

# resume execution of bigcopy kill -CONT "$(cat /var/run/bigcopy.pid)" 

You will need to schedule this for specific times when you want it executed, you can use tools such as cron or systemd timers (or a variety of other similar tools) to get this scheduled. Instead of scheduling based on a time interval, you might choose to monitor the server (perhaps looking at load average, CPU usage or activity from server logs) to make a decision of when to pause/resume the copy.

You also need to manage the PID file (if you use one), make sure your copy is actually still running before pausing it, probably you’ll want to clean up by removing the PID file once the copy is finished, etc.

In other words, you need more around this to make a reliable, but the base idea of using these SIGSTOP and SIGCONT signals to pause/resume execution of a process seems to be what you’re looking for.

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How can I resume a stopped job in Linux?

This is actually a fairly normal work flow for Vim, if you want to keep you commands in your bash history, then you hit Ctrl-z type your commands and then resume. Obviously you can run commands without leaving Vim via the :! ed command

5 Answers 5

The command fg is what you want to use. You can also give it a job number if there are more than one stopped jobs.

The general job control commands in Linux are:

• jobs — list the current jobs
• fg — resume the job that’s next in the queue
• fg %[number] — resume job [number]
• bg — Push the next job in the queue into the background
• bg %[number] — Push the job [number] into the background
• kill %[number] — Kill the job numbered [number]
• kill -[signal] %[number] — Send the signal [signal] to job number [number]
• disown %[number] — disown the process(no more terminal will be owner), so command will be alive even after closing the terminal.

That’s pretty much all of them. Note the % infront of the job number in the commands — this is what tells kill you’re talking about jobs and not processes.

Why use «%» character. Is it required to be prepended before the job number or Is it a unix convention to specify the int type ?

You can also type % ; i.e., you hit Ctrl-Z in emacs, then you can type %emacs in the console and bring it back to the foreground.

Just to add to the other answers, bash lets you skip the fg if you specify a job number.

For example, these are equivalent and resume the latest job:

% is awesome, thanks! As a touch-typist, I find fg very irritating (same finger). But then, so is cd .

in my case when i tried to use fg i see the stopped process appears and disappears quickly and just succeeded to restore it.

If you didn’t launch it from current terminal, use ps aux | grep to find the process number (pid), then resume it with:

(Despite the name, kill is simply a tool to send a signal to the process, allowing processes to communicate with each other. A «kill signal» is only one of many standard signals.)

Bonus tip: wrap the first character of the process name with [] to prevent the grep command itself appearing in the results. e.g. to find emacs process, use ps aux | grep [e]macs

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How to suspend and resume processes

In the bash terminal I can hit Control + Z to suspend any running process. then I can type fg to resume the process. Is it possible to suspend a process if I only have it’s PID? And if so, what command should I use? I’m looking for something like:

suspend-process $PID_OF_PROCESS 

resume-process $PID_OF_PROCESS 

2 Answers 2

You can use kill to stop the process.

For a ‘polite’ stop to the process (prefer this for normal use), send SIGTSTP :

For a ‘hard’ stop, send SIGSTOP :

Note that if the process you are trying to stop by PID is in your shell’s job table, it may remain visible there, but terminated, until the process is fg ‘d again.

To resume execution of the process, sent SIGCONT :

Unless there are other reasons for it, I would prefer SIGTSTP over SIGSTOP, as some applications do handle SIGTSTP specially. For example, if scp is showing a progress bar, SIGTSTP will cause it to clean up the terminal mode before suspending, but if you send SIGSTOP, it will not have a chance to do so.

@ephemient I tried SIGTSTP, I saw what you were saying about it cleaning up the terminal. Thanks for the explanation of SIGTSTP, alawys good to learn new things 🙂

Also useful to note that you can reference the [pid] value by using the % symbol and then the job number (one that you can find by running jobs ). So you’d go: kill -TSTP %1

Why do you mean by it may remain visible there, but terminated ? It’s not terminated but stopped/paused. I tried kill -STOP and it behaves exactly like kill -TSTP for shell jobs.

You should use the kill command for that.

To be more verbose — you have to specify the right signal, i.e.

for suspending the process and

for resuming it. Documented at 24.2.5 Job Control Signals.

I wonder what accident of history led to this answer getting fewer votes? The answers are nearly the same and this one came first.

@Wildcard, when I created the answer I was a bit in a hurry, thus, it basically just contained the first part up to kill -TSTP (i.e. how to suspend). 1/2 year later, i.e. 2011, I revisited my answer and noticed its incompleteness. Thus, I edited it and added also the kill -CONT part. This should explain the difference in votes in comparison with Steve’s answer.

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How to Suspend and Resume Processes in Linux [Quick Tip]

Learn how to suspend a running process in the Linux command line. Also learn how to resume a stopped process.

Have you ever encountered a situation where you have to suspend an ongoing task as you have more important tasks to execute and want to resume the previous task later?

Actually, it is possible and can be done pretty easily using two termination signals: STOP and CONT .

And in this guide, I will walk you through step-by-step to suspend and resume the process again in Linux.

Suspending the process in Linux

You have two options to suspend the process:

1. Using the Ctrl + Z shortcut (for a process running in the foreground)
2. Using the kill command with the STOP signal

A suspended process is denoted as stopped in the terminal. This may confuse you but the ‘stopped process’ can be resumed. After all, there is a difference between stop (SIGSTOP) and termination (SIGTERM).

1. Suspend a foreground process

Let me give you an example you can see. I run Firefox from the command line with this command:

The browser runs and you can see some strange output on the terminal. That’s okay.

No, I suspend the Firefox process which is running in the foreground with the Ctrl+z terminal shortcut:

And here is what happens. The Firefox process is suspended and you can see it in the terminal. After a few seconds of delay, a dialogue box popups to notify that the Firefox browser is not responding anymore.

2. Suspend a process by sending STOP signal

When the process is running in the background, you can not use the Ctrl + Z shortcut. In that case, you’d have to use the kill command.

Let’s see this with an example. I run the sleep command for a really long time. But I am running the command in the background by adding & to it.

You can verify the background processes using the jobs command:

I can see the process ID in the example. There are various ways to find process ID. You can use the ps command and grep on the process name.

Once you have the process ID, you can suspend the process using the kill command in the following manner:

In my case, the PID is 26589, then, I’d have to use the following command:

And as you can see, the process is stopped successfully.

Resuming the process again

To resume the process, you’d have to use the CONT flag with the kill command as shown:

In my case, the PID was 26589 then, the command would be:

And as you can see, after resuming the process, I used the jobs command, which showed the process was running as it should.

Know more about termination signals

Apart from STOP and TERM signals which I explained above, the kill command offers various other termination signals to kill the process with different effects.

And we made a detailed guide for how you can use the most popular ones:

I hope you will find this quick tip helpful.

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