Introduction
In this guide, we’ll talk about how bash, the Linux system, and your terminal come together to offer process and job control. In a previous guide, we discussed how the ps, kill, and nice commands can be used to control processes on your system.
This article will focus on managing foreground and background processes and will demonstrate how to leverage your shell’s job control functions to gain more flexibility in how you run commands.
Managing Foreground Processes
Most processes that you start on a Linux machine will run in the foreground. The command will begin execution, blocking use of the shell for the duration of the process. The process may allow user interaction or may just run through a procedure and then exit. Any output will be displayed in the terminal window by default. We’ll discuss the basic way to manage foreground processes below.
Starting a Process
By default, processes are started in the foreground. Until the program exits or changes state, you will not be able to interact with the shell.
Some foreground commands exit very quickly and return you to a shell prompt almost immediately. For instance, this command:
- echo “Hello World”
This would print “Hello World” to the terminal and then return you to your command prompt.
Other foreground commands take longer to execute, blocking shell access for the duration. This might be because the command is performing a more extensive operation or because it is configured to run until it is explicitly stopped or until it receives other user input.
A command that runs indefinitely is the top utility. After starting, it will continue to run and update its display until the user terminates the process:
- top
You can quit by typing “q”. Some processes don’t have a dedicated quit function. To stop those, you’ll have to use another method.
Terminating a Process
Suppose we start a simple bash loop on the command line. We can start a loop that will print “Hello World” every ten seconds. This loop will continue forever, until explicitly terminated:
- while true; do echo “Hello World”; sleep 10; done
Loops have no “quit” key. We will have to stop the process by sending it a signal. In Linux, the kernel can send processes signals in order to request that they exit or change states. Linux terminals are usually configured to send the “SIGINT” signal (typically signal number 2) to current foreground process when the CTRL-C key combination is pressed. The SIGINT signal tells the program that the user has requested termination using the keyboard.
To stop the loop we’ve started, hold the control key and press the “c” key:
CTRL-C
The loop will exit, returning control to the shell.
The SIGINT signal sent by the CTRL-C combination is one of many signals that can be sent to programs. Most signals do not have keyboard combinations associated with them and must be sent using the kill command instead (we will cover this later).
Suspending Processes
We mentioned above that foreground process will block access to the shell for the duration of their execution. What if we start a process in the foreground, but then realize that we need access to the terminal?
Another signal that we can send is the “SIGTSTP” signal (typically signal number 20). When we hit CTRL-Z, our terminal registers a “suspend” command, which then sends the SIGTSTP signal to the foreground process. This will basically pause the execution of the command and return control to the terminal.
To demonstrate, let’s use ping to connect to google.com every 5 seconds. We will precede the ping command with command, which will allow us to bypass any shell aliases that artificially set a maximum count on the command:
- command ping -i 5 google.com
Instead of terminating the command with CTRL-C, type CTRL-Z instead:
CTRL-Z
You will see output that looks like this:
[1]+ Stopped ping -i 5 google.com
The ping command has been temporarily stopped, giving you access to a shell prompt again. We can use the ps process tool to show this:
- ps T
PID TTY STAT TIME COMMAND
26904 pts/3 Ss 0:00 /bin/bash
29633 pts/3 T 0:00 ping -i 5 google.com
29643 pts/3 R+ 0:00 ps t
We can see that the ping process is still listed, but that the “STAT” column has a “T” in it. The ps man page tells us that this represents a job that has been “stopped by (a) job control signal”.
We will discuss in more depth how to change process states, but for now, we can resume execution of the command in the foreground again by typing:
- fg
Once the process has resumed, terminate it with CTRL-C:
CTRL-C
Managing Background Processes
The main alternative to running a process in the foreground is to allow it to execute in the background. A background process is associated with the specific terminal that started it, but does not block access to the shell. Instead, it executes in the background, leaving the user able to interact with the system while the command runs.
Because of the way that a foreground processes interacts with its terminal, there can be only a single foreground process for every terminal window. Because background processes return control to the shell immediately without waiting for the process to complete, many background processes can run at the same time.
Starting Processes
You can start a background process by appending an ampersand character (“&”) to the end of your commands. This tells the shell not to wait for the process to complete, but instead to begin execution and to immediately return the user to a prompt. The output of the command will still display in the terminal (unless redirected), but you can type additional commands as the background process continues.
For instance, we can start the same ping process from the last section in the background by typing:
- command ping -i 5 google.com &
You will see output from the bash job control system that looks like this:
[1] 4287
You will also see the normal output from the ping command:
PING google.com (74.125.226.71) 56(84) bytes of data.
64 bytes from lga15s44-in-f7.1e100.net (74.125.226.71): icmp_seq=1 ttl=55 time=12.3 ms
64 bytes from lga15s44-in-f7.1e100.net (74.125.226.71): icmp_seq=2 ttl=55 time=11.1 ms
64 bytes from lga15s44-in-f7.1e100.net (74.125.226.71): icmp_seq=3 ttl=55 time=9.98 ms
However, you can also type commands at the same time. The background process’s output will be mixed among the input and output of your foreground processes, but it will not interfere with the execution of the foreground processes.
Listing Background Processes
To see all stopped or backgrounded processes, you can use the jobs command:
- jobs
If you have the ping command running in the background, you will see something that looks like this:
[1]+ Running command ping -i 5 google.com &
This shows that we currently have a single background process running. The [1] represents the command’s “job spec” or job number. We can reference this with other job and process control commands, like kill, fg, and bg by preceding the job number with a percentage sign. In this case, we’d reference this job as %1.
Stopping Background Processes
We can stop the current background process in a few ways. The most straight forward way is to use the kill command with the associate job number. For instance, we can kill our running background process by typing:
- kill %1
Depending on how your terminal is configured, either immediately or the next time you hit ENTER, you will see the job termination status:
[1]+ Terminated command ping -i 5 google.com
If we check the jobs command again, we’ll see no current jobs.
Changing Process States
Now that we know how to start and stop processes in the background, we can talk about how to change their state.
We demonstrated one state change earlier when we described how to stop or suspend a process with CTRL-Z. When processes are in this stopped state, we can move a foreground process to the background or vice versa.
Moving Foreground Processes to the Background
If we forget to end a command with & when we start it, we can still move the process to the background.
The first step is to stop the process with CTRL-Z again:
CTRL-Z
Once the process is stopped, we can use the bg command to start it again in the background:
- bg
You will see the job status line again, this time with the ampersand appended:
[1]+ ping -i 5 google.com &
By default, the bg command operates on the most recently stopped process. If you’ve stopped multiple processes in a row without starting them again, you can reference the process by job number to background the correct process.
Note that not all commands can be backgrounded. Some processes will automatically terminate if they detect that they have been started with their standard input and output directly connected to an active terminal.
Moving Background Processes to the Foreground
We can also move background processes to the foreground by typing fg:
- fg
This operates on your most recently backgrounded process (indicated by the “+” in the jobs output). It immediately suspends the process and puts it into the foreground. To specify a different job, use its job number:
- fg %2
Once a job is in the foreground, you can kill it with CTRL-C, let it complete, or suspend and background it again.
Dealing with SIGHUPs
Whether a process is in the background or in the foreground, it is rather tightly tied with the terminal instance that started it. When a terminal closes, it typically sends a SIGHUP signal to all of the processes (foreground, background, or stopped) that are tied to the terminal. This signals for the processes to terminate because their controlling terminal will shortly be unavailable. What if you want to close a terminal but keep the background processes running?
There are a number of ways of accomplishing this. The most flexible ways are typically to use a terminal multiplexer like screen or tmux, or use a utility that provides at least the detach functionality of those, like dtach.
However, this isn’t always an option. Sometimes these programs aren’t available or you’ve already started the process you need to continue running. Sometimes these are overkill for what you need to accomplish.
Using nohup
If you know when starting the process that you will want to close the terminal before the process completes, you can start it using the nohup command. This makes the started process immune to the SIGHUP signal. It will continue running when the terminal closes. It will be reassigned as a child of the init system:
- nohup ping -i 5 google.com &
You will see a line that looks like this, indicating that the output of the command will be written to a file called nohup.out (in the current directory if writeable, otherwise to your home directory):
nohup: ignoring input and appending output to ‘nohup.out’
This is to ensure that output is not lost if the terminal window is closed.
If you close the terminal window and open another one, the process will still be running. You will not see it in the output of the jobs command because each terminal instance maintains its own independent job queue. The terminal closing caused the ping job to be destroyed even though the ping process is still running.
To kill the ping process, you’ll have to look up its process ID (or “PID”). You can do that with the pgrep command (there is also a pkill command, but this two-part method ensures that we are only killing the intended process). Use pgrep and the -a flag to search for the executable:
- pgrep -a ping
7360 ping -i 5 google.com
You can then kill the process by referencing the returned PID, which is the number in the first column:
- kill 7360
You may wish to remove the nohup.out file if you don’t need it anymore.
Using disown
The nohup command is helpful, but only if you know you will need it at the time you start the process. The bash job control system provides other methods of achieving similar results with the disown built in command.
The disown command, in its default configuration, removes a job from the jobs queue of a terminal. This means that it can no longer be managed using the job control mechanisms discussed in this guide (like fg, bg, CTRL-Z, CTRL-C). It will immediately be removed from the list in the jobs output and no longer associated with the terminal.
The command is called by specifying a job number. For instance, to immediately disown job 2, we could type:
- disown %2
This leaves the process in a state not unlike that of a nohup process after the controlling terminal has been closed. The exception is that any output will be lost when the controlling terminal closes if it is not being redirected to a file.
Usually, you don’t want to remove the process completely from job control if you aren’t immediately closing your terminal window. You can pass the -h flag to the disown process instead in order to mark the process to ignore SIGHUP signals, but to otherwise continue on as a regular job:
- disown -h %1
In this state, you could use normal job control mechanisms to continue controlling the process until closing the terminal. Upon closing the terminal, you will, once again, be stuck with a process with nowhere to output if you didn’t redirect to a file when starting it.
To work around that, you can try to redirect the output of your process after it is already running. This is outside the scope of this guide, but you can take a look at this post to get an idea of how you would do that.
Using the huponexit Shell Option
Bash also has another way of avoiding the SIGHUP problem for child processes. The huponexit shell option controls whether bash will send its child processes the SIGHUP signal when it exits.
The huponexit option only affect the SIGHUP behavior when a shell session termination is initiated from within the shell itself. Some examples of when this applies is when the exit command or CTRL-D is hit within the session.
When a shell session is ended through the terminal program itself (through closing the window, etc.), the command huponexit will have no affect. Instead of bash deciding on whether to send the SIGHUP signal, the terminal itself will send the SIGHUP signal to bash, which will then (correctly) propagate the signal to its child processes.
Despite the above caveats, the huponexit option is perhaps one of the easiest. You can see whether this feature is on or off by typing:
- shopt huponexit
To turn it on, type:
- shopt -s huponexit
Now, if you exit your session by typing exit, your processes will all continue to run:
- exit
This has the same caveats about program output as the last option, so make sure you have redirected your processes’ output if it is important prior to closing your terminal.
Conclusion
Learning job control and how to manage foreground and background processes will give you greater flexibility when running programs on the command line. Instead of having to open up many terminal windows or SSH sessions, you can often get by with a few stop and background commands.