Job control with linux

Job Control

In the previous lesson, we looked at some of the implications of Linux being a multi-user operating system. In this lesson, we will examine the multitasking nature of Linux, and how it is controlled with the command line interface.

As with any multitasking operating system, Linux executes multiple, simultaneous processes. Well, they appear simultaneous, anyway. Actually, a single processor core can only execute one process at a time but the Linux kernel manages to give each process its turn at the processor and each appears to be running at the same time.

There are several commands that are used to control processes. They are:

• ps — list the processes running on the system
• kill — send a signal to one or more processes (usually to «kill» a process)
• jobs — an alternate way of listing your own processes
• bg — put a process in the background
• fg — put a process in the foreground

A Practical Example

While it may seem that this subject is rather obscure, it can be very practical for the average user who mostly works with the graphical user interface. Though it might not be apparent, most (if not all) graphical programs can be launched from the command line. Here’s an example: there is a small program supplied with the X Window system called xload which displays a graph representing system load. We can execute this program by typing the following:

Notice that the small xload window appears and begins to display the system load graph. On systems where xload is not available, try gedit instead. Notice also that our prompt did not reappear after the program launched. The shell is waiting for the program to finish before control returns. If we close the xload window, the xload program terminates and the prompt returns.

Putting a Program into the Background

Now, in order to make life a little easier, we are going to launch the xload program again, but this time we will put it in the background so that the prompt will return. To do this, we execute xload like this:

In this case, the prompt returned because the process was put in the background.

Now imagine that we forgot to use the «&» symbol to put the program into the background. There is still hope. We can type Ctrl-z and the process will be suspended. We can verify this by seeing that the program’s window is frozen. The process still exists, but is idle. To resume the process in the background, type the bg command (short for background). Here is an example:

Listing Running Processes

Now that we have a process in the background, it would be helpful to display a list of the processes we have launched. To do this, we can use either the jobs command or the more powerful ps command.

[me@linuxbox me]$ jobs [1]+ Running xload& [me@linuxbox me]$ ps PID TTY TIME CMD 1211 pts/4 00:00:00 bash 1246 pts/4 00:00:00 xload 1247 pts/4 00:00:00 ps [me@linuxbox me]$

Killing a Process

Suppose that we have a program that becomes unresponsive; how do we get rid of it? We use the kill command, of course. Let’s try this out on xload . First, we need to identify the process we want to kill. We can use either jobs or ps , to do this. If we use jobs we will get back a job number. With ps , we are given a process id (PID). We will do it both ways:

[me@linuxbox me]$ xload & [1] 1292 [me@linuxbox me]$ jobs [1]+ Running xload& [me@linuxbox me]$ kill %1 [me@linuxbox me]$ xload & [2] 1293 [1] Terminated xload [me@linuxbox me]$ ps PID TTY TIME CMD 1280 pts/5 00:00:00 bash 1293 pts/5 00:00:00 xload 1294 pts/5 00:00:00 ps [me@linuxbox me]$ kill 1293 [2]+ Terminated xload [me@linuxbox me]$

A Little More About kill

While the kill command is used to «kill» processes, its real purpose is to send signals to processes. Most of the time the signal is intended to tell the process to go away, but there is more to it than that. Programs (if they are properly written) listen for signals from the operating system and respond to them, most often to allow some graceful method of terminating. For example, a text editor might listen for any signal that indicates that the user is logging off, or that the computer is shutting down. When it receives this signal, it could save the work in progress before it exits. The kill command can send a variety of signals to processes. Typing:

will print a list of the signals it supports. Many are rather obscure, but several are handy to know:

Signal #	Name	Description
1	SIGHUP	Hang up signal. Programs can listen for this signal and act upon it. This signal is sent to processes running in a terminal when you close the terminal.
2	SIGINT	Interrupt signal. This signal is given to processes to interrupt them. Programs can process this signal and act upon it. We can also issue this signal directly by typing Ctrl-c in the terminal window where the program is running.
15	SIGTERM	Termination signal. This signal is given to processes to terminate them. Again, programs can process this signal and act upon it. This is the default signal sent by the kill command if no signal is specified.
9	SIGKILL	Kill signal. This signal causes the immediate termination of the process by the Linux kernel. Programs cannot listen for this signal.

Now let’s suppose that we have a program that is hopelessly hung and we want to get rid of it. Here’s what we do:

1. Use the ps command to get the process id (PID) of the process we want to terminate.
2. Issue a kill command for that PID.
3. If the process refuses to terminate (i.e., it is ignoring the signal), send increasingly harsh signals until it does terminate.

[me@linuxbox me]$ ps x | grep bad_program PID TTY STAT TIME COMMAND 2931 pts/5 SN 0:00 bad_program [me@linuxbox me]$ kill -SIGTERM 2931 [me@linuxbox me]$ kill -SIGKILL 2931

In the example above we used the ps command with the x option to list all of our processes (even those not launched from the current terminal). In addition, we piped the output of the ps command into grep to list only list the program we are interested in. Next, we used kill to issue a SIGTERM signal to the troublesome program. In actual practice, it is more common to do it in the following way since the default signal sent by kill is SIGTERM and kill can also use the signal number instead of the signal name:

Then, if the process does not terminate, force it with the SIGKILL signal:

That’s It!

This concludes the «Learning the Shell» series of lessons. In the next series, «Writing Shell Scripts,» we will look at how to automate tasks with the shell.

Further Reading

• For a more in-depth treatment of the topic, see Chapter 10 in The Linux Command Line.
• 1963 Timesharing: A Solution to Computer Bottlenecks, a fascinating YouTube video from the Computer History Museum describing the first timesharing operating system and how it works. It’s basically the same method used by all modern computers.

© 2000-2023, William E. Shotts, Jr. Verbatim copying and distribution of this entire article is permitted in any medium, provided this copyright notice is preserved.

Linux® is a registered trademark of Linus Torvalds.

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Job Control Commands in Linux | bg, fg, and CTRL+Z

In the Linux operating system, the command shell manages a series of processes known as “jobs.” These jobs are assigned to the shell for execution with three possible states such as “background,” “foreground,” and “stopped.” Linux provides the “jobs” command line tool that interacts directly with the system jobs and allows users to list and manage their states.

This guide describes Linux’s job control commands bg, fg, and CTRL+Z.

• Working of the “bg” Command
• Working of the “fg” Command
• Working of the “Ctrl+Z” Shortcut Key

How Does the “bg” Command Work in Linux?

The “bg” is the short form of the “background” job control command that switches the foreground processes into the background. The process stops for the time being and then resumes in the background.

Syntax:

The generalized syntax of the “bg” command is written here:

The syntax of the “bg” command supports only one parameter, “job.” This parameter can be denoted in four ways:

• %n: Specifies the job number or ID.
• %% or %+: Send the current process in the background.
• %-: Places the previous process in the background.
• %string: Identifies the process that starts from the defined string.

Let’s see with an example how to use the “bg” command to place the foreground process in the background.

A “sleep” command is executed for “40” seconds. After that, press the “Ctrl+Z” key to stop this job:

The output shows that the “sleep” command has now been stopped having job ID [1].

Execute the “jobs” command to list down the background jobs:

To keep the “sleep” command in the background, use the “bg” command with an argument “%1(job ID)” in this way:

$ bg %1 #Place Forground process in background $ jobs # List all jobs

The execution of the “jobs” command confirms the background “sleep” command is in “Running” state. The “&(ampersand)” symbol denotes that “sleep” is a background process.

How Does the “fg” Command Work in Linux?

The “fg” command is opposite to the “bg” command. It is an acronym of the “foreground” jobs control command utilized to resume the background job in the foreground.

Syntax:

The “fg” command follows this basic syntax:

In the above syntax, the user can pass the %n, %% or %+,%-, and the %string parameter in place of “[job]” same as the “bg” command.

If no parameter is passed with the “fg” command, it will print the most recent background process.

List down the jobs that are running in the background using the “jobs” command:

The output contains all the jobs list from which only the first background job, i.e., the “sleep 500” command, is in the “Running” state.

Execute the “fg” command having the “sleep” command job ID “%1” to keep it in the foreground:

The “fg” command has successfully moved the “sleep” command to the foreground. It will take time to complete its execution, and no other command can be executed until its completion.

How Does the “Ctrl+Z” Shortcut Key Work in Linux?

The “Ctrl+Z” is the key to stopping the foreground process and keeping it in the “stopped” state. It can be used at any point of the process/command execution. It is not a command and is a shortcut key in the shell.

There is a foreground process, i.e., installation of the “curl” command line tool. Press the “Ctrl+Z” to stop it:

The above “apt” command has been stopped.

Run the “jobs” command to verify the stopped process:

The output shows that the “sudo apt install curl” foreground process has been stopped.

Conclusion

The “jobs” control command “bg” places the foreground job in the background, and the “fg” resumes the background jobs. Moreover, the “Ctrl+Z” shortcut key keeps the job in a “stopped” state.

All these “job control” utilities handle the jobs directly through the terminal.

This guide has provided a detailed view of jobs control commands “bg,” “fg” and the “Ctrl+Z” in Linux.

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