Process Management

The operating system manages multiple user requests and tasks. In most cases, the operating system comes with only one CPU and one main memory, but it may have multiple tier-2 disks and input/output (I/O) devices. Therefore, users have to share resources, but it appears to users that they are exclusively occupying resources. The operating system places user tasks, OS tasks, emailing, print tasks, and other pending tasks in the queue and schedules the tasks according to predefined rules. In this topic, you will know how the operating system manages processes.

Viewing Processes

Linux is a multi-task system and needs to get process information during process management. To manage processes, you first need to know the number of processes and their statuses. Multiple commands are available to view processes.

who Command

The who command is used to display system user information. For example, before running the talk command to establish instant communication with another user, you need to run the who command to determine whether the target user is online. As another example, the system administrator can run the who command to learn what each login user is doing at the current time. The who command is widely seen in system administration since it is easy to use and can return a comprehensive set of accurate user information.

The following is an example output of the who command, where system users and their status are displayed: The use of the who command is as follows:

$ who
admin     tty1         Jul 28 15:55
admin     pts/0        Aug  5 15:46 (
admin     pts/2        Jul 29 19:52 (
root     pts/3        Jul 30 12:07 (
root     pts/4        Jul 31 10:29 (
root     pts/5        Jul 31 14:52 (
root     pts/6        Aug  6 10:12 (
root     pts/8        Aug  6 11:34 (

ps Command

The ps command is the most basic and powerful command to view process information. The ps command is used to display process information, including which processes are running, terminated, resource-hungry, or stay as zombies.

A common scenario is using the ps command to monitor background processes, which do not interact with your screen, keyboard, and other I/O devices. Table 1 lists the common ps command options.

Table 1 Common ps command options




Displays all processes.


Full output format.


Hides column headings in the listing of process information.


Long output format.


Wide output format.


Lists all processes on a terminal, including those of other users.


Lists only running processes.


Lists all processes without controlling terminals.

For example, to list all processes on a terminal, run the following command:

$ ps -a
  PID TTY          TIME CMD
12175 pts/6    00:00:00 bash
24526 pts/0    00:00:00 vsftpd
29478 pts/5    00:00:00 ps
32461 pts/0    1-01:58:33 sh

top Command

Both the top and the ps commands can display a list of currently running processes, but the top command allows you to update the displayed list of processes repeatedly with the press of a button. If the top command is executed in foreground, it exclusively occupies foreground until it is terminated. The top command provides real-time visibility into system processor status. You can sort the list of CPU tasks by CPU usage, memory usage, or task execution time. Extensive customization of the display, such as choice of columns or sorting method, can be achieved using interactive commands or the customization file.

Figure 1 provides an example output of the top command.

Figure 1 Example command output

kill Command

The kill command is used to terminate a process regardless of whether the process is running in foreground or background. It differs from the combo key Ctrl+c, which can terminate only foreground processes. The kill command is used to terminate a process regardless of whether the process is running in foreground or background. The reason for terminating a background process can be heavy use of CPU resources or deadlock.

The kill command sends a signal to terminate running processes. By default, the TERM signal is used. The TERM signal terminates all processes incapable of capturing the TERM signal. To terminate a process capable of capturing the TERM signal, use the KILL signal (signal ID: 9) instead.

Two types of syntax of the kill command:

kill [-s signal | -p] [-a] PID…
kill -l [signal]

The process ID is retrieved from the ps command. The -s option indicates the signal sent to specified program. The signal details can be viewed by running the kill -l command. The -p option indicates the specified process IDs.

For example, to terminate the process with ID 1409, run the following command as the root user:

# kill -9 1409

Example output of the kill command with the -l option

$ kill -l
 1) SIGHUP       2) SIGINT       3) SIGQUIT      4) SIGILL       5) SIGTRAP
 6) SIGABRT      7) SIGBUS       8) SIGFPE       9) SIGKILL     10) SIGUSR1
11) SIGSEGV     12) SIGUSR2     13) SIGPIPE     14) SIGALRM     15) SIGTERM
16) SIGSTKFLT   17) SIGCHLD     18) SIGCONT     19) SIGSTOP     20) SIGTSTP
21) SIGTTIN     22) SIGTTOU     23) SIGURG      24) SIGXCPU     25) SIGXFSZ
26) SIGVTALRM   27) SIGPROF     28) SIGWINCH    29) SIGIO       30) SIGPWR
31) SIGSYS      34) SIGRTMIN    35) SIGRTMIN+1  36) SIGRTMIN+2  37) SIGRTMIN+3

Scheduling a Process

The time-consuming and resource-demanding part of maintenance work is often performed at late night. You can arrange relevant processes to get started at the scheduled time instead of staying up all night. Here, we will explain the process scheduling commands.

Using the at Command to Run Processes at the Scheduled Time


The at command is used to run a batch of processes (a series of commands) at the scheduled time or time+date.

Syntax of the at command:

at [-V] [-q queue] [-f filename] [-mldbv] time
at -c job [job...]

Time Format

The scheduled time can be in any of the following formats:

  • hh:mm today: If hh:mm is earlier than the current time, the selected commands will be run at hh:mm the next day.
  • midnight, noon, teatime (typically at 16:00), or the like
  • 12-hour format followed by am or pm
  • Time + date (month day, mm/dd/yy, or The scheduled date must follow the scheduled time.

The scheduled time can also be relative time, which is suitable for scheduling commands that are going to be executed soon. For example, now+N minutes, hours, days, or weeks. N is time, which may be a few days or hours. Further, the scheduled time can be words like today, tomorrow, or the like. Here are some examples of the scheduled time.

Imagine the current time is 12:30 June 7 2019 and you want to run a command at 4:30 pm. The scheduled time in the at command can be any of the following:

 at 4:30pm
 at 16:30
 at 16:30 today
 at now+4 hours
 at now+ 240 minutes
 at 16:30 7.6.19
 at 16:30 6/7/19
 at 16:30 Jun 7

Although you can select any of the preceding examples according to your preference, absolute time in 24-hour format, such as at 16:30 6/7/19, is recommended.


Only commands from standard input or from the file specified by the -f option can be scheduled by the at command to be executed. If the su command is executed to switch the operating system from user A to user B and then the at command is executed at the shell prompt of user B, the at command execution result is sent to user B. whereas emails (if any) are sent to user A.

For example, to run the slocate -u command at 10 am on June 8, 2019, perform the following steps as the root user:

# at  10:00  6/8/19
at> slocate -u
[1]+   Stopped    at  10:00  6/8/19

When the at> prompt appears, type slocate -u and press Enter. Repeat substep 2 to add other commands that need to be run at 10 am on 8 June 2015. Then, press Ctrl+d to exit the at command.

The administrator is authorized to run the at command unconditionally. For other users, their privilege to run the at command is defined in /etc/at.allow and /etc/at.deny files.

Using the cron Service to Run Commands Periodically

The at command can run commands at the scheduled time but only once. It means that after the running command is specified, the system completes the task at the specified time. If you need to run commands repeatedly, the cron service is a good helper.

Cron Service

The cron service searches the /var/spool/cron directory for crontab files named by the user name in the /etc/passwd file and loads the search results into memory to execute the commands in the crontab files. Each user has a crontab file, with the file name being the same as the user name. For example, the crontab file of the userexample user is /var/spool/cron/userexample.

The cron service also reads the cron configuration file /etc/crontab every minute, which can be edited in various formats. If no crontab files are found, the cron service enters sleep mode and releases system resources. One minute later, the cron service is awoken to repeat the search work and command execution. Therefore, the background process occupies few resources and is wakened up every minute to check whether there are commands to be executed.

Command execution results are then mailed to users specified by the environment variable MAILTO in the /etc/crontab file. The cron service, once started, does not require manual intervention except when you need to replace periodic commands with new ones.

crontab Command

The crontab command is used to install, edit, remove, list, and perform other operations on crontab files. Each user has its own crontab files and can add commands to be executed to the files.

Here are common crontab command options:

  • crontab -u //Set the cron service of a user. This option is required only when the crontab command is run by the root user.
  • crontab -l //List details of the cron service of a user.
  • crontab -r //Remove the cron service of a user.
  • crontab -e //Edit the cron service of a user.

For example, to list cron service settings of the user root, run the following command:

# crontab -u root -l

crontab Files

Enter the commands to be executed and time in crontab files. Each line in the files contains six fields. The first five fields are the time when the specified command is executed, and the last field is the command to be executed. Fields are separated by spaces or tabs. The format is as follows:

minute hour day-of-month month-of-year day-of-week commands

Each field is described as follows:

Table 2 Parameter description




The minute of the hour at which commands will be executed. Value range: 0–59.


The hour of the day at which periodic commands will be executed. Value range: 0–23.


The day of month at which periodic commands will be executed. Value range: 1–31.


The month of year at which periodic commands will be executed. Value range: 1–12.


The day of week at which periodic commands will be executed. Value range: 0–6.


Periodic commands.

The fields cannot be left unspecified. In addition to numerical values, the following special symbols are allowed: Asterisk (*): a wildcard value. Forward slash (/): followed by a numeral N to indicate that commands will be executed at a regular interval of N. Hyphen (-): used with a range.Comma (,): used to separate discrete numbers. A complete path to the commands shall be provided.

For example, to allow the operating system to add sleepy to the /tmp/test.txt file every two hours from 18 pm to 22 pm, add the following line in a crontab file:

* 18-22/2 * * * echo "sleepy" >> /tmp/test.txt

Each time the cron service settings of a user are edited, the cron service generates in the /var/spool/cron directory a crontab file named after the user. The crontab file can be edited only using the crontab -e command. Alternatively, the user can create a file and run the crontab filename command to import its cron settings into the new file.

For example, to create a crontab file for the userexample user, perform the following steps: The procedure is as follows:

  1. Create a file using any text editor. Add the commands that need to be executed periodically and the command execution interval to the new file. In this example, the new file is ~/userexample.cron.

  2. Run the following command as the root user to install the new file as the crontab file of the userexample user:

    # crontab -u userexample ~/userexample.cron

After the new file is installed, you will find a file named userexample in the /var/spool/cron directory. This file is the required crontab file.

Do not restart the cron service after a crontab file is modified, because the cron service, once started, reads the crontab file every minute to check whether there are commands that need to be executed periodically. You do not need to restart the cron service after modifying the crontab file.

/etc/crontab File

The cron service reads all files in the /var/spool/cron directory and the crontab file in the /etc/crontab directory every minute. Therefore, you can use the cron service by configuring the crontab file. A crontab file contains user-specific commands, whereas the /etc/crontab file contains system-wide commands. Example /etc/crontab file

MAILTO=root //If an error occurs or data is output, the data is sent to the account by email.
#  run-parts
01 * * * * root run-parts /etc/cron.hourly //Run scripts in the /etc/cron.hourly directory once an hour.
02 4 * * *   root run-parts /etc/cron.daily    //Run scripts in the /etc/cron.daily directory once a day.
22 4 * * 0  root run-parts /etc/cron.weekly     //Run scripts in the /etc/cron.weekly directory once a week.
42 4 1  * *  root run-parts /etc/cron.monthly     //Run scripts in the /etc/cron.monthly directory once a month.

If the run-parts parameter is deleted, a script name instead of a directory name is executed.

Suspending/Resuming a Process

A process can be suspended or resumed by job control, and the process will continue to work from the suspended point after being resumed. To suspend a foreground process, press Ctrl+Z. After you press Ctrl+Z, the cat command is suspended together with the foreground process you wish to suspend. You can use the jobs command instead to display a list of shell jobs, including their job names, IDs, and status.

To resume a process in foreground or background, run the fg or bg command, respectively. The process then starts from where it paused previously.


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