Processes are running instances of programs in Linux.
- Linux Processes
- The ps Command
- Killing Processes
- Prioritizing Processes
- Background Processes
- &: include an ampersand at the end of the command you use to run the job
- nohup: stands for no hang up and prevents termination of background processes after shell termination
- screen: runs a background process on a remote server, and keeps it running despite a dropped connection
- jobs: once a process is forked, it can be seen in the jobs list
- bg: resumes suspended jobs in the current environment by running them as background jobs
- fg: runs them in foreground and occupies the current terminal and waits for process to exit
- disown: removes the process from the shell's job control, but leaves it connected to the terminal
- sleep: tells Bash what time to run a command and delays execution to allow a process to start
- wait: waits until the last background process is completed
ps stands for process status and shows the running processes on a system.
To invoke it, run
ps and it will display the following information:
- PID: process ID which identifies the running process
- TTY: is the terminal type
- TIME: Total CPU usage
CMD: the command or program that is running, including options
List all current running processes in the machine
-e will display all the processes, and the
-f option will display in a full format listing.
List the processes of a user
ps -f -u <user>
It displays the process that belongs to user. When you have multiple usernames, separate them using a comma.
Check the execution time of a process
ps -eo comm,etime,user | grep httpd
It shows the command, time, and user(s) related to the "httpd" service. You can replace "httpd" with the service you are looking for.
Find the top five running processes by memory usage
ps -eo pmem,pid,cmd | sort -k 1 -nr | head -5
It displays the top five of the output, organized in three columns with the memory that a process is taking, process ID, and the command, sorted by memory usage.
Display the processes in the form of a tree diagram
pstree -np | less
-p shows process identification numbers (PIDs) and
-n sorts its output in the order of the PIDs.
If you want to see the process tree of any specific user, run
pstree <user>. Use your username instead of
Determine how much memory process uses
It displays the memory usage map of a process 1232. If you need information for multiple processes, you can add the their PID separated by a space.
All processes in Linux respond to signals. Signals are an OS-level way of telling programs to terminate or modify their behavior.
kill: sends the TERM signal to the process to ask the process to terminate and exit smoothly
This terminates a process with a PID of 1734.
If this fails, the stronger signal 9, called SIGKILL can help by doing
kill -9 1734. To see all the options, run
In case you cannot determine the number of the process, you can use the name of the program to make it stop:
kill -9 firefox
killall: if there are multiple instances of a particular command running, the command will terminate them all
In this case
killall is closing a current program(s) that is running a process called
Linux schedules the process and allocates CPU time accordingly for each of them, but you can set the priority to get more CPU time by using the
The process scheduling priority has a nice value that ranges from -20 to 19. The highest priority will consume a lot of CPU and that is not nice, so we set it as -20. On the other hand, the least priority for a process is represented as nicer because it will not take much CPU resources, and a nice value of 19 then is set.
Only the root user can set a negative value. A nice value of a process can be seen in the column
NI after you type
top in your terminal.
top: monitors processes and system resource usage on Linux
It displays the main 30 processes on the system sorted by CPU utilization, memory usage, and routine. See more information here.
If you want to sort processes by CPU usage, you can do so with
top -o %CPU.
To see a list of processes of any user, use
top -u <user>. Remember to replace
<user> with your username or
nice: sets priority on new processes
nice -n 10 apt-get upgrade
It sets a positive 10 as a nice value that gives less priority to a process.
renice: sets a priority on existing processes
renice 10 -p 2187
It sets a priority of 10 to a process with an ID 2187. If its value was 0, you are lowering the priority.
📝 Note: You can set the default nice value of a particular user or group in the
/etc/security/limits.conf file, by using the syntax:
[username] [hard|soft] priority [nice value], e.g.
backupuser hard priority 1.
A background process executes independently of the shell, without user intervention, leaving the terminal free for other work.
This means that you do not have to wait for a command to finish in the terminal to run another one. For further information, click here.
After using commands to run process in the background, you will immediately be returned to the shell, and you will see the shell prompt.
&: include an ampersand at the end of the command you use to run the job
./myscript.py is forked and runs in a separate sub-shell as a job. A process's job number and its PID will be displayed and stored in a special variable
$!. This can be seen later with
nohup: stands for no hang up and prevents termination of background processes after shell termination
nohup ./myscript.py &
The output generated by
./myscript.py will be saved in
nohup.out in the current directory. If you logout, your process will not get killed.
To run more scripts at the same and leave them to be finished in background, run
./script.py & ./script2.py & ./script3.py &.
screen: runs a background process on a remote server, and keeps it running despite a dropped connection
This creates a new session when you log into another server. A screen ID is displayed after running your command(s).
To create a screen session with a name, run
screen -S name. See more screen options on
man screen or here.
To detach from the screen session with
D or if you are remotely logged in, you can do it with
screen -d [SCREENID].
jobs: once a process is forked, it can be seen in the jobs list
jobs + Running ./myscript.py &
It displays the list of the current jobs that are running in the background; there is the script
./myscript.py with the job number:1.
bg: resumes suspended jobs in the current environment by running them as background jobs
dd if=/dev/zero of=myfile bs=1K count=2048000 ^Z + Stopped dd if=/dev/zero of=file bs=1K count=2048000 bg %1
The number 1 is the ID of the job as viewed under a job suspended; then, to use it with
bg it must be preceded with a
fg: runs them in foreground and occupies the current terminal and waits for process to exit
Without any argument,
fg runs the current job in foreground.
To see the ID of the jobs that are running in the background to bring them to the foreground, type
jobs. Then type the ID preceded by a
disown: removes the process from the shell's job control, but leaves it connected to the terminal
./run_script.sh CTRL-Z + Stopped run_script.sh bg + run_script.sh & disown %1
./run_script.sh file is executed, then this job is suspended by pressing
Z, followed by
bg to make it run in the background. Then, by typing
disown %1, the job won't get the SIGHUP signal to be shut down.
sleep: tells Bash what time to run a command and delays execution to allow a process to start
sleep 3h; mplayer game.mp3
This will wait three hours to play game.mp3.
You might consider using
m to set minutes, e.g.
sleep 10m ; your_script, or
d, for days. If you do not specify anything, the sleepy action will happen in seconds.
wait: waits until the last background process is completed
collect-job1.sh & collect-job2.sh & collect-job3.sh & wait process-job-output.sh wait
The three scripts of "collection" that are running in the background will finish before the
wait ensures this process and asks to not exit the containing script until all the execution has finished.