Linux ps command
The ps command on linux is one of the most basic commands for viewing the processes running on the system. It provides a snapshot of the current processes along with detailed information like user id, cpu usage, memory usage, command name etc. It does not display data in real time like top or htop commands. But even though being simpler in features and output it is still an essential process management/monitoring tool that every linux newbie should know about and learn well.
In this post we are going to revise the basics of using the ps command to check the processes and filter and sort them in different ways to suit better.
Note on syntax
The ps command comes with an unusual set of 2 syntax styles. That is BSD and UNIX both. New users are often confused with and mis-interpret the two styles. So here is some basic info to get it clear before moving on.
Note : "ps aux" is not the same as "ps -aux". For example "-u" is used to show process of that user. But "u" means show detailed information.
BSD style - The options in bsd style syntax are not preceded with a dash.
UNIX/LINUX style - The options in linux style syntax are preceded by a dash as usual.
It is okay to mix both the syntax styles on linux systems. For example "ps ax -f". But in this post we shall mostly focus on the unix style syntax.
How to use ps command
1. Display all processes
The following command will give a full list of processes
$ ps ax $ ps -ef
Pipe the output to "less" to make it scrollable.
Use the "u" option or "-f" option to display detailed information about the processes
$ ps aux $ ps -ef -f
Why is the USER column not displaying my username, but showing others like root, www-data etc ? For all usernames (including yours) if the length is greater than 8 characters then ps will fall back to show only the UID instead of username.
2. Display process by user
To filter the processes by the owning user use the "-u" option followed by the username. Multiple usernames can be provided separated by a comma.
$ ps -f -u www-data UID PID PPID C STIME TTY TIME CMD www-data 1329 1328 0 09:32 ? 00:00:00 nginx: worker process www-data 1330 1328 0 09:32 ? 00:00:00 nginx: worker process www-data 1332 1328 0 09:32 ? 00:00:00 nginx: worker process www-data 1377 1372 0 09:32 ? 00:00:00 php-fpm: pool a.localhost www-data 1378 1372 0 09:32 ? 00:00:00 php-fpm: pool a.localhost www-data 4524 2359 0 10:03 ? 00:00:00 /usr/sbin/apache2 -k start www-data 4527 2359 0 10:03 ? 00:00:00 /usr/sbin/apache2 -k start www-data 4528 2359 0 10:03 ? 00:00:00 /usr/sbin/apache2 -k start
3. Show process by name or process id
To search the processes by their name or command use the "-C" option followed by the search term.
$ ps -C apache2 PID TTY TIME CMD 2359 ? 00:00:00 apache2 4524 ? 00:00:00 apache2 4525 ? 00:00:00 apache2 ...
To display processes by process id, use the "-p" option and provides the process ids separated by comma.
$ ps -f -p 3150,7298,6544
The "-C" must be provided with the exact process name and it cannot actually search with a partial name or wildcard. To search the process list more flexibly, the usual grep command has to be used
$ ps -ef | grep apache
4. Sort process by cpu or memory usage
System administrators often want to find out processes that are consuming lots of memory or CPU. The sort option will sort the process list based on a particular field or parameter.
Multiple fields can be specified with the "--sort" option separated by a comma. Additionally the fields can be prefixed with a "-" or "+" symbol indicating descending or ascending sort respectively. There are lots of parameters on which the process list can be sorted. Check the man page for the complete list.
$ ps aux --sort=-pcpu,+pmem
Display the top 5 processes consuming most of the cpu.
$ ps aux --sort=-pcpu | head -5 USER PID %CPU %MEM VSZ RSS TTY STAT START TIME COMMAND root 1 2.6 0.7 51396 7644 ? Ss 02:02 0:03 /usr/lib/systemd/systemd --switched-root --system --deserialize 23 root 1249 2.6 3.0 355800 30896 tty1 Rsl+ 02:02 0:02 /usr/bin/X -background none :0 vt01 -nolisten tcp root 508 2.4 1.6 248488 16776 ? Ss 02:02 0:03 /usr/bin/python /usr/sbin/firewalld --nofork silver 1525 2.1 2.3 448568 24392 ? S 02:03 0:01 /usr/bin/python /usr/share/system-config-printer/applet.py
5. Display process hierarchy in a tree style
Many processes are actually forked out of some parent process, and knowing this parent child relationship is often helpful. The '--forest' option will construct an ascii art style tree view of the process hierarchy.
The following command will search for processes by the name apache2 and construct a tree and display detailed information.
$ ps -f --forest -C apache2 UID PID PPID C STIME TTY TIME CMD root 2359 1 0 09:32 ? 00:00:00 /usr/sbin/apache2 -k start www-data 4524 2359 0 10:03 ? 00:00:00 \_ /usr/sbin/apache2 -k start www-data 4525 2359 0 10:03 ? 00:00:00 \_ /usr/sbin/apache2 -k start www-data 4526 2359 0 10:03 ? 00:00:00 \_ /usr/sbin/apache2 -k start www-data 4527 2359 0 10:03 ? 00:00:00 \_ /usr/sbin/apache2 -k start www-data 4528 2359 0 10:03 ? 00:00:00 \_ /usr/sbin/apache2 -k start
Try not to use any sorting with the tree style display, as they both effect the order of display in different ways.
6. Display child processes of a parent process
Here is an example of finding all forked apache processes.
$ ps -o pid,uname,comm -C apache2 PID USER COMMAND 2359 root apache2 4524 www-data apache2 4525 www-data apache2 4526 www-data apache2 4527 www-data apache2 4528 www-data apache2
The first process that is owned by root is the main apache2 process and all other apache2 processes have been forked out of this main process. The next command lists all child apache2 processes using the pid of the main apache2 process
$ ps --ppid 2359 PID TTY TIME CMD 4524 ? 00:00:00 apache2 4525 ? 00:00:00 apache2 4526 ? 00:00:00 apache2 4527 ? 00:00:00 apache2 4528 ? 00:00:00 apache2
7. Display threads of a process
The "-L" option will display the threads along with the processes. It can be used to display all threads of a particular process or all processes.
The following command shall display all the threads owned by the process with id 3150.
$ ps -p 3150 -L
8. Change the columns to display
The ps command can be configured to show a selected list of columns only. There are a large number of columns to to show and the full list is available in the man pages.
The following command shows only the pid, username, cpu, memory and command columns.
$ ps -e -o pid,uname,pcpu,pmem,comm
It is possible to rename the column labels
$ ps -e -o pid,uname=USERNAME,pcpu=CPU_USAGE,pmem,comm PID USERNAME CPU_USAGE %MEM COMMAND 1 root 0.0 0.0 init 2 root 0.0 0.0 kthreadd 3 root 0.0 0.0 ksoftirqd/0 4 root 0.0 0.0 kworker/0:0 5 root 0.0 0.0 kworker/0:0H 7 root 0.0 0.0 migration/0 8 root 0.0 0.0 rcu_bh 9 root 0.0 0.0 rcuob/0 10 root 0.0 0.0 rcuob/1
9. Display elapsed time of processes
The elapsed time indicates, how long the process has been running for. The column for elapsed time is not shown by default, and has to be brought in using the "-o" option
$ ps -e -o pid,comm,etime
10. Turn ps into an realtime process viewer
As usual, the watch command can be used to turn ps into a realtime process reporter. Simple example is like this
$ watch -n 1 'ps -e -o pid,uname,cmd,pmem,pcpu --sort=-pmem,-pcpu | head -15'
The output on my desktop is something like this.
Every 1.0s: ps -e -o pid,uname,cmd,pmem,pcpu --... Sun Dec 1 18:16:08 2013 PID USER CMD %MEM %CPU 3800 1000 /opt/google/chrome/chrome - 4.6 1.4 7492 1000 /opt/google/chrome/chrome - 2.7 1.4 3150 1000 /opt/google/chrome/chrome 2.7 2.5 3824 1000 /opt/google/chrome/chrome - 2.6 0.6 3936 1000 /opt/google/chrome/chrome - 2.4 1.6 2936 1000 /usr/bin/plasma-desktop 2.3 0.2 9666 1000 /opt/google/chrome/chrome - 2.1 0.8 3842 1000 /opt/google/chrome/chrome - 2.1 0.8 4739 1000 /opt/google/chrome/chrome - 1.8 1.0 3930 1000 /opt/google/chrome/chrome - 1.7 1.0 3911 1000 /opt/google/chrome/chrome - 1.6 0.6 3645 1000 /opt/google/chrome/chrome - 1.5 0.4 3677 1000 /opt/google/chrome/chrome - 1.5 0.4 3639 1000 /opt/google/chrome/chrome - 1.4 0.4
The output would be updated every 1 second to refresh the stats. However do not think that this is similar to top.
You would notice that the output of top/htop command changes much more frequently compared to the above ps command.
This is because the top output sorts on a value that is a mix of cpu usage and memory usage. But the above ps command sorts in a more simpler manner, taking 1 column at a time (like school maths). So it would not update rapidly like top.