As an Operating System which inspired from Unix, Linux has a built-in tool to capture current processes on the system. This tool is available in command line interface.
What is PS Command
From its manual page, PS gives a snapshots of the current process. It will “capture” the system condition at a single time. If you want to have a repetitive updates in a real time, we can use top command.
PS support three (3) type of usage syntax style.
1. UNIX style, which may be grouped and must be preceded by a dash
2. BSD style, which may be grouped and must not be used with a dash
3. GNU long options, which are preceded by two dash
We can mix those style, but conflicts can appear. In this article, will use UNIX style. Here’s are some examples of PS command in a daily use.
1. Run ps without any options
This is a very basic ps usage. Just type ps on your console to see its result.
By default, it will show us 4 columns of information.
- PID is a Process ID of the running command (CMD)
- TTY is a place where the running command runs
- TIME tell about how much time is used by CPU while running the command
- CMD is a command that run as current process
This information is displayed in unsorted result.
2. Show all current processes
To do this, we can use -a options. As we can guess, -a is stand for “all”. While x will show all process even the current process is not associated with any TTY (terminal)
$ ps -ax
This result might be long result. To make it more easier to read, combine it with less command.
$ ps -ax | less
3. Filter processes by its user
For some situation we may want to filter processes by user. To do this, we can use -u option. Let say we want to see what processes which run by user pungki. So the command will be like below
$ ps -u pungki
4. Filter processes by CPU or memory usage
Another thing that you might want to see is filter the result by CPU or memory usage. With this, you can grab information about which processes that consume your resource. To do this, we can use aux options. Here’s an example of it :
$ ps -aux | less
Since the result can be in a long list, we can pipe less command into ps command.
By default, the result will be in unsorted form. If we want to sort by particular column, we can add --sort option into ps command.
Sort by the highest CPU utilization in ascending order
$ ps -aux --sort -pcpu | less
Sort by the highest Memory utilization in ascending order
$ ps -aux --sort -pmem | less
Or we can combine itu a single command and display only the top ten of the result :
$ ps -aux --sort -pcpu,+pmem | head -n 10
5. Filter processes by its name or process ID
To to this, we can use -C option followed by the keyword. Let say, we want to show processes named getty. We can type :
$ ps -C getty
If we want to show more detail about the result, we can add -f option to show it on full format listing. The above command will looks like below :
$ ps -f -C getty
6. Filter processes by thread of process
If we need to know the thread of a particular process, we can use -L option followed by its Process ID (PID). Here’s an example of -L option in action :
$ ps -L 1213
As we can see, the PID remain the same value, but the LWP which shows numbers of thread show different values.
7. Show processes in hierarchy
Sometime we want to see the processes in hierarchical form. To do this, we can use -axjf options.
$ps -axjf
Or, another command which we can use is pstree.
$ pstree
8. Show security information
If we want to see who is currently logged on into your server, we can see it using the ps command. There are some options that we can use to fulfill our needs. Here’s some examples :
$ ps -eo pid,user,args
Option -e will show you all processes while -o option will control the output. Pid, User and Args will show you the Process ID, the User who run the application and the running application.
The keyword / user-defined format that can be used with -e option are args, cmd, comm, command, fname, ucmd, ucomm, lstart, bsdstart and start.
9. Show every process running as root (real & effecitve ID) in user format
System admin may want to see what processes are being run by root and other information related to it. Using ps command, we can do by this simple command :
$ ps -U root -u root u
The -U parameter will select by real user ID (RUID). It selects the processes whose real user name or ID is in the userlist list. The real User ID identifies the user who created the process.
While the -u paramater will select by effective user ID (EUID)
The last u paramater, will display the output in user-oriented format which contains User, PID, %CPU, %MEM, VSZ, RSS, TTY, STAT, START, TIME and COMMAND columns.
Here’s the output of the above command.
10. Use PS in a realtime process viewer
ps will display a report of what happens in your system. The result will be a static report.
Let say, we want to filter processes by CPU and Memory usage as on the point 4 above. And we want the report is updated every 1 second. We can do it by combining ps command with watch command on Linux.
Here’s the command :
$ watch -n 1 ‘ps -aux --sort -pmem, -pcpu’
If you feel the report is too long, we can limit it by - let say - the top 20 processes. We can add head command to do it.
$ watch -n 1 ‘ps -aux --sort -pmem, -pcpu | head 20’
This live reporter is not like top or htop of course. But the advantage of using ps to make live report is that you can custom the field. You can choose which field you want to see.
For example, if you need only the pungki user shown, then you can change the command to become like this :
$ watch -n 1 ‘ps -aux -U pungki u --sort -pmem, -pcpu | head 20’
Conclusion
You may use ps on your daily usage to monitor about what happens your Linux system. But actually, you can generate various types of report using ps command with the use of appropriate paramaters.
Another ps advantage is that ps are installed by default in any kind of Linux. So you can just start to use it.
Don't forget to see ps documentation by typing man ps on you Linux console to explore more options.
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