Pstree Command in Linux

In this article we will discuss about pstree command. On Linux machine, sometimes you need to check which services processes are currently running. You can find out information about the running processes by multiple ways using different commands.

The pstree command is similar to ps, but instead of listing the running processes, it shows them in a tree. It is a more convenient way to show the processes hierarchy.

Use the pstree Command

Below is the basic syntax for the pstree command:

ps [OPTIONS] [USER or PID]

If you will run pstree command without any options it will show the hierarchical tree structure of all running processes:

pstree

systemd─┬─accounts-daemon───2*[{accounts-daemon}]
        ├─2*[agetty]
        ├─atd
        ├─cron
        ├─dbus-daemon
        ├─do-agent───4*[{do-agent}]
        ├─fail2ban-server───2*[{fail2ban-server}]
        ├─lvmetad
        ├─lxcfs───10*[{lxcfs}]
        ├─master─┬─pickup
        │        └─qmgr
...

In the output the systemd is the root of the tree and it’s parent process of all processes. The pstree merges identical branches by putting them between square brackets and prefixing them with an integer that represents the number of branches. This makes the output more readable and visually appealing.

Following is the example showing how the square brackets are used:

├─2*[agetty]

Above is similar to the below:

├─agetty
├─agetty

You can disable the merging the identical branches by using -c option:

pstree -c

The parent process display the threads of a process using the process name inside curly braces. Below is an example:

├─lxcfs───10*[{lxcfs}]

You can use the -t option to show the full threads names and to hide threads and show only processes use the -T option.

Generally, the output of the pstree is in multiple lines so output doesn’t fit on the screen. To view the output page wise use the less command as following:

pstree | less

To view the processes of a particular user, you can pass user name as an argument with the pstree command. For example, if you would like to show process of a user tecnstuff, you would run command as following:

pstree tecnstuff

When PID is specified as an argument, pstree displays a tree with the given process as the root of the tree. Here is an example:

pstree 418

sshd───bash───pstree

By adding -s option followed by the PID with pstree command, it will show the parent process of the given process:

pstree -s 418

systemd───sshd───sshd───bash───pstree

Show PIDs and PGIDs

Sometimes, Linux user required to know the process id of a process to kill the malfunctioning the process.

Use the -p option with pstree to show the PIDs:

pstree -p

PIDs are shown in parentheses after each process or thread.

systemd(1)─┬─accounts-daemon(836)─┬─{accounts-daemon}(850)
           │                      └─{accounts-daemon}(857)
           ├─agetty(873)
           ├─agetty(876)
           ├─atd(838)
           ├─cron(831)
           ├─dbus-daemon(841)
           ├─do-agent(828)─┬─{do-agent}(852)
           │               ├─{do-agent}(853)
           │               ├─{do-agent}(854)
           │               └─{do-agent}(31983)
           ├─fail2ban-server(866)─┬─{fail2ban-server}(1039)
           │                      └─{fail2ban-server}(1040)

The pstree command sorts the processes by the parent name by default. To sort by PIDs (numeric) use -n option along with pstree command:

pstree -pn

The process group ID or PGIDs is the process ID of the first member of the process group. To view PGIDs use the -g option:

pstree -g

PIDs are also shown in parentheses after each process or thread.

├─lxcfs(833)─┬─{lxcfs}(833)                                                                                             │            ├─{lxcfs}(833)
           │            ├─{lxcfs}(833)
           │            ├─{lxcfs}(833)
           │            ├─{lxcfs}(833)
           │            ├─{lxcfs}(833)
           │            ├─{lxcfs}(833)
           │            ├─{lxcfs}(833)
           │            ├─{lxcfs}(833)
           │            └─{lxcfs}(833)

When merging is disabled the PIDs or PGIDs are displayed.

Highlighting

For better visual representation the pstree command allows you to highlight processes.

Use the -h option with pstree to highlight the current process and all its ancestors.

pstree -h

To highlight a specific process, use the -H option followed by the process ID:

pstree -H PID_NUMBER

If the command throw an error and exit that means the highlighting is not supported.

Conclusion

You learned how to view running process in tree view using the pstree command. To know more about all available pstree options, type man pstree in your terminal.

If you have any questions or suggestions, please leave a comment below.

Tags: ps, terminal