Authors Top

If you have a few years of experience in the Linux ecosystem, and you’re interested in sharing that experience with the community, have a look at our Contribution Guidelines.

1. Overview

Linux’s top command-line utility displays information about currently active processes and system resources. Seeing what a Linux system is doing in real time makes it easier to find processes that use a lot of resources or to simply monitor loads. For example, we can use top to monitor the uptime, memory usage, and CPU load of our system or a specific process.

In this tutorial, we’ll see how to use the top command in Linux to filter processes based on their names. First, we can make use of the top built-in facility for our needs. Second, we can write a custom script. It will automatically refresh itself and update the status of the target process.

2. Filtering Processes Using the top Command

To filter processes by name when the top command is running, we can use the O key followed by entering a filter expression.

For example, we’ll single out the apache2 process via a filter expression in the top command prompt.

Let’s first open a terminal window and enter the top command. This displays a list of processes currently running on our system:

$ top

PID  USER  PR NI VIRT  RES  SHR  S  %CPU   %MEM     TIME+   COMMAND
16   root  20 0   0     0    0   R   81.7  0.0    15:53.89  kworker+
1352 root  20 0  7224  1316 704  S   1.0   0.5     0:09.76  Xorg
1373 root -51 0   0     0    0   S   0.7   0.0     0:03.50  irq/132+

Now, let’s press the O key, which brings up a prompt asking us to enter the field and value to filter.

In our case, the field is COMMAND, while the value should be apache2. Hence, we join these strings with an equals sign, so our filter to single out apache2 becomes COMMAND=apache2:

...
add filter #1 (ignoring case) as: [!]FLD?VAL COMMAND=apache2

Finally, the top command should now only display the statistics of the apache2 process:

top - 03:46:28 up 9 min, 1 user, load average: 0.00, 0.08, 0.08
Tasks: 110 total, 1 running, 109 sleeping, 0 stopped, 0 zombie
%Cpu(s): 0.0 us, 0.0 sy, 0.0 ni,100.0 id, 0.0 wa, 0.0 hi, 0.0 si, 0.0 st
MiB Mem : 976.6 total, 189.8 free, 146.9 used, 639.9 buff/cache
MiB Swap: 1953.0 total, 1953.0 free, 0.0 used. 678.2 avail Mem

 PID  USER    PR NI VIRT   RES  SHR  S  %CPU %MEM  TIME+   COMMAND

1825 root     20 0 6524    4412 3220 S  0.0  0.4  0:00.00  apache2
1691 www-data 20 0 1211412 4548 2988 S  0.0  0.5  0:00.00  apache2
1692 www-data 20 0 1211412 4552 2992 S  0.0  0.5  0:00.00  apache2

Notably, there can be several processes with the same name, but different PIDs.

3. Filtering Processes Using a Script

To filter a process, we can also use the pgrep command along with the top command. The -f flag of pgrep gets the matching PIDs of the target processes from their command line:

$ top -c -p $(pgrep -d',' -f matching_string_in_cmdline_output)

However, when we restart a process, its PID changes. Consequently, the top command needs to restart to retrieve the full list of target process PIDs.

To resolve this problem, we can use a script to filter the results of the top command by process name. For example, let’s consider the script (my_script.sh) below:

#!/bin/bash
__process_name=$1
while (true)
do
  clear
  __arg=$(pgrep -d',' -f $__process_name)
  if [ -z "$__arg" ]; then
    top | head -n 5
  else
    top -c -n 1 -p $__arg
  fi
  sleep 5
done

Let’s break down this script and see the meaning of the various commands used here:

  • __process_name: name of the variable which sets the name of the process from the first command line argument ($1)
  • while loop: runs the process list iteration indefinitely until the script is interrupted
  • clear: clears the terminal screen for each iteration, similar to watch
  • pgrep: the -d option specifies a delimiter to use when printing the process IDs, while -f is used to match the full command line of a process instead of just the process name
  • __arg: stores the output (the list of processes) from the pgrep command
  • if block: the -z option checks if a given string is empty, so we know whether any PIDs are returned and only use them as a filter when they are returned

Basically, the script keeps the output updated with data about the process with the given name. Here, the refresh interval is set to 5 seconds using the sleep command.

Let’s take the example of the apache2 process and run our script as:

$ ./my_script.sh apache2

Consequently, the top command continuously displays all the apache2 processes:

Tasks:   3 total,   0 running,   3 sleeping,   0 stopped,   0 zombie
%Cpu(s):  0.0 us,  0.0 sy,  0.0 ni,100.0 id,  0.0 wa,  0.0 hi,  0.0 si,  0.0 st
MiB Mem :    976.6 total,    169.3 free,    155.1 used,    652.3 buff/cache
MiB Swap:   1953.0 total,   1953.0 free,      0.0 used.    669.1 avail Mem 

 PID USER      PR  NI    VIRT    RES    SHR S  %CPU  %MEM     TIME+ COMMAND         
3364 root      20   0    6524   4484   3296 S   0.0   0.4   0:00.01 /usr/sbin/apac+ 
3365 www-data  20   0 1211412   4544   2984 S   0.0   0.5   0:00.00 /usr/sbin/apac+ 
3366 www-data  20   0 1211412   4544   2984 S   0.0   0.5   0:00.00 /usr/sbin/apac+ 

Also, when a process restarts, we don’t have to press the Q key or CTRL+C and run top again and again. The above script takes care of this.

4. Conclusion

In this article, we’ve seen how to filter by process name the output of the top command.

First, we used the top command prompt to interactively enter the filter expression for filtering processes based on their names. Second, we created a custom script that displays a specific process in top’s output and refreshes the output at intervals.

Authors Bottom

If you have a few years of experience in the Linux ecosystem, and you’re interested in sharing that experience with the community, have a look at our Contribution Guidelines.

guest
1 Comment
Oldest
Newest
Inline Feedbacks
View all comments