Simple (but still a bit too complex) way to show an updated transmit packet count for an interface in your shell prompt

This utilizes sar. There are a lot of clever ways to do this, some are lightweight, some are heavy, some are more accurate than others, etc, etc. This is just the quickest way I hacked it together that works well enough for me. You can probably do it much better with a small .c reading from /proc/net. You could certainly use a shell script, perl script, etc. There are a million ways to do this

While this can of course be done with one-liners, you don't want to wait a second each time your prompt rendors to pull a second's worth of sample data, so the best way to do this seems to have a simple service/long-running process in the background, keeping a flat file up to date, and then accessing that in the prompt using cat

A very simple shell script that continuously samples the transmit rate using sar. It does not spin the CPU since sar does polling with sleeps. The content:

$ cat net-tx-mon-daemon
#!/bin/bash
export TEMP_TX=/dev/shm/tx-tmp
export DONE_TX=/dev/shm/tx-now
export RAW_TX=/dev/shm/tx-raw
export INTERFACE=eth0
while [ 1 ]; do
  sar -n DEV 1 1 > "${RAW_TX}"
  grep -E "^Average: +${INTERFACE}" "${RAW_TX}" | awk '{print $4}' > "${TEMP_TX}"
  cp "${TEMP_TX}" "${DONE_TX}"
done

You might be curious ... "why so many files?" or "why not a one-liner?" .. the answer is because you don't want to get caught reading a truncated file, or one that's in the process of being updated. So..

1. Get the sample data by running sar and output it to $RAW_RX
2. Use grep to pull out the line you want and awk to grab the column and output it to $TEMP_TX
3. Finally use cp, which is "atomic enough" to make this all work cleanly, copying the final result into $DONE_TX

This process should run as a low-privileged user, ideally a dedicated user with no ownership of any files. You can run it however you like. If you want to do it the "clean" and "proper" way, you can use a simple systemd service

First, a caveat. There are a significant amount of security features now available in systemd, especially if your kernel supports namespaces. You can essentially create a container for this simple little service. Just remember you'll need to be able to access the one file it produces outside of that container. I'm not covering all the hardening options here because it's a very simple script and when running with minimal privileges it's really not much of a security issue. But if you're nervous, don't use it :>

OK, here is the barebones service file

$ cat /etc/systemd/system/net-tx-mon.service
[Unit]
Description=Network Transmission Packet Counter Service
After=network.target

[Service]
User=nobody     # It is safer to use a dedicated user, e.g. nettx-nobody
Group=nogroup   # It is safer to use a dedicated group, e.g. nettx-nogroup
ExecStart=/usr/bin/net-tx-mon-daemon
ExecStop=/bin/kill -s QUIT $MAINPID

[Install]
WantedBy=multi-user.target

You can now start the daemon and enable it for run at boot-time

$ sudo systemctl daemon-reload
$ sudo systemctl enable net-tx-mon
$ sudo systemctl start net-tx-mon

As long as this glorified script "service" is running, you should be able to check the average transmitted packets per second using:

$ cat /dev/shm/tx-now 
620673.00

This is most useful if you have an interactive shell and always want to know this rate. You can simple embed the following in your prompt:

$(cat /dev/shm/tx-now)

It depends on what shell you are using and if you're using a framework for your shell rc files when it comes to doing this properly. Here is a simple example of how to use this with an oh-my-zsh theme file

Simply comment out the current prompt, and insert how/where you please the cat statement. For example:

$ cat ~/.oh-my-zsh/themes/aussiegeek.zsh-theme | head -2
# PROMPT='$fg_bold[blue][ $fg[red]%t $fg_bold[blue]] $fg_bold[blue] [ $fg[red]%n@%m:%~$(git_prompt_info)$fg[yellow]$(rvm_prompt_info)$fg_bold[blue] ]$reset_color
PROMPT='$fg_bold[green]TX PPS:($(cat /dev/shm/tx-now)) $fg_bold[blue][ $fg[red]%t $fg_bold[blue]] $fg_bold[blue] [ $fg[red]%n@%m:%~$(git_prompt_info)$fg[yellow]$(rvm_prompt_info)$fg_bold[yellow] $fg_bold[blue] ]$reset_color

You can now login again and you will see the packet rate reflected on the left side of your prompt in green, like this (except with color):

TX PPS:(623136.00) [ 10:09PM ]  [ user@host:~  ]
 $ 

I don't know. I needed to do it because I do heavy network I/O and I want to know if things are crashed or melting.

Sure. Change the value in the awk column selection

Yeah. Same thing.