Tmux is one of my favorite utilities: it’s a terminal multiplexer that lets you create persistent shell sessions, panes, windows, etc. all within a single terminal. It’s a great way to organize your shell sessions and natively give you multi-shell environments to work in without having to rely on a terminal program for those features.

You’d think in a world of modern applications and fancy terminals like iTerm 2 and Kitty, you wouldn’t need such a utility. But time and time again, tmux has proven itself to be a powerful and essential tool. Especially when working with remote machines in the cloud or across SSH sessions, tmux is critical in maintaining my organization and getting things done.

Beyond multiplexing, tmux has some incredible capabilities that extend its functionality to be able to run and schedule jobs, automatically execute scripts within given contexts, and much more.

Let’s look at a few use cases where we can schedule jobs to run and even create a whole production like environment, all organized and managed from tmux!

Running commands

Tmux offers a way to run scripts in new sessions automatically:

tmux new -s my-session -c /path/to/directory 'echo "Hello Tmux!" && sleep 100'

Let’s break this down: this arbitrary example creates a new session named “my-session”, sets the session directory using the -c flag, and then executes a command.

This command will echo “Hello Tmux!” and then sleep for 100 seconds.

When running this tmux command, we are automatically attached to the session and see “Hello Tmux!” printed at the top of the screen and then the sleep command takes over. Once the sleep command is done, the session exits.

If we wanted to run this in the background, we could provide the -d flag: this will keep the new session detached and run the given commands behind the scenes in the background.

$ tmux new -s my-session -d -c ~/workspace 'echo "hello world!" && sleep 1000'

$ tmux ls
my-session: 1 windows (created Mon Jan 15 11:02:21 2024)

Using tmux ls we can list out the current sessions and see my-session is running with 1 window in the background. This is part of the power of tmux: you can have sessions exist and persist outside of the current shell or session you are attached to. The sky is really the limit here and using multiple sessions, windows, and panes has become a cornerstone of my workflows.

If we wanted to attach to the session and see the progress of the command we gave it, we could run tmux a -t my-session. This will attach to the session named my-session.

Persisting sessions

This is all great, but not all that useful when need to latter observe the results of our command or persist the history: running a script for a new session or window or pane will automatically close once it’s completed.

Instead, we can use a regular session we create and send it some commands remotely:

As an example, let’s say we needed to run some tests in the background on our Typescript project with npm run test and latter observe the results. We can do this with the send-keys command for sessions. Here, I’ll be using the OpenSauced API as my playground:

  1. Create a new named session:
# Create a new named, detached session
# that starts in the given directory
tmux new -s my-npm-tests -d -c ~/workspace/opensauced/api
  1. Send the command
# Send the test command to the session
tmux send-keys -t my-npm-tests "npm run test" Enter

A few things to note here:

Enter uses the special “key binding syntax” for sending a literal Enter key at the end of the command. If we needed to send something else, like “control c”, we could do that with C-c or M-c for “alt c”. Check the official man page where this has a full description of what’s possible with sending key bindings to sessions.

  1. Attach to the session:
tmux a -t my-npm-tests

Now that we’ve sent our test command to the session, at any point in the future we can attach to the session to see how it did and check the results. Since the session will be persisted after the command has run, there’s no rush to observe the results! The shell’s full history for that session will be right there when we need it!

  1. Check results

Within the attached session, we can see the full history of the npm command that was sent and check the results! This session is persisted so we can use the shell from this session to do additional work, detach, close it, etc.

$ npm run test
npm info using npm@9.6.7
npm info using node@v18.17.1

> @open-sauced/api@2.3.0-beta.2 test
> jest

npm info ok


Script it!

What if there are 5 or 6 things I want to do behind the scenes? Maybe I have a build and test process that can run many things in parallel at once? Instead of using send-keys manually, let’s create a small script that can do this all for us!

#!/usr/bin/env bash

# Create named, detached sessions
tmux new -s npm-test -d -c ~/workspace/opensauced/api
tmux new -s npm-build -d -c ~/workspace/opensauced/api

# Send commands to the detached sessions
tmux send-keys -t npm-test "npm run test" Enter
tmux send-keys -t npm-build "npm run build" Enter

Running this script yields the following tmux sessions:

❯ tmux ls
npm-build: 1 windows (created Mon Jan 15 11:31:28 2024)
npm-test: 1 windows (created Mon Jan 15 11:31:28 2024)

and can be attached to in order to inspect the results of each command.

If the commands to run within individual sessions is more complex than just a sole one liner, send-keys can also run a script or make command!

tmux send-keys -t kubernetes "make build" Enter

In this article, I’m assuming you always want to create a new session. But many of the same rules, flags, and syntaxes also apply to creating new windows, panes, etc. Tmux has a strong paradigm that is consistent across different ways to multi-plex shells so it’d be just as simple to create 2 windows instead of two panes that we then send commands to:

#!/usr/bin/env bash

# Create named windows
tmux new-window -n npm-test -d -c ~/workspace/opensauced/api
tmux new-window -n npm-build -d -c ~/workspace/opensauced/api

# Send commands to the detached sessions
tmux send-keys -t 0:npm-test "npm run test" Enter
tmux send-keys -t 0:npm-build "npm run build" Enter

A few things to note here: instead of -s for the session name, we provide -n for the new window name. You’ll also notice the send-keys syntax now includes a :. The first part is the name of the session (in my case, session named 0) and the name of the window to send the keys to.

Setting env variables for sessions

An important and powerful thing to remember here is environment variables: tmux provides the ability to denote global environment variables (env vars available to all new sessions) and session based env vars. In newer versions of tmux, I recommend setting the local session variable with the -e flag:

tmux new -s my-session -d -e MYVAR=myvalue -c /dir

This session named my-session will have access to the MYVAR environment variable we provided when creating the new session:

$ echo $MYVAR

Scheduling jobs with at and scripts

One of the more powerful things I’ve used this all for is local job scheduling. Let’s look at 2 examples using at and scripts:

One off at scheduling

at is a very basic command line utility that comes packaged with many desktop Linux distros and lets you do very simple one off scheduling.

For example, let’s say that you needed to do a git push 3 hours from now in a specific directory:

tmux new -d -s git-push-later \
  -c /path/to/your/repo 'echo "git push" | at now + 3 hours'

This will create a new detached session named git-push-later within the directory for your git repo and it sends git push to the at command via a pipe with the argument “now + 3 hours”.

Looking at scheduled jobs via at:

$ at -l
1       Mon Jan 15 14:46:00 2024

I can see there is a scheduled job! Cool!! This isn’t too much different than just running at manually from the given current directory, but it can be really useful and powerful if I’m working in a different directory or need to quickly load up some env vars. Better yet, you can easily combine this into a script that loads some global tmux environments to then execute many at commands in sequence.

Shell script scheduling

There are alot of ways in Linux to do what I’m suggesting here, primarily through cron and crontab but sometimes for a quick and dirty job that needs to run on repeat every so often in a background shell, it can be quick and dirty to just wrap what I’m doing in a loop with a sleep command:

while true; do
    # The command to continously run
    npm run test

    # Sleep for 5 minutes between runs
    sleep 5m

This can then be thrown in a script and executed via a tmux send-keys command like we’ve seen:

tmux send-keys -t my-npm-tests "./" Enter

Why do it this way and not just have a cron job in the background?

For observable things, like builds, tests, etc., I really like to have a persistent shell session that I can attach to, detach from, and occasionally keep track of.

Usually with this method, these aren’t things that are too important, so if the tmux server dies, it’s nothing I can’t quickly spin back up with a little tmux script. It’s nice having a sort of “location” where these jobs are running in the background but always reachable from a different tmux window or tab. I sometimes find I’ve lost track of things Linux abstracts away with cron, systemd, etc. (which is generally a good thing: I don’t want to have to think about the things systemd is managing!) So, instead, for the little things I need to keep an eye on, I choose to keep track of them in a tmux session!

Building production like environments

Using all of this and with my weird tendency to keep track of things in tmux sessions, let’s build a simple production like environment using a starter script, docker, and a few tmux sessions!

Let’s again look at an OpenSauced example: this starts a postgres database in docker, boots up the API (which will then attach to that database), and then starts the frontend:

#!/usr/bin/env bash

# Create named, detached sessions
tmux new -s database -d -c ~/workspace/opensauced/api
tmux new -s api -d -c ~/workspace/opensauced/api
tmux new -s frontend -d -c ~/workspace/opensauced/app

# Start the database up
tmux send-keys -t database "docker run -it --rm --name database -p 25060:5432 my_postgres_image:latest" Enter

# Start the API
tmux send-keys -t api "npm run start" Enter

# Start the frontend app
tmux send-keys -t frontend "npm run start" Enter

Horrifying, I know.

But surprisingly, I’ve found this to be a really great way to keep the various components of our system organized in a system I know well and can easily wrap my head around.

Then, when I’m done with this environment, I can easily tear it down by stopping the tmux sessions:

tmux kill-session database
tmux kill-session api
tmux kill-session frontend

And that’s it! Easy organization, job scheduling, and multi-tasking with tmux!