tmux

Mark Volkmann, Partner at Object Computing, Inc. (OCI), mark@ociweb.com

Introduction

This article provides an overview of tmux. tmux is short for "terminal multiplexer." It dramatically expands what can be done with terminal windows. It is highly configurable and scriptable.

tmux is an alternative to an older, similar program named "screen". It was created by Nicholas Marriott. The first version was released in 2007.

One benefit of both tmux and screen is realized when working on remote machines where network communication can fail. Both maintain the state of sessions so the user can pick up where they left off by reconnecting and reattaching to the session.

I first learned about tmux from the excellent book "tmux: Productive Mouse-Free Development" by Brian Hogan at http://pragprog.com/book/bhtmux/tmux. If this article piques your interest in tmux, I highly recommend getting this book.

Another excellent source of information about tmux is its long man page. Studying the tmux man page will enable you to maximize the benefits derived from its use. If you decide to print it, generate a nicely formatted PostScript version with man -t tmux > tmux.ps.

This article contains many tables. It is intended to be printed and used as a cheat sheet. The tables have been carefully organized and styled so none of them split across a page break.

For You?

Like any software tool, tmux doesn't appeal to everyone. There are three key indicators that tmux is a fit for you.

1. You work in a Unix-like environment like Linux or Mac OS X. tmux is not supported under Windows, but can run under Cygwin when installed like other Cygwin tools.
2. You frequently work in terminal windows performing command-line activities.
3. You value using keyboard shortcuts as alternatives to using a mouse. An indicator of this is that you prefer editors like Vim or emacs, or that you have memorized many keyboard shortcuts in an IDE.

Benefits over plain terminal windows

1. Keyboard shortcuts can be used for everything. Once learned, this is more efficient than using a mouse.
2. The keyboard shortcuts for every operation can be customized.
3. Copy mode supports copying and pasting between windows without using a mouse.
4. Windows can be split into panes, horizontally and vertically. These panes can be rearranged and resized.
5. Tmux can be exited (detach) and restarted again later (attach) without losing the state of the windows.
6. Pair programming is supported. Multiple people can share a set of windows and concurrently modify them.

Terminology

Typically it is desirable to view certain panes simultaneously. This is accomplished by putting those panes in the same window. The pane border color indicates which pane is active.

Consider creating a separate session for each group of related activities. For example, when working on multiple projects, create a session for each. Each project can have its own script that attaches to an existing session for that project if one already exists, or creates a new session. The script can create a set of windows, each with a set of panes, that are configured in a way that is convenient for that project. The script can also run commands (ex. ssh) in specific panes.

On the other hand, using a single session containing sets of unrelated windows has the advantage that all windows are always visible in the status bar of the one session. It is somewhat easier to switch between these windows, that to switch between both sessions and windows.

Sessions and windows can have names, but panes typically only have indexes. A pane can be given a title by running this command from the pane: printf '\e]2;title\e\\'. Pane titles are only displayed in customized status bars using #T. Only the title of the active pane is displayed.

Installing

To install tmux on a Mac:

1. Open a Terminal window.
2. Install Homebrew by running
   ruby -e "$(curl -fsSkL raw.github.com/mxcl/homebrew/go)"
3. brew install tmux

Additional Mac-specific advice appears near the end of this article.

The steps to install tmux in a Linux environment vary based on the package manager used.
Here are the steps when using apt-get:

1. Open a terminal window.
2. sudo apt-get install libevent-dev
3. sudo apt-get install libncurses5-dev
4. Download the latest version of tmux from https://tmux.github.io/
5. tar -zxvf tmux-{version}.tar.gz
6. cd tmux-{version}
7. ./configure
8. make
9. sudo make install

To verify the version of tmux that is installed, enter tmux -V. Note that the flag is a capital V.

Commands

There are many tmux commands. The most useful subset and their options (in my opinion) are covered in this article. To see a list of all of them, enter tmux list-commands or tmux lscm.

Many commands can be executed in several ways that include:

• command-line / shell script
• command mode in a tmux client
• keyboard shortcut (default or custom) in a tmux client
• tmux configuration file

Every keyboard shortcut is bound to a tmux command. Some tmux commands can only be used from the command-line or a shell script. Some tmux commands can only be used from inside a tmux client. Some tmux commands can be used from anywhere.

Most keyboard shortcuts are case-sensitive and begin with what tmux refers to as a "prefix key". By default this is ctrl-b, also written as C-b. The control key is abbreviated as C. The tmux man page refers to control key sequences as C-{key} and this article follows that convention. The meta key is abbreviated as M. On a Mac, this is cmd-option by default. See the "Mac-specific Advice" section later for steps that must be taken to enable this. Many users bind the prefix key to C-a for convenience and to match the prefix key in the "screen" program. This is done in the sample tmux configuration file shown later.

Some commands briefly replace the status bar with a message. The duration of the display is controlled by the display-time option which defaults to 750ms. Previously displayed messages can be viewed by running the show-messages command whose keyboard shortcut is prefix ~.

From a command-line or shell script, tmux commands can be executed by entering "tmux {command}". From inside a tmux client, tmux commands can be executed by pressing prefix : to enter command mode and then entering a command in the status bar. Commands can be specified by name (full or shortest unique prefix) or alias for commands that have one. Tab completion can be used to enter command names. The up and down arrows can be used to recall previously entered commands.

A common command flag is -t which specifies a target client, session, window or pane.

Sample Session

After installing tmux, following along with this sample session to get a feel for what tmux can do. Where "prefix" appears, press C-a.

1. Create the file ~/.tmux.conf with the following content that will be explained later:
   set -g prefix C-a # changes prefix key sequence from C-b to C-a
bind | split-window -h
bind - split-window -v


2. Create a session named "foo": tmux new -s foo
3. Split the window horizontally: prefix |
4. Split the right pane vertically: prefix -
5. Run the "top" command in the lower-right pane just created: top
6. Switch to the left pane: prefix left-arrow
7. cd to the /tmp directory and list the files there: cd /tmp; ls
8. Create a second window in this session: prefix c
9. Name the window "editor": prefix , (press the comma key)
   and change the default name (ex. bash) to "editor"
10. Start a text editor in the only pane in this window: vim
11. Notice that there are two window names in the status bar at the bottom.
12. Switch back to the first window: prefix l
13. Detach from this session: prefix d


14. Create a session named "bar": tmux new -s bar
15. Split the window vertically: prefix -
16. Start a text editor in the bottom pane: vim
17. Split the bottom pane horizontally two times: prefix | prefix |
18. Split the current pane vertically: prefix -
19. Display a clock in the current pane: prefix : clock-mode or prefix t
20. Change the layout of the panes in the current window several times: prefix space
21. Note how the pane borders indicate which pane is active.
22. Change the active pane several times: prefix {arrow-key}
23. Make the clock pane the active pane: prefix {arrow-key}
24. Swap the clock pane with the previous pane several times: prefix {
25. Rotate all the panes in this window up: prefix C-o
26. Make the vim pane the active one: prefix {arrow-key}
27. Move the vim pane to a new window within this session: prefix !
28. Detach from this session: prefix d


29. List the existing sessions: tmux ls
30. Attach to the foo session: tmux attach -t foo
31. Switch to the bar session: prefix s, use arrow keys to move to "bar" and press return
32. Detach from this session: prefix d


33. Kill the bar session: tmux kill-session -t bar
34. List the existing sessions: tmux ls
35. Note that only the foo session remains.
36. Kill the foo session: tmux kill-session -t foo

Predefined Pane Layouts

Layout Name	Example
even-horizontal
even-vertical
main-horizontal
main-vertical
tiled

Buffers

Each pane has a history buffer that stores the shell commands entered and their output. The size of this buffer is specified by the history-limit session option and defaults to 2000 lines.

To scroll through the content of a pane buffer and optionally copy some of its content, enter copy mode using the buffer command shown below. While in copy mode, the upper-right corner of the pane will display "[m/n]" where n is the number lines in the history and m is the current line number within that starting from zero and numbering back in time.

Each copy creates a new "paste buffer" on a stack. There is one paste buffer stack per session. The stack has a limited size that is specified by the buffer-limit server option and defaults to 20. When the limit is reached and a new paste buffer is created, the oldest one is discarded.

Configuration

tmux is highly configurable. There are three kinds of options: server, session, and window. Options can be global (the common case) or apply to a specific session or window.

There are WAY too many options to describe in detail in this article. The table below is provided simply to give a feel for the configurability of tmux. See the tmux man page for details.

tmux configuration comes from two files. If it exists, tmux reads /etc/tmux.conf which is shared by all users. Then it reads .tmux.conf in the user's home directory (~/.tmux.conf) to supplement and override settings in the first file.

After upgrading to a new version of tmux, kill the tmux server and restart it. To do this, exit tmux, enter tmux kill-server, and enter tmux again.

Here's an example configuration file that is well-commented to describe each piece. This barely scratches the surface of possible status bar customizations. All characters from a # to the end of a line are treated as a comment and ignored.

# Session options
# reduce time tmux waits to send commands after an escape is input
set -sg escape-time 1

# Window options
# allow multiple clients to display windows from the same session in different sizes
setw -g aggressive-resize-on
setw -g mode-keys vi # enable vi keys for working with buffers
# watch for activity in windows so it can be indicated in the status bar
# (see visual-activity option below)
setw -g monitor-activity on
setw -g renumber-windows on # when one is deleted
setw -g window-status-current-bg magenta # in status bar

# Prefix key
set -g prefix C-a # instead of default C-b
unbind C-b # don't want two prefix keys
bind C-a send-prefix # press C-a twice to send to an app

# Key bindings
bind r source-file ~/.tmux.conf \; display "reloaded" # reload this file
# The line above replaces the default binding to the "refresh-client" command.
bind | split-window -h # in addition to %
bind - split-window -v # in addition to "

# Mouse options
set -g mouse on # allow mouse drags to enter copy mode and select text

# Color customizations
set -g default-terminal "screen-256color" # use 256 colors instead of 16
# Colors can be specified by name or hex #rrggbb.
set -g pane-border-fg black # default is "default" which is gray
set -g pane-active-border-fg red # default is green

# Status bar
set -g status-bg blue # default is green
set -g status-fg white # default is black
set -g status-interval 60 # update status bar every 60 seconds
set -g status-justify centre # display window list in center of status bar
set -g status-left "session #S window #I pane #P -"
set -g status-left-length 40
set -g status-right "#h %a %m/%d/%y" # hostname, day abbrev., month, day, year
set -g status-right-length 45
set -g status-utf8 on # allow status bar to contain Unicode characters
set -g visual-activity on # highlight window names in status bar on activity
    

Changes made to a configuration file are not automatically picked up by existing tmux clients. To load configuration changes into a running client, use the command source-file {path} (alias source). This is needed often when testing configuration changes, so it is convenient to bind this to a key as shown in the configuration file above. With that binding in place, changes can be loaded by pressing prefix r.

Scripting

Here's an example bash shell script for starting a tmux session named mytmux. A key feature of this type of script is only creating a new session if one with a given name doesn't already exist. If it does exist, the script just attaches to it.

!/bin/bash
# If this file is modified, kill the existing session
# with "tmux kill-session -t mytmux" and run this again.

# If the "mytmux" session does not exist ...
if ! tmux has-session -t mytmux; then
  # Create a new session named "mytmux" and detach from it.
  # The name of the first window will be the command running in it (vim).
  tmux new-session -s mytmux -d

  # In first window, cd to Node labs directory and
  # start Vim with vertical split and in explore mode.
  tmux send-keys -t mytmux:0.0 'cd ~/Node.js/nodejs-labs; vim +vs +E' C-m

  # Create a second window named "console" for entering shell commands.
  tmux new-window -n console -t mytmux

  tmux split-window -h -t mytmux:1 # split into two horizontal panes
  tmux split-window -v -t mytmux:1.1 # split right pane into two vertical panes

  # In upper-right pane, cd to Node labs directory and start "less-watch".
  tmux send-keys -t mytmux:1.1 'cd ~/Node.js/labs; less-watch .' C-m

  # In lower-right pane, run "top".
  tmux send-keys -t mytmux:1.2 'top' C-m

  tmux select-window -t mytmux:0 # move focus to first window
  #tmux select-pane -t 0 # move focus to vim pane; only one, so not needed
fi

# Attach to the "mytmux" session.
tmux attach -t mytmux
    

Multiple Clients, One Session

Sometimes it is useful to attach multiple tmux clients to the same session. This can be done on a single machine using multiple terminal windows. It enables simultaneous viewing of multiple tmux windows that belong to the same session. This is especially useful on computers that have more than one monitor or on a laptop that also has an external monitor. After creating a session or attaching to one in the normal way from the first terminal window, create new sessions from other terminal windows using tmux new -t {session-name}. The new session is "grouped" with the target session which enables the sessions to share the same set of tmux windows, but display different ones.

If multiple monitors are being used and they are different sizes (the typical case when one is a laptop screen), tmux will only display the same number of cells (rows and columns) in each terminal window. Excess cells in larger terminal windows are filled with period characters. To avoid this, set the window option aggressive-resize to "on", preferably in a tmux configuration file.

Having multiple clients attached to the same session can be used for collaborative work, including pair programming. To do this, login to a machine where ssh is enabled and start a tmux session. Then ask each participant to ssh to that machine and attach to the tmux session in the normal way, tmux attach -t {session-name}. Each user will see the results of what others do in the shared session.

Mac-specific Advice

The Mac OS X Terminal program in Snow Leopard and earlier only supports 16 colors. To get 256 colors, iTerm2 must be used. This is a free Terminal replacement that can be downloaded from http://www.iterm2.com/. After starting iTerm2, activate the iTerm menu and select Preferences ... Profiles ... Terminal. In the "Default" profile, set "Report Terminal Type" to "xterm-256color". Quit iTerm2, restart it, and verify that the TERM environment variable is set to "xterm-256color" instead of "xterm". In addition, add the following command to ~/.tmux.conf:
set -g default-terminal "screen-256color"

To verify correct Terminal color configuration, enter these commands in a Terminal window:
wget http://www.vim.org/scripts/download_script.php?src_id=4568 -O colortest
perl colortest -w

There are some preference changes that are needed to allow certain tmux shortcut keys to be used on a Mac. In System Preferences, select Keyboard ... Shortcuts ... Mission Control and uncheck these: "Mission Control", "Application windows", "Move left a space", and "Move right a space". The keyboard shortcuts for those are the tmux defaults for resizing a pane up, down, left, or right by one cell. In iTerm2 preferences, select Profiles ... Keys and select "+Esc" for "Left option key" and "Right option key". This allows cmd-option to be used as the meta key in tmux.

Earlier it was recommended to change the prefix key to C-a. If this is done, also consider changing the caps lock key to function as the control key. There are two reasons for this. The caps lock key is larger than the control key and it is next to the "A" key. To make this change, open System Preferences, select Keyboard ... Keyboard tab ... Modifier Keys, and select "^ Control" from the drop-down list next to "Caps Lock Key".

To make an iTerm2 window fullscreen, press cmd-enter. Press it again to toggle out of fullscreen mode.

Final Thoughts

tmux certainly has a learning curve. However, the effort pays off in increased productivity.

Several people provided feedback on this article. I especially thank Charles Sharp and Brian Hicks who provided the most extensive reviews.

Additional feedback on this article is welcome. Please email it to mark@ociweb.com.

References

• tmux website - https://tmux.github.io/
• tmux book - http://pragprog.com/book/bhtmux/tmux