TweetFollow Us on Twitter

Screen: Living In A Virtual World

Volume Number: 21 (2005)
Issue Number: 9
Column Tag: Programming

Mac In The Shell

Screen: Living In A Virtual World

by Edward Marczak

Hello ladies and gentlemen! Step right up! Don't be afraid. Gather 'round! Come closer...that's it. Did you know that there are those who are hooked on the CLI? That some of us have to work on a monitor with less than 1024x768 of space? Or perhaps you know people that use ssh to perform a lot of work remotely? Today, I'll demonstrate the magic of "screen". For many, many reasons, screen is an indispensable utility. It's the answer - to several questions, for anyone doing command-line work, from the 12" iBook user, to the dual-G5 30" monitor owner. Watch, and be amazed!

The more things change...

My first job after graduation was at Computer Associates ("Hello, Doug!...who wouldn't know a Mac if he mistakenly walked into an Apple store!"). While CA has attempted to make a push into the PC world, in their heyday they had some of the best selling software for IBM "big iron" - mainframes. Of course, after thinking that I was going to get a job programming games or graphics (not that I had any real experience to speak of), this was an awakening into the things that companies really need. So, I sat down at my desk, and took a gander at the dumb-terminal green screen that was sitting on my desk, staring back at me. Good 'ol 5250 in all its glory. After a little introduction to the world of REXX, JCL and HLLAPI, I thought, "it would be great if I had another terminal here, so I could compare code on one screen without having to quit out and get back into what I'm working on." Well, Mr. Doug, who was a mainframe guy, showed me a great trick shortly after.

"Hey, Ed, I want to show you something really cool. Look at my terminal. Here, I'm logged into System B, but if I press Fn-F2, there's e-mail, and Fn-F3 brings up my text editor. And when I press Fn-F1, there's System B again! We use a system here called virtual terminals. Did anyone ever show you that?" Well, no, they hadn't. Now I had a way to be logged into several systems at once and not lose my place when I needed to switch to another. All with a single physical terminal sitting on my desk.

The more they stay the same!

What does this tale of mainframe tutelage have to do with modern day OS X? Well, virtual terminals, of course! Thanks to the hard work of the GNU volunteers that have contributed to "screen", we have fantastic virtual terminal facilities on our Macs (and Linux, and IRIX, etc.). So what, right? I can open up multiple terminal windows! This is 2005, buddy, and I'm not working on some crummy old green-screen. I've got a dual-mega-hyper-super-secret-quad-X-on-Intel-benchmark-smoking-egg-frying-electricity-sucking machine! I think 16GB of RAM can handle a terminal window or two!

Whoa! I'll show you why screen is more than just a way to have multiple terminals. Apple sees the light on this one, too: screen has shipped with OS X since 10.2. Thanks to this, we can even skip any discussion about acquiring the source or a package and getting it running on your system. We can get right to the good stuff!

What's that in the road? A Head?

What exactly is a virtual terminal? Instead of having physical screens attached to a system for each session, screen allows us to create a session that lives virtually. That is, not attached to any physical console. To even drive that point home, screen allows us to detach the session, and pick it up from anyplace else! Even better, you can have multiple people attached to a single session from anywhere, like a poor man's VNC.

Unix already supports job control. However, once you background a job and logout, you can't bring it back to the foreground. More than that, if you don't take special measures, a background job will die with the parent shell when you log out. No longer will you have to say, "I'm Mike Jones and I use nohup." (Especially if your name isn't Mike Jones).

For the impatient, pop open (or, the excellent iTerm, GLTerm, or other favorite), and type screen. You should get a simple welcome screen as shown here:

Screen version 4.00.02 (FAU) 5-Dec-03

Copyright (c) 1993-2002 Juergen Weigert, Michael Schroeder
Copyright (c) 1987 Oliver Laumann

This program is free software; you can redistribute it and/or modify it under the 
terms of the GNU General Public License as published by the Free Software Foundation; 
either version 2, or (at your option) any later version.

This program is distributed in the hope that it will be useful, but WITHOUT ANY 
WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A 
PARTICULAR PURPOSE. See the GNU General Public License for more details.

You should have received a copy of the GNU General Public License along with this 
program (see the file COPYING); if not, write to the Free Software Foundation, Inc., 
59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.

Send bugreports, fixes, enhancements, t-shirts, money, beer & pizza to

[Press Space or Return to end.]

Once you press space or return, you'll get back a prompt. That's it, you're running under screen! If you expected something fancier, sorry to disappoint, but you'll see that it's worth it!

^a (ctrl-a) becomes the important key when using screen. You use it to give screen commands. Now that you're in screen, here's a quick demonstration. Type ^a, followed by ":" (that's ^a, let go, type the colon). You will see the command mode prompt, ready for input - much like vi. Type "caption always" and press return. You'll see a status line along the bottom of your terminal. It should read, in reverse video, "0 bash". "0" is the window number and "bash" is the default window name. Let's give it something more descriptive. Type ^a-A, and you'll be prompted with:

Set window's title to: bash

Backspace over "bash" and fill in your own title for this window - I'm going to call mine "window0" - and press return. While you're there, get a file listing with ls -l, just to fill the window. Now for the magic: type ^a-c. This will create a new window. Your file listing will seem to disappear, and the status line will change to read "1 bash". This is simply a new terminal - all in the same terminal window. You can vaguely compare this to fast user switching without the cube effect, or switching users. window0 is still there, of course. Let's give this window a title right now. ^a-A, and I title mine window1. On this window, get a process listing with ps ax. Great. Now, how about another window? ^a-c, and you've got "2 bash". I'm going to title this one "topwin". Of course, then I'm going to run top:

top -ocpu.

It's great that we've created a few windows, but it's even more useful if we can move between them! There's a few ways to do that. Here's a quick list, followed by an explanation:

  • ^a-n - next window
  • a-p - previous window
  • ^a-# - Jump to window number "#"
  • ^a-^a - Jump to last window (like a 'back' button)

You can step through each window in order with ^a-n. Try it: you'll wrap to window0 and see the file listing that you left there. Press ^a-n again and you'll see window1 with its old process listing. One more time and you'll get our top display running on topwin (you left it running, didn't you?).

One more trick in the magic show. Press ^a-d. This will detach you from this session of screen. You'll be back to where you were before starting screen, with the exception of a note that says, "[detached]". Now log out. Yep, you heard me: Apple-Shift-Q and log out. This normally crushes all processes for your user. But screen will protect us here. Now, log back in. Open a terminal, and type screen -r. The -r flag instructs screen to "re-attach" to a detached screen, rather than start a new session. When you press return, you'll be magically transported right back to where you left off. Yeah, it's magic.

No! It's "What's that in the road ahead?"!

Anxious to learn more, are you? It would be difficult to present every single thing that screen can do in one column. Even two columns would be tough. So here are my favorite, practical tricks of screen, and how to perform them.

  • Multiplex - Ah, the wonders of modern technology! Look at this miniature marvel, the 12" PowerBook! Light and portable, it's everything you need in a traveling laptop. That is, of course, besides the screen real estate. One Terminal window looks great, but how many are you going to open? Just one, of course! Then run screen, and ^a-c to your heart's content. Even on a larger monitor, how many individual windows do you want?

  • Go home - Nothing like having a personal transporter to get you from place to place. Well, aside from the fact that they don't exist yet! But there is a way to end your workday on time and continue later on. I had a large copy job to run at the end of the day: over a Terabyte. But I was at the client's site, and not at home. Everyone else had gone for the day. Here's the trick: use a machine at the client's site that is accessible via ssh. Pop open and immediately run screen. You can then start your copy job, and disconnect using ^a-d, and go home. Once you're home (or wherever you're going), ssh in and run screen -r to reconnect and check on, or finish, the job. Why sit and stare at file copy status when you could be using that time to travel home?

  • Protect against an unexpected disconnect - In a world of broadband and high-speed access, there are still some secrets. The secret of the unreliable ISP is the most insidious. It's true! You may ssh into a remote machine over the Internet, only to find that you unceremoniously get dropped - Connection reset by peer, connection closed. Dial-up still exists, my friends! If you're managing via dialup, or find that you get dropped connections, run screen immediately after ssh-ing into a remote host. This way, if you get dropped while you're in the middle of editing an rc file with vi, you simply ssh back in, and type screen -r. You'll be right where you left off.

  • Have screen setup my environment for me - Amaze at the automatic setup! Swoon over the customized messages! screen looks for a .screenrc file in your home directory. You can customize the way screen starts up. Here's my startup file:

    defscrollback          20000
    activity 'Activity in local window %n'
    defmonitor                    on
    nethack                       on
    startup_message               off
    caption always "%{= g}%50-%t%{= g} %{= r}| %{= y}%c:%s %Y-%m-%d %{= r}| %{= g}%W"
    screen           -t           irc
    screen           -t           rootwin
    screen           -t           remotewin1
    screen           -t           remotewin2
    screen           -t           shamewin
    screen           -t           localwin
    screen           -t           sshtun

    Here's the explanations:

    defscrollback defines the scrollback buffer for each window.

    activity...defines the activity message. This goes hand-in-hand with the following line, defmonitor on. With this set, when a window you're not watching has activity in it - like when you're waiting for a compile to complete...or your irc session to show some activity - screen will display a status message to let you know. You can customize the message by following the directive with a string. "%n" will substitute the window number.

    nethack on is in my .screenrc file because, well, I'm a geek! With this option on, the standard screen messages are replaced with nethack-like ones. If you've just started using screen, this has the potential to confuse you. Otherwise, it's fun!...if you're me, I suppose.

    startup message off simply suppresses the intro screen.

    caption always, which we saw a simple example of earlier, can also accept a string that will define the status-line display.

    The final screen -t... lines simply fire up new windows for me, titled with the title I supply using the -t switch.

    There's much more about all of these features in the screen man page.

  • Screen sharing - There are many times that people call me for help, and solving the problem involves the command-line. Well, I really want them to watch what I'm doing so they can learn. For this exercise, I may be in place A, they're in place B and the server is in place C. Once again, I sprinkle a little magic dust over my keyboard and I can type in the same session as my remote friend! Of course, I'm just using screen's -x flag. Much like the -r switch, -x also re-connects to a session, although one that is not detached. It's called multi-display mode. Here's the easy way to use it. Have person A ssh into the server and start screen. Have person B ssh into the same server as the same user and run screen -x. Person A and person B will now be sharing the same session. Either user can type and both will see all screen output. What's nice about this is that if there are any sensitive passwords to type, and only one party knows them, you can let the prompt come up and let the appropriate person type it.

All good things...

Those 5 uses are the most common ways I use screen. Of course, there's plenty more in its top hat of tricks.

Since you can detach from a running session of screen, what happens if you start a new session of screen after detaching? Well, you'll have two separate sessions of screen running. If you detach from the second session, and try to reconnect, you'll realize that you have to tell screen which one to reconnect to:

$ screen -r
There are several suitable screens on:
        3848.ttyp1.Jack-Kerouak (Detached)
        574.ttyp2.Jack-Kerouak  (Detached)
Type "screen [-d] -r [pid.]" to resume one of them.

If I want to reconnect to the second session, I can tell screen that with the command screen -r 574. You can also find out this list anytime by using the -list flag:

$ screen -list
There are screens on:
        3848.ttyp1.Jack-Kerouak (Detached)
        574.ttyp2.Jack-Kerouak  (Detached)
2 Sockets in /tmp/uscreens/S-marczak.

Note that the sessions here are exclusive to a single user id. You won't pick up the detached sessions of other users. If something goes awry, and instead of "Detached" one session says "Dead", you can clear out it out with the "-wipe" flag: screen -wipe 3848.

screen will let you split your terminal. Create a few screens (^a-c), create some output on each, and then press ^a-S: Boom! Split-screen! ^a-Tab hops between segments. Each ^a-n (or -p, or -#) is independent of the other window. So, you could have top running in one split, with a long running app in the other. Of course, on my local Mac, I'd probably just open another terminal window, but this is pretty handy on a remote system. ^a-X removes the region that currently has the focus. This is why I never use's split bar. Thank you, screen!

You can screen into an app immediately by passing the program name into screen. screen vi somefile.txt will start vi under screen. This avoids spawning another copy of your shell, too. Naturally, all of the meta keys are then active, and you can create another window if you do need some other functionality.

screen can perform logging for you with the ^a-H keystroke combo. Again, this is of more use on a remote system that you may detach from, but want the output of some long running command. One pressed, ^a-H starts logging, tells you the name of the file it's logging into, and you're done. Pressed again turns off the log. Thought you'd like to know.

screen also sports a history buffer with copy and paste. Nethack mode gives some particularly good messages here. I'll be honest, this is a feature I barely use anymore locally. I have's unlimited scrollback buffer and a mouse. But remember, you can run screen and then disconnect. All of the output will accumulate in screen's scrollback, not on your terminal while in this state. ^a-[ gets you into copy/scrollback mode. Use the arrow keys to move around. Additionally, ^u and ^d will page up and down respectively. The escape key will bail you out.

There are plenty of other options and expansions on what is contained in this month's column. The man page happens to be excellent, so once you're comfortable with the basics, go dig in! One thing that is not possible as-is with the Apple-supplied screen is the ability to do anything fancy with multi-user. ACLs only work if the screen binary is set-uid. Naturally, any set-uid binary brings security implications, so I like the default of 'off' for this. If multiuser support with fine-grain ACLs is what you need, get out of screen, get root, and add the suid bit to screen: chmod 4755 /usr/bin/screen. Run screen again, and get a screen command line with ^a-: and type multiuser on. Then, grant another user full rights with acladd username. Then, "username" from the same system can access your session with "screen -x yourusername/". Otherwise, you can always set up a dummy account that both users can access, let on run screen first, and the second can connect with screen -x. Hit the man page for more information regarding ACLs.

Is there more wonderment? Isn't there always?!? While Apple does place the source to screen on the Darwin source site, there really isn't too much they had to change. The original source compiles cleanly and runs nicely. And just when I was about to say that there was a dearth of documentation regarding screen, I find these while Googling about: .9-to/ and The former is a deeper look at most commands, while the latter is a really nice way to ease yourself into screen.

Must come to an end!

Is screen actually magic? I hope it feels like it to you. I think all technology seems like a small bit of reality (physics, electrical engineering, etc.) and magic. The people who created and continue to work on screen certainly have my humble thanks (plus, I sent them some Nastro Azzuro when I first found screen). The real magic of screen is certainly in the code itself; it allows me to be in more than one place at a time!

Next month, we'll get back to basics. bash basics: scripting for the admin.

Ed Marczak, owns and operates Radiotope, a technology consulting company that implements mail servers and mail automation. When not typing furiously, he spends time with his wife and two daughters. Get your mail on at


Community Search:
MacTech Search:

Software Updates via MacUpdate

MacPilot 8.0 - Enable over 1,200 hidden...
MacPilot gives you the power of UNIX and the simplicity of Macintosh, which means a phenomenal amount of untapped power in your hands! Use MacPilot to unlock over 1,200 features, and access them all... Read more
Typinator 6.7 - Speedy and reliable text...
Typinator turbo-charges your typing productivity. Type a little. Typinator does the rest. We've all faced projects that require repetitive typing tasks. With Typinator, you can store commonly used... Read more
Adobe Lightroom 6.2 - Import, develop, a...
Adobe Lightroom is available as part of Adobe Creative Cloud for as little as $9.99/month bundled with Photoshop CC as part of the photography package. Lightroom 6 is also available for purchase as a... Read more
ForeverSave 2.1.4 - Universal auto-save...
ForeverSave auto-saves all documents you're working on while simultaneously doing backup versioning in the background. Lost data can be quickly restored at any time. Losing data, caused by... Read more
VueScan 9.5.27 - Scanner software with a...
VueScan is a scanning program that works with most high-quality flatbed and film scanners to produce scans that have excellent color fidelity and color balance. VueScan is easy to use, and has... Read more
AirPort Utility 6.3.6 - Set up and manag...
Note: Most recent release available only within OS X 10.11 El Capitan update. Use AirPort Utility to set up and manage your Wi-Fi network and AirPort base stations, including AirPort Express, AirPort... Read more
Quicksilver 1.3.1 - Application launcher...
Quicksilver is a light, fast and free Mac application that gives you the power to control your Mac with keystrokes alone. Quicksilver allows you to find what you need quickly and easily, then act... Read more
Tidy Up (Five Users) 4.1.5 - Find duplic...
Tidy Up is a complete duplicate finder and disk-tidiness utility. With Tidy Up you can search for duplicate files and packages by the owner application, content, type, creator, extension, time... Read more
Mellel 3.4.3 - The word processor of cho...
Mellel is the leading word processor for OS X and has been widely considered the industry standard since its inception. Mellel focuses on writers and scholars for technical writing and multilingual... Read more
Skype - Voice-over-internet p...
Skype allows you to talk to friends, family and co-workers across the Internet without the inconvenience of long distance telephone charges. Using peer-to-peer data transmission technology, Skype... Read more

Swords & Crossbones: An Epic Pirate...
Swords & Crossbones: An Epic Pirate Story 1.0 Device: iOS Universal Category: Games Price: $4.99, Version: 1.0 (iTunes) Description: | Read more »
Camel Up (Games)
Camel Up 1.0.0 Device: iOS Universal Category: Games Price: $4.99, Version: 1.0.0 (iTunes) Description: | Read more »
The Martian: Bring Him Home (Games)
The Martian: Bring Him Home 1.0 Device: iOS Universal Category: Games Price: $2.99, Version: 1.0 (iTunes) Description: Based on the best selling novel and critically acclaimed film, THE MARTIAN tells the story of Astronaut Mark... | Read more »
This Week at 148Apps: September 21-30, 2...
Leap Into Fall With 148Apps How do you know what apps are worth your time and money? Just look to the review team at 148Apps. We sort through the chaos and find the apps you're looking for. The ones we love become Editor’s Choice, standing out above... | Read more »
Tweetbot 4 for Twitter (Social Networki...
Tweetbot 4 for Twitter 4.0 Device: iOS Universal Category: Social Networking Price: $4.99, Version: 4.0 (iTunes) Description: *** 50% off for a limited time. *** | Read more »
Mori (Games)
Mori 1.0 Device: iOS Universal Category: Games Price: $2.99, Version: 1.0 (iTunes) Description: Stop, rewind and unwind with Mori. Time is always running, take a moment to take control. Mori is an action puzzle game about infinitely... | Read more »
100 Years' War (Games)
100 Years' War 1.0 Device: iOS Universal Category: Games Price: $3.99, Version: 1.0 (iTunes) Description: | Read more »
Tower in the Sky (Games)
Tower in the Sky 0.0.60 Device: iOS Universal Category: Games Price: $1.99, Version: 0.0.60 (iTunes) Description: | Read more »
hocus. (Games)
hocus. 1.0.0 Device: iOS Universal Category: Games Price: $.99, Version: 1.0.0 (iTunes) Description: New, polished, mind-bending, minimal puzzle game with dozens of levels and extra-ordinary design Features:- Beautifully crafted... | Read more »
Mos Speedrun 2 (Games)
Mos Speedrun 2 1.0 Device: iOS Universal Category: Games Price: $1.99, Version: 1.0 (iTunes) Description: Mos is back, in her biggest and most exciting adventure ever! Wall-jump to victory through 30 mysterious, action packed levels... | Read more »

Price Scanner via

13-inch 2.5GHz MacBook Pro (refurbished) avai...
Apple has Certified Refurbished 13″ 2.5GHz MacBook Pros available for $829, or $270 off the cost of new models. Apple’s one-year warranty is standard, and shipping is free: - 13″ 2.5GHz MacBook Pros... Read more
27-inch 3.2GHz iMac on sale for $1689, save $...
Adorama has the 27″ 3.2GHz iMac on sale for $1689 including free shipping plus NY & NJ sales tax only. Their price is $110 off MSRP. Read more
12-inch Retina MacBooks on sale for up to $12...
B&H Photo has 12″ Retina MacBooks in stock today and on sale for up to $120 off MSRP. B&H will include free shipping, and there is NY sales tax only: - 12″ 1.1GHz Gray Retina MacBook: $1224 $... Read more
Tablets Shaping Up for Growth in 2016 – Strat...
Observing that Apple, Samsung, and Microsoft have refocused what tablet computers can do, market analysis firm Strategy Analytics believes there is immense opportunity for new and replacement sales... Read more
Apple Interbrand’s Number One Most Valuable G...
Apple and Google hold aced #1 and #2 spots respectively in Interbrand’s 2015 Best Global Brands Report, leading all tech brands that now comprise more than a third of the entire rankings value.... Read more
Apple offering refurbished 2015 13-inch Retin...
Apple is offering Certified Refurbished 2015 13″ Retina MacBook Pros for up to $270 (15%) off the cost of new models. An Apple one-year warranty is included with each model, and shipping is free: -... Read more
Apple refurbished 2015 MacBook Airs available...
Apple has Certified Refurbished 2015 11″ and 13″ MacBook Airs (the latest models), available for up to $180 off the cost of new models. An Apple one-year warranty is included with each MacBook, and... Read more
Adobe Photoshop Elements 14 Gets Haze Removal...
The latest iteration of Adobe’s powerful consumer image editing appliction Photoshop Elements 14 analyzes your photo and removes background haze, so your shot looks sharp all the way to the horizon... Read more
Apple refurbished 15-inch Retina MacBook Pros...
Apple has Certified Refurbished 2015 15″ Retina MacBook Pros available for up to $380 off the cost of new models. An Apple one-year warranty is included with each model, and shipping is free: - 15″ 2... Read more
21-inch iMacs on sale for up to $120 off MSRP
B&H Photo has 21″ iMacs on sale for up to $100 off MSRP including free shipping plus NY sales tax only: - 21″ 1.4GHz iMac: $1029.99 $70 off - 21″ 2.7GHz iMac: $1229 $70 off - 21″ 2.9GHz iMac: $... Read more

Jobs Board

Touch Validation Design (EE) - *Apple* Watc...
**Job Summary** Help launch next-generation Touch Technologies in Apple products. The Touch Technology team develops cutting-edge Touch solutions and technologies that Read more
WW Sales Strategy & Program Manager, *Ap...
**Job Summary** Imagine what you could do here. At Apple , great ideas have a way of becoming great products, services, and customer experiences very quickly. Bring Read more
*Apple* TV Product Design Internship (Spring...
…the mechanical design effort associated with creating world-class products with the Apple TV PD Group. Responsibilities will include working closely with manufacturing, Read more
Product Design Engineer - *Apple* Watch - A...
**Job Summary** Product Design Engineer - WATCH ( Apple Watch) Be an integral part of a small and visible team of world-class Mechanical Engineers making Apple 's Read more
Senior Software System App Engineer, *Apple*...
**Job Summary** The Apple Watch system application team is looking for great software engineers who are comfortable working across all levels of the software stack. From Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.