2001.03_Terminal Emulation.pdf

101command.qxd 02.02.2001 16:52 Uhr Seite 101

AT YOUR COMMAND

BEGINNERS

A TERMINAL

CASE

HEIKE JURZIK

Even though lots of things can

easily be controlled by means

of graphical user interfaces

such as KDE or GNOME –

anyone really wanting to tax

his Linux system cannot avoid

using the command line. Apart

from this, there are also many

situations where it’s good to

know your way around a bit in

the jungle of command lines.

Have you ever got fed up of switching back and

forth between lots of terminals because you wanted

to run several applications at once in the

foreground? Or have you been downcast because a

process which you had to start on a computer at the

workstation was not ready in time to go to the ball,

but you wanted to check the output of the

program? screen is an extremely powerful tool

which can make many tasks easier for you.

Before starting the program for the first time it’s

best to check to what value the environmental

variable TERM ( terminal emulation ) has been set,

since this will be evaluated immediately when the

program is started:

along best with vt100 – so it’s best to check first.

With screen you can simulate up to ten virtual

windows in a single Xterm (or on the console). You

can then run programs in all these windows – with

each of the virtual windows being independent of

the others. Simply type screen – after a short

greeting text there are instructions on what

happens next:

Terminal emulation:

The program responsible for

screen output, appears to the

system as a terminal. Linux

consoles or an Xterm for

example can use certain

control sequences for

highlighting, cursor

positioning etc. Sometimes

these emulate real hardware

terminals, e.g. those of the

type DEC vt100. If the

environmental variable TERM

is set to vt100, the program

can be controlled like a vt100

terminal.

[Press Space or Return to end.]

So using the space bar, you can now enter the realm

of infinite expanses of terminals. You have access to

a whole range of commands, all starting with [Ctrl-

a]: Hold down the [Ctrl] key and type [a]. Now the

program waits for the next command inputs: [Ctrl-

a] [?] for example gives a complete overview of the

key configuration (see Table 1).

huhn@asteroid:# echo $TERM

xterm

huhn@asteroid:# export TERM=vt100

huhn@asteroid:# echo $TERM

vt100

Out of the blue – screen!

The programmers of screen point out explicitly in

their documentation to the fact that this tool gets

Apart from all the control commands within the

window it is of course also possible to provide the

6 · 2001 LINUX MAGAZINE 101

At your command

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BEGINNERS

AT YOUR COMMAND

program with various parameters at the start. In

case you have started screen several times and no

longer know how many and whether these are

currently active, there is the option -ls (stands for:

-list ):

defining your own commands. E.g. if you write in

the .screenrc

bindkey ^f screen ssh marvin.cologne.de

(and not, as described in the Man page, bind xy !),

when [Ctrl-f] is pressed in the screen window, a new

screen will automatically be opened with an ssh

connection to the computer marvin.cologne.de . In

this way you can define lots of useful aliases. If you

would like to have more than 100 lines as standard

buffer, you can use the entry

huhn@asteroid:# screen -list

There are screens on:

1200.pts-10.asteroid (Attached)

1203.pts-14.asteroid (Detached)

Here you can see the process ID ( pid ), then the

virtual terminal ( tty ) in which the screen was started,

the host ( asteroid ), and as the last piece of

information whether it is currently active

(”attached”) or has been put to sleep (”detached”).

Inactive screens can be brought back to life with

screen -r [pid.tty.host] . It is only necessary to specify

the process number and terminal if several screens

are inactive. You can make this task much easier for

yourself by giving the session a name right from the

start: screen -S petronella names your screen

”petronella”. In the overview this is then called:

1364.petronella – the name thus replaces terminal

and host. By the way, if ever a screen process hangs,

you can detect this in the overview from the status

flag ”dead”. You can get rid of it elegantly with the

parameter screen -wipe .

When you revive a screen which has been put to

sleep, you may sometimes want to scroll back to

look at the last outputs from current programs. 100

lines are standard for the buffer. You can alter this

with the aid of the option -h number of lines . So

with screen -h 1000 you can now go back 1000

lines. To move around in this buffer there is a range

of keyboard commands. To do this first go into

Copy/Scrollback mode (see Table 1, [Ctrl-a] [Esc]). If

you already know and use the editor vi , you will

certainly be familiar with the commands for cursor

movement. Otherwise you can find a short

reference on the commands in Table 2.

defscrollback 1000

to define your own buffer size. Another nice feature

is the so-called vbell_msg . For this you must first

define: vbell on , then the desired message which is

to appear when a window receives a ”beep” ([Ctrl-

g]), e.g.

vbell_msg "Hello! Here’s a beep!"

There is a whole range of tips and tricks on this

subject in the very comprehensive Man page. It is

also worthwhile taking a look into the default

configuration file /etc/screenrc . If you would like to

read more on this subject, most distributions have a

very well-written README and an FAQ . The

directory in which these files are located depends

on the distribution. For Red Hat it’s /usr/share/doc/

screen-3.9.5/ , for Debian /usr/doc/screen/ , and for

Mandrake /usr/doc/screen-3.9.5/ .

The author

(Tip: you can read these files, if they end in .gz , thus

are gzip -compressed, with the program zless – this

is the case for example with Debian Linux.)

Otherwise the following applies:

Heike Jurzik works at the

computing centre of the

University of Cologne as

Administrator of the local

news-server. She has been

working on Linux systems since

1996. And because the

computer keyboard is enough

when it comes to keyboard

instruments, instead of piano

she prefers to play the violin in

a symphony orchestra and

when she gets the chance

enjoys reading a good book.

Send bugreports, fixes, enhancements,

t-shirts, money, beer & pizza to

screen@uni-erlangen.de

(hge)

Stars of the small screen

Screen URLs

Homepage of the GNU Project screen is

http://www.gnu.org/software/screen/

A nice collection of information can be

found at http://www.math.fu-berlin.de/

~guckes/screen/ .

You can create a configuration file in your home

directory, .screenrc , in which you can enter specific

wishes for program behaviour. For example if you enter

startup_message off

the greeting message at the start of the program

will be left out. Another practical option is that of

Table 2: The most important commands for Movement command

h, j, k, l

move the cursor line by line or column by column, left, right, up, down.

0, $

go to extreme left or right end of line.

H, L, M

moves the cursor in the column on the far left to the top, bottom or middle.

+, -

line up or down.

jumps to end of buffer.

jumps to start of buffer.

w,b,e

jump word by word: back, forward and to end of word.

102 LINUX MAGAZINE 6 · 2001

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FAVOURITE COMMANDS

BEGINNERS

Ta ble 1: Key combinations in screen

Keyboard shortcut

Command

Meaning

[Ctrl-a] [?]

help

Lists all key configurations.

[Ctrl-a] [c]

screen

Opens an additional virtual window.

[Ctrl-a] [space bar]

Changes to next window, and if the command is repeated, one can ”run through” all the

windows.

[Ctrl-a] [Ctrl-a]

other

Constantly changes back and forth between two windows.

[Ctrl-a] [0...9]

select n

Changes to window with no. n.

[Ctrl-a] [w]

windows

Shows in a line on the lower edge for a short time how many windows have been started,

the current one being highlighted with *.

[Ctrl-a] [a], [s] or [q]

meta/xoff/xon

Sends a [Ctrl-a], [Ctrl-s] or [Ctrl-q] direct to the window, needed for some programs (e.g.

Emacs), which also have [Ctrl-a] control sequences

[Ctrl-a] [x]

lockscreen

Locks the screen – after entering a valid password you can carry on working

[Ctrl-a] [H]

log

Logs the standard output in a file, and depending on the number of the window (1-10) the

logfile is called screenlog.n, calling up [Ctrl-a H] again ends the logging

[Ctrl-a] [Esc]

copy

Changes to copy mode: If there is no mouse to mark text, one can now go with the letters h,

j, k, l to the desired point on the screen, make the marking with the space bar, then go to

the next point, press the space bar again, to store it on the ”clipboard”. With [Ctrl-a] ”]”

( [Ctrl-A] followed by a closing square bracket) incidentally one inserts the marked text, with

[Esc] the action is interrupted.

[Ctrl-a] [d]

detach

”Releases” the screen, all processes started therein continue to run, but the program

detaches itself from the terminal: Now you can log out. With screen -r the screen can be

called up again (complete explanations follow in the text).

[Ctrl-a] [D] [D]

pow_detach

”Power Detach” – not only detaches the screen, but also immediately logs out of the

terminal.

[Ctrl-a] [K]

kill

Destroys the whole screen – fortunately there is a safety challenge at this point: Really kill

this window [y/n]

AT YOUR

COMMAND

RICHARD SMEDLEY

| less [pipe, less]

The | [pipe] is the Unix tool for gluing together

commands by sending the output of one to the

input of the next. Piping several screens to the

pager less enables easy back-and-forth scrolling

with <b> and <f>. For more details of the scrolling

commands available type

If you are fairly new to the command line, you may

have been occasionally frustrated by a list

command, such as ‘ls /etc’ scrolling off the screen

past the information that you wanted. Perhaps you

have used <RtShift><PgUp> and <RtShift><PgDn>,

a very useful utility if your shell allows it – but

annoying over several pages. The answer is:

less —help

If you have never used less before, then you need to

know that ‘q’ will exit the program (and this is also

the command to leave man pages).

ls /etc | less

6 · 2001 LINUX MAGAZINE 103

A series of tips on command

line favourites.

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